Solar System
Encyclopedia
The Solar System consists of the Sun
and the astronomical objects gravitationally bound in orbit
around it, all of which formed
from the collapse of a giant molecular cloud
approximately 4.6 billion years ago. The vast majority of the system's mass (well over 99%) is in the Sun. Of the many objects that orbit
the Sun, most of the mass
is contained within eight relatively solitary planets whose orbits are almost circular and lie within a nearly flat disc called the ecliptic plane
. The four smaller inner planets, Mercury
, Venus
, Earth
and Mars
, also called the terrestrial planets, are primarily composed of rock and metal. The four outer planets, the gas giant
s, are substantially more massive than the terrestrials. The two largest, Jupiter
and Saturn
, are composed mainly of hydrogen and helium; the two outermost planets, Uranus
and Neptune
, are composed largely of ices
, such as water, ammonia and methane, and are often referred to separately as "ice giants".
The Solar System is also home to a number of regions populated by smaller objects. The asteroid belt
, which lies between Mars and Jupiter, is similar to the terrestrial planets as it is composed mainly of rock and metal. Beyond Neptune's orbit lie the Kuiper belt
and scattered disc
; linked populations of trans-Neptunian object
s composed mostly of ices
such as water, ammonia and methane. Within these populations, five individual objects, Ceres, Pluto
, Haumea, Makemake and Eris
, are recognized to be large enough to have been rounded by their own gravity, and are thus termed dwarf planets. In addition to thousands of small bodies
in those two regions, various other small body populations, such as comet
s, centaurs and interplanetary dust, freely travel between regions.
Six of the planets and three of the dwarf planets are orbited by natural satellite
s, usually termed "moons" after Earth's Moon
. Each of the outer planets is encircled by planetary ring
s of dust and other particles.
The solar wind
, a flow of plasma
from the Sun, creates a bubble
in the interstellar medium
known as the heliosphere
, which extends out to the edge of the scattered disc
. The hypothetical Oort cloud
, which acts as the source for long-period comets, may also exist at a distance roughly a thousand times further than the heliosphere.
The Solar System is located in the Milky Way
galaxy
, which contains about 200 billion stars.
and categorically different from the divine or ethereal objects that moved through the sky. Although the Greek
philosopher Aristarchus of Samos
had speculated on a heliocentric reordering of the cosmos, Nicolaus Copernicus
was the first to develop a mathematically predictive heliocentric system. His 17th-century successors, Galileo Galilei
, Johannes Kepler
and Isaac Newton
, developed an understanding of physics
that led to the gradual acceptance of the idea that the Earth moves around the Sun and that the planets are governed by the same physical laws that governed the Earth. Additionally, the invention of the telescope led to the discovery of further planets and moons. In more recent times, improvements in the telescope and the use of unmanned spacecraft
have enabled the investigation of geological phenomena such as mountains and craters
, and seasonal meteorological phenomena such as clouds, dust storms and ice caps on the other planets.
The principal component of the Solar System is the Sun, a main-sequence
G2
star
that contains 99.86 percent of the system's known mass and dominates it gravitationally. The Sun's four largest orbiting bodies, the gas giants
, account for 99 percent of the remaining mass, with Jupiter and Saturn together comprising more than 90 percent.
Most large objects in orbit around the Sun lie near the plane of Earth's orbit, known as the ecliptic
. The planets are very close to the ecliptic while comets and Kuiper belt
objects are frequently at significantly greater angles to it. All the planets and most other objects orbit the Sun in the same direction that the Sun is rotating (counter-clockwise, as viewed from above the Sun's north pole). There are exceptions
, such as Halley's Comet.
The overall structure of the charted regions of the Solar System consists of the Sun, four relatively small inner planets surrounded by a belt of rocky asteroids, and four gas giants surrounded by the outer Kuiper belt of icy objects. Astronomers sometimes informally divide this structure into separate regions. The inner Solar System includes the four terrestrial planets and the asteroid belt. The outer Solar System is beyond the asteroids, including the four gas giant planets. Since the discovery of the Kuiper belt, the outermost parts of the Solar System are considered a distinct region consisting of the objects beyond Neptune.
Kepler's laws of planetary motion
describe the orbits of objects about the Sun. Following Kepler's laws, each object travels along an ellipse
with the Sun at one focus
. Objects closer to the Sun (with smaller semi-major axes
) travel more quickly, as they are more affected by the Sun's gravity. On an elliptical orbit, a body's distance from the Sun varies over the course of its year. A body's closest approach to the Sun is called its perihelion, while its most distant point from the Sun is called its aphelion. The orbits of the planets are nearly circular, but many comets, asteroids and Kuiper belt objects follow highly elliptical orbits.
Due to the vast distances involved, many representations of the Solar System show orbits the same distance apart. In reality, with a few exceptions, the farther a planet or belt is from the Sun, the larger the distance between it and the previous orbit. For example, Venus is approximately 0.33 astronomical unit
s (AU) farther out from the Sun than Mercury, while Saturn is 4.3 AU out from Jupiter, and Neptune lies 10.5 AU out from Uranus. Attempts have been made to determine a relationship between these orbital distances (for example, the Titius–Bode law), but no such theory has been accepted.
Most of the planets in the Solar System possess secondary systems of their own, being orbited by planetary objects called natural satellite
s, or moons (two of which are larger than the planet Mercury
), or, in the case of the four gas giant
s, by planetary ring
s; thin bands of tiny particles that orbit them in unison. Most of the largest natural satellites are in synchronous rotation
, with one face permanently turned toward their parent.
and Saturn
, which comprise nearly all the remaining matter, possess atmospheres composed of roughly 99% of those same elements. A composition gradient exists in the Solar System, created by heat and light pressure from the Sun; those objects closer to the Sun, which are more affected by heat and light pressure, are composed of elements with high melting points. Objects farther from the Sun are composed largely of materials with lower melting points. The boundary in the Solar System beyond which those volatile substances could condense is known as the frost line
, and it lies at roughly 4 AU from the Sun.
The objects of the inner Solar System are composed mostly of rock, the collective name for compounds with high melting points, such as silicates, iron or nickel, that remained solid under almost all conditions in the protoplanetary nebula
. Jupiter and Saturn are composed mainly of gases, the astronomical term for materials with extremely low melting points and high vapor pressure
such as molecular hydrogen, helium
, and neon
, which were always in the gaseous phase in the nebula. Ices, like water
, methane
, ammonia
, hydrogen sulfide
and carbon dioxide
, have melting points up to a few hundred kelvins, while their phase depends on the ambient pressure and temperature. They can be found as ices, liquids, or gases in various places in the Solar System, while in the nebula they were either in the solid or gaseous phase. Icy substances comprise the majority of the satellites of the giant planets, as well as most of Uranus and Neptune (the so-called "ice giant
s") and the numerous small objects that lie beyond Neptune's orbit. Together, gases and ices are referred to as volatiles
.
The Sun is the Solar System's star
, and by far its chief component. Its large mass (332,900 Earth masses) produces temperatures and densities in its core great enough to sustain nuclear fusion
, which releases enormous amounts of energy
, mostly radiated
into space
as electromagnetic radiation
, peaking in the 400–700 nm band we call visible light.
The Sun is classified as a type G2 yellow dwarf
, but this name is misleading as, compared to the majority of stars in our galaxy
, the Sun is rather large and bright. Stars are classified by the Hertzsprung–Russell diagram
, a graph that plots the brightness of stars with their surface temperature
s. Generally, hotter stars are brighter. Stars following this pattern are said to be on the main sequence
, and the Sun lies right in the middle of it. However, stars brighter and hotter than the Sun are rare, while substantially dimmer and cooler stars, known as red dwarf
s, are common, making up 85 percent of the stars in the galaxy.
Evidence suggests that the Sun's position on the main sequence puts it in the "prime of life" for a star, in that it has not yet exhausted its store of hydrogen for nuclear fusion. The Sun is growing brighter; early in its history it was 70 percent as bright as it is today.
The Sun is a population I star; it was born in the later stages of the universe's evolution
, and thus contains more elements heavier than hydrogen and helium ("metals
" in astronomical parlance) than older population II stars. Elements heavier than hydrogen and helium were formed in the cores
of ancient and exploding stars, so the first generation of stars had to die before the universe could be enriched with these atoms. The oldest stars contain few metals, while stars born later have more. This high metallicity is thought to have been crucial to the Sun's developing a planetary system
, because planets form from accretion of "metals".
, the Sun radiates a continuous stream of charged particles (a plasma
) known as the solar wind
. This stream of particles spreads outwards at roughly 1.5 million kilometres per hour, creating a tenuous atmosphere (the heliosphere) that permeates the Solar System out to at least 100 AU (see heliopause). This is known as the interplanetary medium
. Activity on the Sun's surface, such as solar flare
s and coronal mass ejection
s, disturb the heliosphere, creating space weather
and causing geomagnetic storm
s. The largest structure within the heliosphere is the heliospheric current sheet
, a spiral form created by the actions of the Sun's rotating magnetic field on the interplanetary medium.
Earth's magnetic field
stops its atmosphere
from being stripped away by the solar wind. Venus and Mars do not have magnetic fields, and as a result, the solar wind causes their atmospheres to gradually bleed away into space. Coronal mass ejection
s and similar events blow a magnetic field and huge quantities of material from the surface of the Sun. The interaction of this magnetic field and material with Earth's magnetic field funnels charged particles into the Earth's upper atmosphere, where its interactions create aurorae
seen near the magnetic poles.
Cosmic rays originate outside the Solar System. The heliosphere partially shields the Solar System, and planetary magnetic fields (for those planets that have them) also provide some protection. The density of cosmic rays in the interstellar medium
and the strength of the Sun's magnetic field change on very long timescales, so the level of cosmic radiation in the Solar System varies, though by how much is unknown.
The interplanetary medium is home to at least two disc-like regions of cosmic dust
. The first, the zodiacal dust cloud
, lies in the inner Solar System and causes zodiacal light
. It was likely formed by collisions within the asteroid belt brought on by interactions with the planets. The second extends from about 10 AU to about 40 AU, and was probably created by similar collisions within the Kuiper belt
.
s and metals, the objects of the inner Solar System are relatively close to the Sun; the radius of this entire region is shorter than the distance between Jupiter and Saturn.
The four inner or terrestrial planets have dense, rocky
compositions, few or no moons
, and no ring systems
. They are composed largely of refractory
minerals, such as the silicate
s, which form their crusts
and mantles
, and metals such as iron
and nickel
, which form their cores
. Three of the four inner planets (Venus, Earth and Mars) have atmosphere
s substantial enough to generate weather
; all have impact crater
s and tectonic
surface features such as rift valley
s and volcano
es. The term inner planet should not be confused with inferior planet, which designates those planets that are closer to the Sun than Earth is (i.e. Mercury and Venus).
Asteroid
s are mostly small Solar System bodies composed mainly of refractory
rocky and metallic mineral
s, with some ice.
The asteroid belt occupies the orbit between Mars and Jupiter, between 2.3 and 3.3 AU from the Sun. It is thought to be remnants from the Solar System's formation that failed to coalesce because of the gravitational interference of Jupiter.
Asteroids range in size from hundreds of kilometres across to microscopic. All asteroids except the largest, Ceres, are classified as small Solar System bodies, but some asteroids such as Vesta
and Hygiea
may be reclassed as dwarf planets if they are shown to have achieved hydrostatic equilibrium
.
The asteroid belt contains tens of thousands, possibly millions, of objects over one kilometre in diameter. Despite this, the total mass of the asteroid belt is unlikely to be more than a thousandth of that of the Earth. The asteroid belt is very sparsely populated; spacecraft
routinely pass through without incident. Asteroids with diameters between 10 and 10−4 m are called meteoroid
s.
, and a dwarf planet. It has a diameter of slightly under 1000 km, and a mass large enough for its own gravity to pull it into a spherical shape. Ceres was considered a planet when it was discovered in the 19th century, but was reclassified as an asteroid in the 1850s as further observations revealed additional asteroids. It was classified in 2006 as a dwarf planet.
s are asteroids that orbit larger asteroids. They are not as clearly distinguished as planetary moons, sometimes being almost as large as their partners. The asteroid belt also contains main-belt comet
s, which may have been the source of Earth's water.
Trojan asteroid
s are located in either of Jupiter's L4 or L5 points (gravitationally stable regions leading and trailing a planet in its orbit); the term "Trojan" is also used for small bodies in any other planetary or satellite Lagrange point. Hilda asteroids
are in a 2:3 resonance
with Jupiter; that is, they go around the Sun three times for every two Jupiter orbits.
The inner Solar System is also dusted with rogue asteroids, many of which cross the orbits of the inner planets.
, also orbit in this region. Due to their greater distance from the Sun, the solid objects in the outer Solar System contain a higher proportion of volatiles such as water, ammonia and methane, than the rocky denizens of the inner Solar System, as the colder temperatures allow these compounds to remain solid.
The four outer planets, or gas giants (sometimes called Jovian planets), collectively make up 99 percent of the mass known to orbit the Sun. Jupiter and Saturn are each many tens of times the mass of the Earth and consist overwhelmingly of hydrogen and helium; Uranus and Neptune are far less massive (<20 Earth masses) and possess more ices in their makeup. For these reasons, some astronomers suggest they belong in their own category, “ice giants.” All four gas giants have rings
, although only Saturn's ring system is easily observed from Earth. The term outer planet should not be confused with superior planet, which designates planets outside Earth's orbit and thus includes both the outer planets and Mars.
Comets are small Solar System bodies, typically only a few kilometres across, composed largely of volatile ices. They have highly eccentric orbits, generally a perihelion within the orbits of the inner planets and an aphelion far beyond Pluto. When a comet enters the inner Solar System, its proximity to the Sun causes its icy surface to sublimate and ion
ise, creating a coma
: a long tail of gas and dust often visible to the naked eye.
Short-period comets have orbits lasting less than two hundred years. Long-period comets have orbits lasting thousands of years. Short-period comets are believed to originate in the Kuiper belt, while long-period comets, such as Hale–Bopp, are believed to originate in the Oort cloud. Many comet groups, such as the Kreutz Sungrazers
, formed from the breakup of a single parent. Some comets with hyperbolic
orbits may originate outside the Solar System, but determining their precise orbits is difficult. Old comets that have had most of their volatiles driven out by solar warming are often categorised as asteroids.
, has a diameter of about 250 km. The first centaur discovered, 2060 Chiron
, has also been classified as comet (95P) since it develops a coma just as comets do when they approach the Sun.
", is still largely unexplored
. It appears to consist overwhelmingly of small worlds (the largest having a diameter only a fifth that of the Earth and a mass far smaller than that of the Moon) composed mainly of rock and ice. This region is sometimes known as the "outer Solar System", though others use that term to mean the region beyond the asteroid belt.
The Kuiper belt, the region's first formation, is a great ring of debris similar to the asteroid belt, but composed mainly of ice. It extends between 30 and 50 AU from the Sun. Though it contains at least three dwarf planets, it is composed mainly of small Solar System bodies. However, many of the largest Kuiper belt objects, such as Quaoar
, Varuna
, and Orcus
, may be reclassified as dwarf planets. There are estimated to be over 100,000 Kuiper belt objects with a diameter greater than 50 km, but the total mass of the Kuiper belt is thought to be only a tenth or even a hundredth the mass of the Earth. Many Kuiper belt objects have multiple satellites, and most have orbits that take them outside the plane of the ecliptic.
The Kuiper belt can be roughly divided into the "classical" belt and the resonances
. Resonances are orbits linked to that of Neptune (e.g. twice for every three Neptune orbits, or once for every two). The first resonance begins within the orbit of Neptune itself. The classical belt consists of objects having no resonance with Neptune, and extends from roughly 39.4 AU to 47.7 AU. Members of the classical Kuiper belt are classified as cubewanos, after the first of their kind to be discovered, , and are still in near primordial, low-eccentricity orbits.
(68 AU average) is the largest known scattered disc object, and caused a debate about what constitutes a planet, since it is 25% more massive than Pluto and about the same diameter. It is the most massive of the known dwarf planets. It has one moon, Dysnomia
. Like Pluto, its orbit is highly eccentric, with a perihelion of 38.2 AU (roughly Pluto's distance from the Sun) and an aphelion of 97.6 AU, and steeply inclined to the ecliptic plane.
. However, the Sun's Roche sphere
, the effective range of its gravitational dominance, is believed to extend up to a thousand times farther.
. The collision occurs at the termination shock, which is roughly 80–100 AU from the Sun upwind of the interstellar medium and roughly 200 AU from the Sun downwind. Here the wind slows dramatically, condenses and becomes more turbulent, forming a great oval structure known as the heliosheath. This structure is believed to look and behave very much like a comet's tail, extending outward for a further 40 AU on the upwind side but tailing many times that distance downwind; but evidence from the Cassini and Interstellar Boundary Explorer
spacecraft has suggested that it is in fact forced into a bubble shape by the constraining action of the interstellar magnetic field. Both Voyager 1
and Voyager 2
are reported to have passed the termination shock and entered the heliosheath, at 94 and 84 AU from the Sun, respectively. The outer boundary of the heliosphere, the heliopause, is the point at which the solar wind finally terminates and is the beginning of interstellar space.
The shape and form of the outer edge of the heliosphere is likely affected by the fluid dynamics
of interactions with the interstellar medium as well as solar magnetic fields prevailing to the south, e.g. it is bluntly shaped with the northern hemisphere extending 9 AU farther than the southern hemisphere. Beyond the heliopause, at around 230 AU, lies the bow shock
, a plasma "wake" left by the Sun as it travels through the Milky Way
.
No spacecraft have yet passed beyond the heliopause, so it is impossible to know for certain the conditions in local interstellar space. It is expected that NASA
's Voyager spacecraft
will pass the heliopause some time in the next decade and transmit valuable data on radiation levels and solar wind back to the Earth. How well the heliosphere shields the Solar System from cosmic rays is poorly understood. A NASA-funded team has developed a concept of a "Vision Mission" dedicated to sending a probe to the heliosphere.
The hypothetical Oort cloud is a spherical cloud of up to a trillion icy objects that is believed to be the source for all long-period comets and to surround the Solar System at roughly 50,000 AU (around 1 light-year
(LY)), and possibly to as far as 100,000 AU (1.87 LY). It is believed to be composed of comets that were ejected from the inner Solar System by gravitational interactions with the outer planets. Oort cloud objects move very slowly, and can be perturbed by infrequent events such as collisions, the gravitational effects of a passing star, or the galactic tide
, the tidal force
exerted by the Milky Way
.
(525.86 AU average) is a large, reddish Pluto-like object with a gigantic, highly elliptical orbit that takes it from about 76 AU at perihelion to 928 AU at aphelion and takes 12,050 years to complete. Mike Brown
, who discovered the object in 2003, asserts that it cannot be part of the scattered disc
or the Kuiper belt as its perihelion is too distant to have been affected by Neptune's migration. He and other astronomers consider it to be the first in an entirely new population, which also may include the object , which has a perihelion of 45 AU, an aphelion of 415 AU, and an orbital period of 3,420 years. Brown terms this population the "Inner Oort cloud," as it may have formed through a similar process, although it is far closer to the Sun. Sedna is very likely a dwarf planet, though its shape has yet to be determined with certainty.
The Solar System is located in the Milky Way
galaxy
, a barred spiral galaxy
with a diameter of about 100,000 light-year
s containing about 200 billion stars. Our Sun resides in one of the Milky Way's outer spiral arms, known as the Orion Arm
or Local Spur. The Sun lies between 25,000 and 28,000 light years from the Galactic Centre, and its speed within the galaxy is about 220 kilometres per second
, so that it completes one revolution every 225–250 million years. This revolution is known as the Solar System's galactic year
. The solar apex
, the direction of the Sun's path through interstellar space, is near the constellation of Hercules
in the direction of the current location of the bright star Vega
. The plane of the ecliptic lies at an angle of about 60° to the galactic plane
.
The Solar System's location in the galaxy is very likely a factor in the evolution
of life
on Earth. Its orbit is close to being circular and is at roughly the same speed as that of the spiral arms, which means it passes through them only rarely. Since spiral arms are home to a far larger concentration of potentially dangerous supernova
e, this has given Earth long periods of interstellar stability for life to evolve. The Solar System also lies well outside the star-crowded environs of the galactic centre. Near the centre, gravitational tugs from nearby stars could perturb bodies in the Oort Cloud and send many comets into the inner Solar System, producing collisions with potentially catastrophic implications for life on Earth. The intense radiation of the galactic centre could also interfere with the development of complex life. Even at the Solar System's current location, some scientists have hypothesised that recent supernovae may have adversely affected life in the last 35,000 years by flinging pieces of expelled stellar core towards the Sun as radioactive dust grains and larger, comet-like bodies.
or Local Fluff, an area of denser cloud in an otherwise sparse region known as the Local Bubble
, an hourglass-shaped cavity in the interstellar medium
roughly 300 light years across. The bubble is suffused with high-temperature plasma that suggests it is the product of several recent supernovae.
There are relatively few stars within ten light years (95 trillion km) of the Sun. The closest is the triple star system Alpha Centauri
, which is about 4.4 light years away. Alpha Centauri A and B are a closely tied pair of Sun-like stars, while the small red dwarf
Alpha Centauri C (also known as Proxima Centauri
) orbits the pair at a distance of 0.2 light years. The stars next closest to the Sun are the red dwarfs Barnard's Star
(at 5.9 light years), Wolf 359 (7.8 light years) and Lalande 21185
(8.3 light years). The largest star within ten light years is Sirius
, a bright main-sequence
star roughly twice the Sun's mass and orbited by a white dwarf
called Sirius B. It lies 8.6 light years away. The remaining systems within ten light years are the binary red dwarf system Luyten 726-8
(8.7 light years) and the solitary red dwarf Ross 154
(9.7 light years). Our closest solitary sun-like star is Tau Ceti
, which lies 11.9 light years away. It has roughly 80 percent the Sun's mass, but only 60 percent its luminosity. The closest known extrasolar planet
to the Sun lies around the star Epsilon Eridani
, a star slightly dimmer and redder than the Sun, which lies 10.5 light years away. Its one confirmed planet, Epsilon Eridani b
, is roughly 1.5 times Jupiter's mass and orbits its star every 6.9 years.
4.568 billion years ago. This initial cloud was likely several light-years across and probably birthed several stars.
As the region that would become the Solar System, known as the pre-solar nebula
, collapsed, conservation of angular momentum
made it rotate faster. The centre, where most of the mass collected, became increasingly hotter than the surrounding disc. As the contracting nebula rotated, it began to flatten into a spinning protoplanetary disc with a diameter of roughly 200 AU
and a hot, dense protostar
at the centre. At this point in its evolution
, the Sun is believed to have been a T Tauri star
. Studies of T Tauri stars show that they are often accompanied by discs of pre-planetary matter with masses of 0.001–0.1 solar mass
es, with the vast majority of the mass of the nebula in the star itself. The planets formed by accretion
from this disk.
Within 50 million years, the pressure and density of hydrogen
in the centre of the protostar became great enough for it to begin thermonuclear fusion
. The temperature, reaction rate, pressure, and density increased until hydrostatic equilibrium
was achieved, with the thermal energy countering the force of gravitational contraction. At this point the Sun became a full-fledged main-sequence
star.
The Solar System as we know it today will last until the Sun begins its evolution off the main sequence of the Hertzsprung–Russell diagram
. As the Sun burns through its supply of hydrogen fuel, the energy output supporting the core tends to decrease, causing it to collapse in on itself. This increase in pressure heats the core, so it burns even faster. As a result, the Sun is growing brighter at a rate of roughly ten percent every 1.1 billion years.
Around 5.4 billion years from now, the hydrogen in the core of the Sun will have been entirely converted to helium, ending the main-sequence phase. As the hydrogen reactions shut down, the core will contract further, increasing pressure and temperature, causing fusion to commence via the helium process. Helium in the core burns at a much hotter temperature, and the energy output will be much greater than during the hydrogen process. At this time, the outer layers of the Sun will expand to roughly up to 260 times its current diameter; the Sun will become a red giant
. Because of its vastly increased surface area, the surface of the Sun will be considerably cooler than it is on the main sequence (2600 K at the coolest).
Eventually, helium in the core will exhaust itself at a much faster rate than the hydrogen, and the Sun's helium burning phase will be but a fraction of the time compared to the hydrogen burning phase. The Sun is not massive enough to commence fusion of heavier elements, and nuclear reactions in the core will dwindle. Its outer layers will fall away into space, leaving a white dwarf
, an extraordinarily dense object, half the original mass of the Sun but only the size of the Earth. The ejected outer layers will form what is known as a planetary nebula
, returning some of the material that formed the Sun to the interstellar medium.
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
and the astronomical objects gravitationally bound in orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
around it, all of which formed
Formation and evolution of the Solar System
The formation and evolution of the Solar System is estimated to have begun 4.568 billion years ago with the gravitational collapse of a small part of a giant molecular cloud...
from the collapse of a giant molecular cloud
Molecular cloud
A molecular cloud, sometimes called a stellar nursery if star formation is occurring within, is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen ....
approximately 4.6 billion years ago. The vast majority of the system's mass (well over 99%) is in the Sun. Of the many objects that orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
the Sun, most of the mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
is contained within eight relatively solitary planets whose orbits are almost circular and lie within a nearly flat disc called the ecliptic plane
Plane of the ecliptic
The plane of the ecliptic is the plane of the Earth's orbit around the Sun. It is the primary reference plane when describing the position of bodies in the Solar System, with celestial latitude being measured relative to the ecliptic plane. In the course of a year, the Sun's apparent path through...
. The four smaller inner planets, Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...
, Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...
, Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
and Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
, also called the terrestrial planets, are primarily composed of rock and metal. The four outer planets, the gas giant
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...
s, are substantially more massive than the terrestrials. The two largest, Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
and Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...
, are composed mainly of hydrogen and helium; the two outermost planets, Uranus
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...
and Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...
, are composed largely of ices
Volatiles
In planetary science, volatiles are that group of chemical elements and chemical compounds with low boiling points that are associated with a planet's or moon's crust and/or atmosphere. Examples include nitrogen, water, carbon dioxide, ammonia, hydrogen, and methane, all compounds of C, H, O...
, such as water, ammonia and methane, and are often referred to separately as "ice giants".
The Solar System is also home to a number of regions populated by smaller objects. The asteroid belt
Asteroid belt
The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets...
, which lies between Mars and Jupiter, is similar to the terrestrial planets as it is composed mainly of rock and metal. Beyond Neptune's orbit lie the Kuiper belt
Kuiper belt
The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...
and scattered disc
Scattered disc
The scattered disc is a distant region of the Solar System that is sparsely populated by icy minor planets, a subset of the broader family of trans-Neptunian objects. The scattered-disc objects have orbital eccentricities ranging as high as 0.8, inclinations as high as 40°, and perihelia greater...
; linked populations of trans-Neptunian object
Trans-Neptunian object
A trans-Neptunian object is any minor planet in the Solar System that orbits the Sun at a greater distance on average than Neptune.The first trans-Neptunian object to be discovered was Pluto in 1930...
s composed mostly of ices
Volatiles
In planetary science, volatiles are that group of chemical elements and chemical compounds with low boiling points that are associated with a planet's or moon's crust and/or atmosphere. Examples include nitrogen, water, carbon dioxide, ammonia, hydrogen, and methane, all compounds of C, H, O...
such as water, ammonia and methane. Within these populations, five individual objects, Ceres, Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...
, Haumea, Makemake and Eris
Eris (dwarf planet)
Eris, formal designation 136199 Eris, is the most massive known dwarf planet in the Solar System and the ninth most massive body known to orbit the Sun directly...
, are recognized to be large enough to have been rounded by their own gravity, and are thus termed dwarf planets. In addition to thousands of small bodies
Small Solar System body
A small Solar System body is an object in the Solar System that is neither a planet nor a dwarf planet, nor a satellite of a planet or dwarf planet:...
in those two regions, various other small body populations, such as comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...
s, centaurs and interplanetary dust, freely travel between regions.
Six of the planets and three of the dwarf planets are orbited by natural satellite
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....
s, usually termed "moons" after Earth's Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
. Each of the outer planets is encircled by planetary ring
Planetary ring
A planetary ring is a ring of cosmic dust and other small particles orbiting around a planet in a flat disc-shaped region.The most notable planetary rings known in Earth's solar system are those around Saturn, but the other three gas giants of the solar system possess ring systems of their...
s of dust and other particles.
The solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...
, a flow of plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
from the Sun, creates a bubble
Stellar wind bubble
Stellar-wind bubble is the astronomical term usually used to describe a cavity light years across filled with hot gas blown into the interstellar medium by the high-velocity stellar wind from a single massive star of type O or B. Weaker stellar winds still blow bubble structures though, and these...
in the interstellar medium
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
known as the heliosphere
Heliosphere
The heliosphere is a bubble in space "blown" into the interstellar medium by the solar wind. Although electrically neutral atoms from interstellar volume can penetrate this bubble, virtually all of the material in the heliosphere emanates from the Sun itself...
, which extends out to the edge of the scattered disc
Scattered disc
The scattered disc is a distant region of the Solar System that is sparsely populated by icy minor planets, a subset of the broader family of trans-Neptunian objects. The scattered-disc objects have orbital eccentricities ranging as high as 0.8, inclinations as high as 40°, and perihelia greater...
. The hypothetical Oort cloud
Oort cloud
The Oort cloud , or the Öpik–Oort cloud , is a hypothesized spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Sun. This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun...
, which acts as the source for long-period comets, may also exist at a distance roughly a thousand times further than the heliosphere.
The Solar System is located in the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
galaxy
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
, which contains about 200 billion stars.
Discovery and exploration
For many thousands of years, humanity, with a few notable exceptions, did not recognize the existence of the Solar System. People believed the Earth to be stationary at the centre of the universeUniverse
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...
and categorically different from the divine or ethereal objects that moved through the sky. Although the Greek
Ancient Greece
Ancient Greece is a civilization belonging to a period of Greek history that lasted from the Archaic period of the 8th to 6th centuries BC to the end of antiquity. Immediately following this period was the beginning of the Early Middle Ages and the Byzantine era. Included in Ancient Greece is the...
philosopher Aristarchus of Samos
Aristarchus of Samos
Aristarchus, or more correctly Aristarchos , was a Greek astronomer and mathematician, born on the island of Samos, in Greece. He presented the first known heliocentric model of the solar system, placing the Sun, not the Earth, at the center of the known universe...
had speculated on a heliocentric reordering of the cosmos, Nicolaus Copernicus
Nicolaus Copernicus
Nicolaus Copernicus was a Renaissance astronomer and the first person to formulate a comprehensive heliocentric cosmology which displaced the Earth from the center of the universe....
was the first to develop a mathematically predictive heliocentric system. His 17th-century successors, Galileo Galilei
Galileo Galilei
Galileo Galilei , was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations and support for Copernicanism...
, Johannes Kepler
Johannes Kepler
Johannes Kepler was a German mathematician, astronomer and astrologer. A key figure in the 17th century scientific revolution, he is best known for his eponymous laws of planetary motion, codified by later astronomers, based on his works Astronomia nova, Harmonices Mundi, and Epitome of Copernican...
and Isaac Newton
Isaac Newton
Sir Isaac Newton PRS was an English physicist, mathematician, astronomer, natural philosopher, alchemist, and theologian, who has been "considered by many to be the greatest and most influential scientist who ever lived."...
, developed an understanding of physics
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...
that led to the gradual acceptance of the idea that the Earth moves around the Sun and that the planets are governed by the same physical laws that governed the Earth. Additionally, the invention of the telescope led to the discovery of further planets and moons. In more recent times, improvements in the telescope and the use of unmanned spacecraft
Unmanned spacecraft
Unmanned spacecraft are spacecraft without people on board, and probably includes unmanned resupply spacecraft, space probes, and most space observatories. A difference between robotic spacecraft and unmanned spacecraft, is that unmanned spacecraft is inclusive to non-robotic unmanned spacecraft,...
have enabled the investigation of geological phenomena such as mountains and craters
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...
, and seasonal meteorological phenomena such as clouds, dust storms and ice caps on the other planets.
Structure
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...
G2
Stellar classification
In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...
star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
that contains 99.86 percent of the system's known mass and dominates it gravitationally. The Sun's four largest orbiting bodies, the gas giants
Gas Giants
Gas Giants were a pop rock band from Tempe, Arizona, formed as a successor project to the Gin Blossoms. The group was known as The Pharaohs when they formed in 1997, but changed their name after their label, A&M Records, merged with Universal Records and the band changed hands, re-signing with...
, account for 99 percent of the remaining mass, with Jupiter and Saturn together comprising more than 90 percent.
Most large objects in orbit around the Sun lie near the plane of Earth's orbit, known as the ecliptic
Ecliptic
The ecliptic is the plane of the earth's orbit around the sun. In more accurate terms, it is the intersection of the celestial sphere with the ecliptic plane, which is the geometric plane containing the mean orbit of the Earth around the Sun...
. The planets are very close to the ecliptic while comets and Kuiper belt
Kuiper belt
The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...
objects are frequently at significantly greater angles to it. All the planets and most other objects orbit the Sun in the same direction that the Sun is rotating (counter-clockwise, as viewed from above the Sun's north pole). There are exceptions
Retrograde motion
Retrograde motion is motion in the direction opposite to the movement of something else, and is the contrary of direct or prograde motion. This motion can be the orbit of one body about another body or about some other point, or the rotation of a single body about its axis, or other phenomena such...
, such as Halley's Comet.
The overall structure of the charted regions of the Solar System consists of the Sun, four relatively small inner planets surrounded by a belt of rocky asteroids, and four gas giants surrounded by the outer Kuiper belt of icy objects. Astronomers sometimes informally divide this structure into separate regions. The inner Solar System includes the four terrestrial planets and the asteroid belt. The outer Solar System is beyond the asteroids, including the four gas giant planets. Since the discovery of the Kuiper belt, the outermost parts of the Solar System are considered a distinct region consisting of the objects beyond Neptune.
Kepler's laws of planetary motion
Kepler's laws of planetary motion
In astronomy, Kepler's laws give a description of the motion of planets around the Sun.Kepler's laws are:#The orbit of every planet is an ellipse with the Sun at one of the two foci....
describe the orbits of objects about the Sun. Following Kepler's laws, each object travels along an ellipse
Ellipse
In geometry, an ellipse is a plane curve that results from the intersection of a cone by a plane in a way that produces a closed curve. Circles are special cases of ellipses, obtained when the cutting plane is orthogonal to the cone's axis...
with the Sun at one focus
Focus (geometry)
In geometry, the foci are a pair of special points with reference to which any of a variety of curves is constructed. For example, foci can be used in defining conic sections, the four types of which are the circle, ellipse, parabola, and hyperbola...
. Objects closer to the Sun (with smaller semi-major axes
Semi-major axis
The major axis of an ellipse is its longest diameter, a line that runs through the centre and both foci, its ends being at the widest points of the shape...
) travel more quickly, as they are more affected by the Sun's gravity. On an elliptical orbit, a body's distance from the Sun varies over the course of its year. A body's closest approach to the Sun is called its perihelion, while its most distant point from the Sun is called its aphelion. The orbits of the planets are nearly circular, but many comets, asteroids and Kuiper belt objects follow highly elliptical orbits.
Due to the vast distances involved, many representations of the Solar System show orbits the same distance apart. In reality, with a few exceptions, the farther a planet or belt is from the Sun, the larger the distance between it and the previous orbit. For example, Venus is approximately 0.33 astronomical unit
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
s (AU) farther out from the Sun than Mercury, while Saturn is 4.3 AU out from Jupiter, and Neptune lies 10.5 AU out from Uranus. Attempts have been made to determine a relationship between these orbital distances (for example, the Titius–Bode law), but no such theory has been accepted.
Most of the planets in the Solar System possess secondary systems of their own, being orbited by planetary objects called natural satellite
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....
s, or moons (two of which are larger than the planet Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...
), or, in the case of the four gas giant
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...
s, by planetary ring
Planetary ring
A planetary ring is a ring of cosmic dust and other small particles orbiting around a planet in a flat disc-shaped region.The most notable planetary rings known in Earth's solar system are those around Saturn, but the other three gas giants of the solar system possess ring systems of their...
s; thin bands of tiny particles that orbit them in unison. Most of the largest natural satellites are in synchronous rotation
Synchronous rotation
In astronomy, synchronous rotation is a planetological term describing a body orbiting another, where the orbiting body takes as long to rotate on its axis as it does to make one orbit; and therefore always keeps the same hemisphere pointed at the body it is orbiting...
, with one face permanently turned toward their parent.
Composition
The Sun, which comprises nearly all the matter in the Solar System, is composed of roughly 98% hydrogen and helium. JupiterJupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
and Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...
, which comprise nearly all the remaining matter, possess atmospheres composed of roughly 99% of those same elements. A composition gradient exists in the Solar System, created by heat and light pressure from the Sun; those objects closer to the Sun, which are more affected by heat and light pressure, are composed of elements with high melting points. Objects farther from the Sun are composed largely of materials with lower melting points. The boundary in the Solar System beyond which those volatile substances could condense is known as the frost line
Frost line (astrophysics)
In astronomy or planetary science, the frost line, also known as the snow line or ice line, refers to a particular distance in the solar nebula from the central protosun where it is cool enough for hydrogen compounds such as water, ammonia, and methane to condense into solid ice grains. Depending...
, and it lies at roughly 4 AU from the Sun.
The objects of the inner Solar System are composed mostly of rock, the collective name for compounds with high melting points, such as silicates, iron or nickel, that remained solid under almost all conditions in the protoplanetary nebula
Protoplanetary nebula
A protoplanetary nebula or preplanetary nebula is an astronomical object which is at the short-lived episode during a star's rapid stellar evolution between the late asymptotic giant branch phase and the subsequent planetary nebula phase. A PPN emits strongly in infrared radiation, and is a...
. Jupiter and Saturn are composed mainly of gases, the astronomical term for materials with extremely low melting points and high vapor pressure
Vapor pressure
Vapor pressure or equilibrium vapor pressure is the pressure of a vapor in thermodynamic equilibrium with its condensed phases in a closed system. All liquids have a tendency to evaporate, and some solids can sublimate into a gaseous form...
such as molecular hydrogen, helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
, and neon
Neon
Neon is the chemical element that has the symbol Ne and an atomic number of 10. Although a very common element in the universe, it is rare on Earth. A colorless, inert noble gas under standard conditions, neon gives a distinct reddish-orange glow when used in either low-voltage neon glow lamps or...
, which were always in the gaseous phase in the nebula. Ices, like water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
, methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
, ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
, hydrogen sulfide
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...
and carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
, have melting points up to a few hundred kelvins, while their phase depends on the ambient pressure and temperature. They can be found as ices, liquids, or gases in various places in the Solar System, while in the nebula they were either in the solid or gaseous phase. Icy substances comprise the majority of the satellites of the giant planets, as well as most of Uranus and Neptune (the so-called "ice giant
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...
s") and the numerous small objects that lie beyond Neptune's orbit. Together, gases and ices are referred to as volatiles
Volatiles
In planetary science, volatiles are that group of chemical elements and chemical compounds with low boiling points that are associated with a planet's or moon's crust and/or atmosphere. Examples include nitrogen, water, carbon dioxide, ammonia, hydrogen, and methane, all compounds of C, H, O...
.
Sun
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
, and by far its chief component. Its large mass (332,900 Earth masses) produces temperatures and densities in its core great enough to sustain nuclear fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
, which releases enormous amounts of energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
, mostly radiated
Radiant energy
Radiant energy is the energy of electromagnetic waves. The quantity of radiant energy may be calculated by integrating radiant flux with respect to time and, like all forms of energy, its SI unit is the joule. The term is used particularly when radiation is emitted by a source into the...
into space
Outer space
Outer space is the void that exists between celestial bodies, including the Earth. It is not completely empty, but consists of a hard vacuum containing a low density of particles: predominantly a plasma of hydrogen and helium, as well as electromagnetic radiation, magnetic fields, and neutrinos....
as electromagnetic radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
, peaking in the 400–700 nm band we call visible light.
The Sun is classified as a type G2 yellow dwarf
Yellow dwarf
A G-type main-sequence star , often called a yellow dwarf, is a main-sequence star of spectral type G and luminosity class V. Such a star has about 0.8 to 1.2 solar masses and surface temperature of between 5,300 and 6,000 K., Tables VII, VIII...
, but this name is misleading as, compared to the majority of stars in our galaxy
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
, the Sun is rather large and bright. Stars are classified by the Hertzsprung–Russell diagram
Hertzsprung–Russell diagram
The Hertzsprung–Russell diagram is a scatter graph of stars showing the relationship between the stars' absolute magnitudes or luminosities versus their spectral types or classifications and effective temperatures. Hertzsprung–Russell diagrams are not pictures or maps of the locations of the stars...
, a graph that plots the brightness of stars with their surface temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
s. Generally, hotter stars are brighter. Stars following this pattern are said to be on the main sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...
, and the Sun lies right in the middle of it. However, stars brighter and hotter than the Sun are rare, while substantially dimmer and cooler stars, known as red dwarf
Red dwarf
According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type....
s, are common, making up 85 percent of the stars in the galaxy.
Evidence suggests that the Sun's position on the main sequence puts it in the "prime of life" for a star, in that it has not yet exhausted its store of hydrogen for nuclear fusion. The Sun is growing brighter; early in its history it was 70 percent as bright as it is today.
The Sun is a population I star; it was born in the later stages of the universe's evolution
Timeline of the Big Bang
This timeline of the Big Bang describes the history of the universe according to the prevailing scientific theory of how the universe came into being, using the cosmological time parameter of comoving coordinates...
, and thus contains more elements heavier than hydrogen and helium ("metals
Metallicity
In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...
" in astronomical parlance) than older population II stars. Elements heavier than hydrogen and helium were formed in the cores
Solar core
The core of the Sun is considered to extend from the center to about 0.2 to 0.25 solar radius. It is the hottest part of the Sun and of the Solar System. It has a density of up to 150 g/cm³ and a temperature of close to 15,000,000 kelvin...
of ancient and exploding stars, so the first generation of stars had to die before the universe could be enriched with these atoms. The oldest stars contain few metals, while stars born later have more. This high metallicity is thought to have been crucial to the Sun's developing a planetary system
Planetary system
A planetary system consists of the various non-stellar objects orbiting a star such as planets, dwarf planets , asteroids, meteoroids, comets, and cosmic dust...
, because planets form from accretion of "metals".
Interplanetary medium
Along with lightSunlight
Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...
, the Sun radiates a continuous stream of charged particles (a plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
) known as the solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...
. This stream of particles spreads outwards at roughly 1.5 million kilometres per hour, creating a tenuous atmosphere (the heliosphere) that permeates the Solar System out to at least 100 AU (see heliopause). This is known as the interplanetary medium
Interplanetary medium
The interplanetary medium is the material which fills the solar system and through which all the larger solar system bodies such as planets, asteroids and comets move.-Composition and physical characteristics:...
. Activity on the Sun's surface, such as solar flare
Solar flare
A solar flare is a sudden brightening observed over the Sun surface or the solar limb, which is interpreted as a large energy release of up to 6 × 1025 joules of energy . The flare ejects clouds of electrons, ions, and atoms through the corona into space. These clouds typically reach Earth a day...
s and coronal mass ejection
Coronal mass ejection
A coronal mass ejection is a massive burst of solar wind, other light isotope plasma, and magnetic fields rising above the solar corona or being released into space....
s, disturb the heliosphere, creating space weather
Space weather
Space weather is the concept of changing environmental conditions in near-Earth space or thespace from the Sun's atmosphere to the Earth's atmosphere. It is distinct from the concept ofweather within the Earth's planetary atmosphere...
and causing geomagnetic storm
Geomagnetic storm
A geomagnetic storm is a temporary disturbance of the Earth's magnetosphere caused by a disturbance in the interplanetary medium. A geomagnetic storm is a major component of space weather and provides the input for many other components of space weather...
s. The largest structure within the heliosphere is the heliospheric current sheet
Heliospheric current sheet
The heliospheric current sheet is the surface within the Solar System where the polarity of the Sun's magnetic field changes from north to south. This field extends throughout the Sun's equatorial plane in the heliosphere. The shape of the current sheet results from the influence of the Sun's...
, a spiral form created by the actions of the Sun's rotating magnetic field on the interplanetary medium.
Earth's magnetic field
Earth's magnetic field
Earth's magnetic field is the magnetic field that extends from the Earth's inner core to where it meets the solar wind, a stream of energetic particles emanating from the Sun...
stops its atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
from being stripped away by the solar wind. Venus and Mars do not have magnetic fields, and as a result, the solar wind causes their atmospheres to gradually bleed away into space. Coronal mass ejection
Coronal mass ejection
A coronal mass ejection is a massive burst of solar wind, other light isotope plasma, and magnetic fields rising above the solar corona or being released into space....
s and similar events blow a magnetic field and huge quantities of material from the surface of the Sun. The interaction of this magnetic field and material with Earth's magnetic field funnels charged particles into the Earth's upper atmosphere, where its interactions create aurorae
Aurora (astronomy)
An aurora is a natural light display in the sky particularly in the high latitude regions, caused by the collision of energetic charged particles with atoms in the high altitude atmosphere...
seen near the magnetic poles.
Cosmic rays originate outside the Solar System. The heliosphere partially shields the Solar System, and planetary magnetic fields (for those planets that have them) also provide some protection. The density of cosmic rays in the interstellar medium
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
and the strength of the Sun's magnetic field change on very long timescales, so the level of cosmic radiation in the Solar System varies, though by how much is unknown.
The interplanetary medium is home to at least two disc-like regions of cosmic dust
Cosmic dust
Cosmic dust is a type of dust composed of particles in space which are a few molecules to 0.1 µm in size. Cosmic dust can be further distinguished by its astronomical location; for example: intergalactic dust, interstellar dust, interplanetary dust and circumplanetary dust .In our own Solar...
. The first, the zodiacal dust cloud
Interplanetary dust cloud
The interplanetary dust cloud is cosmic dust which pervades the space between planets in the Solar System and in other planetary systems...
, lies in the inner Solar System and causes zodiacal light
Zodiacal light
Zodiacal light is a faint, roughly triangular, whitish glow seen in the night sky which appears to extend up from the vicinity of the sun along the ecliptic or zodiac. Caused by sunlight scattered by space dust in the zodiacal cloud, it is so faint that either moonlight or light pollution renders...
. It was likely formed by collisions within the asteroid belt brought on by interactions with the planets. The second extends from about 10 AU to about 40 AU, and was probably created by similar collisions within the Kuiper belt
Kuiper belt
The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...
.
Inner Solar System
The inner Solar System is the traditional name for the region comprising the terrestrial planets and asteroids. Composed mainly of silicateSilicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...
s and metals, the objects of the inner Solar System are relatively close to the Sun; the radius of this entire region is shorter than the distance between Jupiter and Saturn.
Inner planets
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
compositions, few or no moons
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....
, and no ring systems
Planetary ring
A planetary ring is a ring of cosmic dust and other small particles orbiting around a planet in a flat disc-shaped region.The most notable planetary rings known in Earth's solar system are those around Saturn, but the other three gas giants of the solar system possess ring systems of their...
. They are composed largely of refractory
Refractory (astronomy)
In planetary science, any material that has a relatively high equilibrium condensation temperature is called refractory. The opposite of refractory is volatile....
minerals, such as the silicate
Silicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...
s, which form their crusts
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...
and mantles
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
, and metals such as iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
and nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
, which form their cores
Planetary core
The planetary core consists of the innermost layer of a planet.The core may be composed of solid and liquid layers, while the cores of Mars and Venus are thought to be completely solid as they lack an internally generated magnetic field. In our solar system, core size can range from about 20% to...
. Three of the four inner planets (Venus, Earth and Mars) have atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
s substantial enough to generate weather
Weather
Weather is the state of the atmosphere, to the degree that it is hot or cold, wet or dry, calm or stormy, clear or cloudy. Most weather phenomena occur in the troposphere, just below the stratosphere. Weather refers, generally, to day-to-day temperature and precipitation activity, whereas climate...
; all have impact crater
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...
s and tectonic
Tectonics
Tectonics is a field of study within geology concerned generally with the structures within the lithosphere of the Earth and particularly with the forces and movements that have operated in a region to create these structures.Tectonics is concerned with the orogenies and tectonic development of...
surface features such as rift valley
Rift valley
A rift valley is a linear-shaped lowland between highlands or mountain ranges created by the action of a geologic rift or fault. This action is manifest as crustal extension, a spreading apart of the surface which is subsequently further deepened by the forces of erosion...
s and volcano
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...
es. The term inner planet should not be confused with inferior planet, which designates those planets that are closer to the Sun than Earth is (i.e. Mercury and Venus).
Mercury
- MercuryMercury (planet)Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...
(0.4 AUAstronomical unitAn astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
from the Sun) is the closest planet to the Sun and the smallest planet in the Solar System (0.055 Earth masses). Mercury has no natural satellites, and its only known geological features besides impact craters are lobed ridges or rupesRupesRupes is the Latin word for 'cliff'. It is used in planetary geology to refer to escarpments on other planets, such as Mercury, and moons, such as Luna, Earth's natural satellite.How rupes are formed is, as of 2008, a matter of speculation...
, probably produced by a period of contraction early in its history. Mercury's almost negligible atmosphere consists of atoms blasted off its surface by the solar wind. Its relatively large iron core and thin mantle have not yet been adequately explained. Hypotheses include that its outer layers were stripped off by a giant impact, and that it was prevented from fully accreting by the young Sun's energy.
Venus
- VenusVenusVenus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...
(0.7 AU from the Sun) is close in size to Earth (0.815 Earth masses), and, like Earth, has a thick silicate mantle around an iron core, a substantial atmosphere and evidence of internal geological activity. However, it is much drier than Earth and its atmosphere is ninety times as dense. Venus has no natural satellites. It is the hottest planet, with surface temperatures over 400 °CCelsiusCelsius is a scale and unit of measurement for temperature. It is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...
, most likely due to the amount of greenhouse gasGreenhouse gasA greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...
es in the atmosphere. No definitive evidence of current geological activity has been detected on Venus, but it has no magnetic field that would prevent depletion of its substantial atmosphere, which suggests that its atmosphere is regularly replenished by volcanic eruptions.
Earth
- EarthEarthEarth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
(1 AU from the Sun) is the largest and densest of the inner planets, the only one known to have current geological activity, and is the only place in the Solar System where lifeLifeLife is a characteristic that distinguishes objects that have signaling and self-sustaining processes from those that do not, either because such functions have ceased , or else because they lack such functions and are classified as inanimate...
is known to exist. Its liquid hydrosphereHydrosphereA hydrosphere in physical geography describes the combined mass of water found on, under, and over the surface of a planet....
is unique among the terrestrial planets, and it is also the only planet where plate tectonicsPlate tectonicsPlate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
has been observed. Earth's atmosphere is radically different from those of the other planets, having been altered by the presence of life to contain 21% free oxygenOxygenOxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
. It has one natural satellite, the MoonMoonThe Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
, the only large satellite of a terrestrial planet in the Solar System.
Mars
- MarsMarsMars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
(1.5 AU from the Sun) is smaller than Earth and Venus (0.107 Earth masses). It possesses an atmosphere of mostly carbon dioxideCarbon dioxideCarbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
with a surface pressure of 6.1 millibars (roughly 0.6 percent that of the Earth's). Its surface, peppered with vast volcanoes such as Olympus MonsOlympus MonsOlympus Mons is a large volcanic mountain on the planet Mars. At a height of almost , it is one of the tallest mountains in the Solar System, three times as tall as Mount Everest and more than twice the height of Mauna Kea the tallest mountain on Earth. Olympus Mons is the youngest of the large...
and rift valleys such as Valles MarinerisValles MarinerisValles Marineris is a system of canyons that runs along the Martian surface east of the Tharsis region...
, shows geological activity that may have persisted until as recently as 2 million years ago. Its red colour comes from iron oxideIron(III) oxideIron oxide or ferric oxide is the inorganic compound with the formula Fe2O3. It is one of the three main oxides of iron, the other two being iron oxide , which is rare, and iron oxide , which also occurs naturally as the mineral magnetite. As the mineral known as hematite, Fe2O3 is the main...
(rust) in its soil. Mars has two tiny natural satellites (DeimosDeimos (moon)Deimos is the smaller and outer of Mars's two moons . It is named after Deimos, a figure representing dread in Greek Mythology. Its systematic designation is '.-Discovery:Deimos was discovered by Asaph Hall, Sr...
and PhobosPhobos (moon)Phobos is the larger and closer of the two natural satellites of Mars. Both moons were discovered in 1877. With a mean radius of , Phobos is 7.24 times as massive as Deimos...
) thought to be captured asteroidAsteroidAsteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...
s.
Asteroid belt
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...
s are mostly small Solar System bodies composed mainly of refractory
Refractory (astronomy)
In planetary science, any material that has a relatively high equilibrium condensation temperature is called refractory. The opposite of refractory is volatile....
rocky and metallic mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s, with some ice.
The asteroid belt occupies the orbit between Mars and Jupiter, between 2.3 and 3.3 AU from the Sun. It is thought to be remnants from the Solar System's formation that failed to coalesce because of the gravitational interference of Jupiter.
Asteroids range in size from hundreds of kilometres across to microscopic. All asteroids except the largest, Ceres, are classified as small Solar System bodies, but some asteroids such as Vesta
4 Vesta
Vesta, formally designated 4 Vesta, is one of the largest asteroids, with a mean diameter of about . It was discovered by Heinrich Wilhelm Olbers on March 29, 1807, and is named after the Roman virgin goddess of home and hearth, Vesta....
and Hygiea
10 Hygiea
10 Hygiea is an asteroid located in the asteroid belt. With somewhat oblong diameters of 350–500 km, and a mass estimated to be 2.9% of the total mass of the belt, it is the fourth largest asteroid by volume and mass...
may be reclassed as dwarf planets if they are shown to have achieved hydrostatic equilibrium
Hydrostatic equilibrium
Hydrostatic equilibrium or hydrostatic balance is the condition in fluid mechanics where a volume of a fluid is at rest or at constant velocity. This occurs when compression due to gravity is balanced by a pressure gradient force...
.
The asteroid belt contains tens of thousands, possibly millions, of objects over one kilometre in diameter. Despite this, the total mass of the asteroid belt is unlikely to be more than a thousandth of that of the Earth. The asteroid belt is very sparsely populated; spacecraft
Space probe
A robotic spacecraft is a spacecraft with no humans on board, that is usually under telerobotic control. A robotic spacecraft designed to make scientific research measurements is often called a space probe. Many space missions are more suited to telerobotic rather than crewed operation, due to...
routinely pass through without incident. Asteroids with diameters between 10 and 10−4 m are called meteoroid
Meteoroid
A meteoroid is a sand- to boulder-sized particle of debris in the Solar System. The visible path of a meteoroid that enters Earth's atmosphere is called a meteor, or colloquially a shooting star or falling star. If a meteoroid reaches the ground and survives impact, then it is called a meteorite...
s.
Ceres
Ceres (2.77 AU) is the largest asteroid, a protoplanetProtoplanet
Protoplanets are large planetary embryos that originate within protoplanetary discs and have undergone internal melting to produce differentiated interiors. They are believed to form out of kilometer-sized planetesimals that attract each other gravitationally and collide...
, and a dwarf planet. It has a diameter of slightly under 1000 km, and a mass large enough for its own gravity to pull it into a spherical shape. Ceres was considered a planet when it was discovered in the 19th century, but was reclassified as an asteroid in the 1850s as further observations revealed additional asteroids. It was classified in 2006 as a dwarf planet.
Asteroid groups
Asteroids in the asteroid belt are divided into asteroid groups and families based on their orbital characteristics. Asteroid moonAsteroid moon
A minor planet moon is an astronomical body that orbits a minor planet as its natural satellite. It is thought that many asteroids and Kuiper belt objects may possess moons, in some cases quite substantial in size...
s are asteroids that orbit larger asteroids. They are not as clearly distinguished as planetary moons, sometimes being almost as large as their partners. The asteroid belt also contains main-belt comet
Main-belt comet
Main-belt comets are bodies orbiting within the main asteroid belt which have shown cometary activity during part of their orbit. The Jet Propulsion Laboratory defines a main-belt asteroid as an asteroid with a semi-major axis of more than 2 AU but less than 3.2 AU, and a perihelion of no less...
s, which may have been the source of Earth's water.
Trojan asteroid
Trojan asteroid
The Jupiter Trojans, commonly called Trojans or Trojan asteroids, are a large group of objects that share the orbit of the planet Jupiter around the Sun. Relative to Jupiter, each Trojan librates around one of the planet's two Lagrangian points of stability, and , that respectively lie 60° ahead...
s are located in either of Jupiter's L4 or L5 points (gravitationally stable regions leading and trailing a planet in its orbit); the term "Trojan" is also used for small bodies in any other planetary or satellite Lagrange point. Hilda asteroids
Hilda family
The Hilda asteroids consists of asteroids with a semi-major axis between 3.7 AU and 4.2 AU, an eccentricity less than 0.3, and an inclination less than 20°. They do not form a true asteroid family, in the sense that they do not descend from a common parent object. Instead, this is a dynamical...
are in a 2:3 resonance
Orbital resonance
In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...
with Jupiter; that is, they go around the Sun three times for every two Jupiter orbits.
The inner Solar System is also dusted with rogue asteroids, many of which cross the orbits of the inner planets.
Outer Solar System
The outer region of the Solar System is home to the gas giants and their large moons. Many short-period comets, including the centaursCentaur (planetoid)
Centaurs are an unstable orbital class of minor planets that behave with characteristics of both asteroids and comets. They are named after the mythological race of beings, centaurs, which were a mixture of horse and human...
, also orbit in this region. Due to their greater distance from the Sun, the solid objects in the outer Solar System contain a higher proportion of volatiles such as water, ammonia and methane, than the rocky denizens of the inner Solar System, as the colder temperatures allow these compounds to remain solid.
Outer planets
Planetary ring
A planetary ring is a ring of cosmic dust and other small particles orbiting around a planet in a flat disc-shaped region.The most notable planetary rings known in Earth's solar system are those around Saturn, but the other three gas giants of the solar system possess ring systems of their...
, although only Saturn's ring system is easily observed from Earth. The term outer planet should not be confused with superior planet, which designates planets outside Earth's orbit and thus includes both the outer planets and Mars.
Jupiter
- JupiterJupiterJupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
(5.2 AU), at 318 Earth masses, is 2.5 times the mass of all the other planets put together. It is composed largely of hydrogenHydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
and heliumHeliumHelium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
. Jupiter's strong internal heat creates a number of semi-permanent features in its atmosphere, such as cloud bands and the Great Red Spot. - Jupiter has 64 known satellites. The four largest, GanymedeGanymede (moon)Ganymede is a satellite of Jupiter and the largest moon in the Solar System. It is the seventh moon and third Galilean satellite outward from Jupiter. Completing an orbit in roughly seven days, Ganymede participates in a 1:2:4 orbital resonance with the moons Europa and Io, respectively...
, CallistoCallisto (moon)Callisto named after the Greek mythological figure of Callisto) is a moon of the planet Jupiter. It was discovered in 1610 by Galileo Galilei. It is the third-largest moon in the Solar System and the second largest in the Jovian system, after Ganymede. Callisto has about 99% the diameter of the...
, IoIo (moon)Io ) is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of , the fourth-largest moon in the Solar System. It was named after the mythological character of Io, a priestess of Hera who became one of the lovers of Zeus....
, and EuropaEuropa (moon)Europa Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock and probably has an iron core. It has a tenuous atmosphere composed primarily of oxygen. Its surface is composed of ice and is one of the smoothest in the Solar System. This surface is striated by cracks and...
, show similarities to the terrestrial planets, such as volcanism and internal heating. Ganymede, the largest satellite in the Solar System, is larger than Mercury.
Saturn
- SaturnSaturnSaturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...
(9.5 AU), distinguished by its extensive ring systemRings of SaturnThe rings of Saturn are the most extensive planetary ring system of any planet in the Solar System. They consist of countless small particles, ranging in size from micrometres to metres, that form clumps that in turn orbit about Saturn...
, has several similarities to Jupiter, such as its atmospheric composition and magnetosphere. Although Saturn has 60% of Jupiter's volume, it is less than a third as massive, at 95 Earth masses, making it the least dense planet in the Solar System. The rings of Saturn are made up of small ice and rock particles. - Saturn has 62 confirmed satellites; two of which, TitanTitan (moon)Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....
and EnceladusEnceladus (moon)Enceladus is the sixth-largest of the moons of Saturn. It was discovered in 1789 by William Herschel. Until the two Voyager spacecraft passed near it in the early 1980s very little was known about this small moon besides the identification of water ice on its surface...
, show signs of geological activity, though they are largely made of iceCryovolcanoA cryovolcano is a volcano that erupts volatiles such as water, ammonia or methane, instead of molten rock. Collectively referred to as cryomagma or ice-volcanic melt, these substances are usually liquids and form plumes, but can also be in vapour form...
. Titan, the second-largest moon in the Solar System, is larger than Mercury and the only satellite in the Solar System with a substantial atmosphere.
Uranus
- UranusUranusUranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...
(19.6 AU), at 14 Earth masses, is the lightest of the outer planets. Uniquely among the planets, it orbits the Sun on its side; its axial tiltAxial tiltIn astronomy, axial tilt is the angle between an object's rotational axis, and a line perpendicular to its orbital plane...
is over ninety degrees to the eclipticEclipticThe ecliptic is the plane of the earth's orbit around the sun. In more accurate terms, it is the intersection of the celestial sphere with the ecliptic plane, which is the geometric plane containing the mean orbit of the Earth around the Sun...
. It has a much colder core than the other gas giants, and radiates very little heat into space. - Uranus has 27 known satellites, the largest ones being TitaniaTitania (moon)Titania is the largest of the moons of Uranus and the eighth largest moon in the Solar System at a diameter of 1578 km. Discovered by William Herschel in 1787, Titania is named after the queen of the fairies in Shakespeare's A Midsummer Night's Dream...
, OberonOberon (moon)Oberon , also designated ', is the outermost major moon of the planet Uranus. It is the second largest and second most massive of the Uranian moons, and the ninth most massive moon in the Solar System. Discovered by William Herschel in 1787, Oberon is named after the mythical king of the fairies...
, UmbrielUmbriel (moon)Umbriel is a moon of Uranus discovered on October 24, 1851, by William Lassell. It was discovered at the same time as Ariel and named after a character in Alexander Pope's poem The Rape of the Lock. Umbriel consists mainly of ice with a substantial fraction of rock, and may be differentiated into a...
, ArielAriel (moon)Ariel is the brightest and fourth-largest of the 27 known moons of Uranus. Ariel orbits and rotates in the equatorial plane of Uranus, which is almost perpendicular to the orbit of Uranus, and so has an extreme seasonal cycle....
and MirandaMiranda (moon)-External links:* at * at The Nine8 Planets* at Views of the Solar System* * from the...
.
Neptune
- NeptuneNeptuneNeptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...
(30 AU), though slightly smaller than Uranus, is more massive (equivalent to 17 Earths) and therefore more denseDensityThe mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
. It radiates more internal heat, but not as much as Jupiter or Saturn. - Neptune has 13 known satellites. The largest, TritonTriton (moon)Triton is the largest moon of the planet Neptune, discovered on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2,700 km in diameter, it is...
, is geologically active, with geyserGeyserA geyser is a spring characterized by intermittent discharge of water ejected turbulently and accompanied by a vapour phase . The word geyser comes from Geysir, the name of an erupting spring at Haukadalur, Iceland; that name, in turn, comes from the Icelandic verb geysa, "to gush", the verb...
s of liquid nitrogenLiquid nitrogenLiquid nitrogen is nitrogen in a liquid state at a very low temperature. It is produced industrially by fractional distillation of liquid air. Liquid nitrogen is a colourless clear liquid with density of 0.807 g/mL at its boiling point and a dielectric constant of 1.4...
. Triton is the only large satellite with a retrograde orbit. Neptune is accompanied in its orbit by a number of minor planetMinor planetAn asteroid group or minor-planet group is a population of minor planets that have a share broadly similar orbits. Members are generally unrelated to each other, unlike in an asteroid family, which often results from the break-up of a single asteroid...
s, termed Neptune TrojanNeptune TrojanNeptune trojans are Kuiper belt object-like bodies in solar orbit that have the same orbital period as Neptune and follow roughly the same orbital path...
s, that are in 1:1 resonanceOrbital resonanceIn celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...
with it.
Comets
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
ise, creating a coma
Coma (cometary)
frame|right|The [[153P/Ikeya-Zhang|comet Ikeya-Zhang]] exhibiting a bright, condensed coma In astronomy, a coma is the nebulous envelope around the nucleus of a comet. It is formed when the comet passes close to the Sun on its highly elliptical orbit; as the comet warms, parts of it sublimate...
: a long tail of gas and dust often visible to the naked eye.
Short-period comets have orbits lasting less than two hundred years. Long-period comets have orbits lasting thousands of years. Short-period comets are believed to originate in the Kuiper belt, while long-period comets, such as Hale–Bopp, are believed to originate in the Oort cloud. Many comet groups, such as the Kreutz Sungrazers
Kreutz Sungrazers
The Kreutz Sungrazers are a family of sungrazing comets, characterized by orbits taking them extremely close to the Sun at perihelion. They are believed to be fragments of one large comet that broke up several centuries ago and are named for German astronomer Heinrich Kreutz, who first...
, formed from the breakup of a single parent. Some comets with hyperbolic
Hyperbolic trajectory
In astrodynamics or celestial mechanics a hyperbolic trajectory is a Kepler orbit with the eccentricity greater than 1. Under standard assumptions a body traveling along this trajectory will coast to infinity, arriving there with hyperbolic excess velocity relative to the central body. Similarly to...
orbits may originate outside the Solar System, but determining their precise orbits is difficult. Old comets that have had most of their volatiles driven out by solar warming are often categorised as asteroids.
Centaurs
The centaurs are icy comet-like bodies with a semi-major axis greater than Jupiter's (5.5 AU) and less than Neptune's (30 AU). The largest known centaur, 10199 Chariklo10199 Chariklo
10199 Chariklo is the largest known centaur. It orbits the Sun between Saturn and Uranus, grazing Uranus.Chariklo was discovered by James V. Scotti of the Spacewatch program on February 15, 1997. Chariklo is named after the nymph Chariclo , the wife of Chiron and the daughter of Apollo.A...
, has a diameter of about 250 km. The first centaur discovered, 2060 Chiron
2060 Chiron
2060 Chiron is a minor planet in the outer Solar System. Discovered in 1977 by Charles T. Kowal , it was the first-known member of a new class of objects now known as centaurs, with an orbit between Saturn and Uranus.Although it was initially classified as an asteroid, it was later found to...
, has also been classified as comet (95P) since it develops a coma just as comets do when they approach the Sun.
Trans-Neptunian region
The area beyond Neptune, or the "trans-Neptunian regionTrans-Neptunian object
A trans-Neptunian object is any minor planet in the Solar System that orbits the Sun at a greater distance on average than Neptune.The first trans-Neptunian object to be discovered was Pluto in 1930...
", is still largely unexplored
Timeline of solar system exploration
This is a timeline of Solar System exploration ordered by date of spacecraft launch. It includes:*All spacecraft that have left Earth orbit for the purposes of Solar System exploration , including lunar probes....
. It appears to consist overwhelmingly of small worlds (the largest having a diameter only a fifth that of the Earth and a mass far smaller than that of the Moon) composed mainly of rock and ice. This region is sometimes known as the "outer Solar System", though others use that term to mean the region beyond the asteroid belt.
Kuiper belt
50000 Quaoar
50000 Quaoar is a rocky trans-Neptunian object in the Kuiper belt with one known moon. Discovered on June 4, 2002 by astronomers Chad Trujillo and Michael Brown at the California Institute of Technology from images acquired at the Samuel Oschin Telescope at Palomar Observatory, it is thought by...
, Varuna
20000 Varuna
20000 Varuna is a large classical Kuiper belt object and a probable dwarf planet. It previously had the provisional designation ' and has been precovered in plates dating back to 1953.-Name:Varuna is named after the Hindu deity,...
, and Orcus
90482 Orcus
90482 Orcus is a trans-Neptunian object in the Kuiper belt with a large moon. It was discovered on February 17, 2004 by Michael Brown of Caltech, Chad Trujillo of the Gemini Observatory, and David Rabinowitz of Yale University. Precovery images as early as November 8, 1951 were later identified...
, may be reclassified as dwarf planets. There are estimated to be over 100,000 Kuiper belt objects with a diameter greater than 50 km, but the total mass of the Kuiper belt is thought to be only a tenth or even a hundredth the mass of the Earth. Many Kuiper belt objects have multiple satellites, and most have orbits that take them outside the plane of the ecliptic.
The Kuiper belt can be roughly divided into the "classical" belt and the resonances
Resonant trans-Neptunian object
In astronomy, a resonant trans-Neptunian object is a trans-Neptunian object in mean motion orbital resonance with Neptune. The orbital periods of the resonant objects are in a simple integer relations with the period of Neptune e.g. 1:2, 2:3 etc...
. Resonances are orbits linked to that of Neptune (e.g. twice for every three Neptune orbits, or once for every two). The first resonance begins within the orbit of Neptune itself. The classical belt consists of objects having no resonance with Neptune, and extends from roughly 39.4 AU to 47.7 AU. Members of the classical Kuiper belt are classified as cubewanos, after the first of their kind to be discovered, , and are still in near primordial, low-eccentricity orbits.
Pluto and Charon
- PlutoPlutoPluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...
(39 AU average), a dwarf planet, is the largest known object in the Kuiper belt. When discovered in 1930, it was considered to be the ninth planet; this changed in 2006 with the adoption of a formal definition of planetDefinition of planetThe definition of planet, since the word was coined by the ancient Greeks, has included within its scope a wide range of celestial bodies. Greek astronomers employed the term asteres planetai , "wandering stars", for objects which apparently move over the sky...
. Pluto has a relatively eccentric orbit inclined 17 degrees to the ecliptic plane and ranging from 29.7 AU from the Sun at perihelion (within the orbit of Neptune) to 49.5 AU at aphelion.
- CharonCharon (moon)Charon is the largest satellite of the dwarf planet Pluto. It was discovered in 1978 at the United States Naval Observatory Flagstaff Station. Following the 2005 discovery of two other natural satellites of Pluto , Charon may also be referred to as Pluto I...
, Pluto's largest moon, is sometimes described as part of a binary systemBinary system (astronomy)A binary system is an astronomical term referring to two objects in space which are so close that their gravitational interaction causes them to orbit about a common center of mass. Some definitions A binary system is an astronomical term referring to two objects in space (usually stars, but also...
with Pluto, as the two bodies orbit a barycenter of gravity above their surfaces (i.e., they appear to "orbit each other"). Beyond Charon, three much smaller moons, NixNix (moon)Nix is a natural satellite of Pluto. It was discovered along with Hydra in June 2005, and is to be visited along with Pluto by the New Horizons mission in July 2015.- Discovery :...
, P4S/2011 P 1S/2011 P 1 is a small natural satellite of Pluto whose existence was announced on July 20, 2011...
and HydraHydra (moon)Hydra is the second outermost known natural satellite of Pluto. It was discovered along with Nix in June 2005, and is to be visited along with Pluto by the New Horizons mission in July 2015.- Discovery :...
, orbit within the system.
- Pluto has a 3:2 resonanceOrbital resonanceIn celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...
with Neptune, meaning that Pluto orbits twice round the Sun for every three Neptunian orbits. Kuiper belt objects whose orbits share this resonance are called plutinoPlutinoIn astronomy, a plutino is a trans-Neptunian object in 2:3 mean motion resonance with Neptune. For every 2 orbits that a plutino makes, Neptune orbits 3 times. Plutinos are named after Pluto, which follows an orbit trapped in the same resonance, with the Italian diminutive suffix -ino...
s.
Haumea and Makemake
- Haumea (43.34 AU average), and Makemake (45.79 AU average), while smaller than Pluto, are the largest known objects in the classical Kuiper belt (that is, they are not in a confirmed resonanceResonant trans-Neptunian objectIn astronomy, a resonant trans-Neptunian object is a trans-Neptunian object in mean motion orbital resonance with Neptune. The orbital periods of the resonant objects are in a simple integer relations with the period of Neptune e.g. 1:2, 2:3 etc...
with Neptune). Haumea is an egg-shaped object with two moons. Makemake is the brightest object in the Kuiper belt after Pluto. Originally designated 2003 EL61 and 2005 FY9 respectively, they were given names and designated dwarf planets in 2008. Their orbits are far more inclined than Pluto's, at 28° and 29°.
Scattered disc
The scattered disc, which overlaps the Kuiper belt but extends much further outwards, is thought to be the source of short-period comets. Scattered disc objects are believed to have been ejected into erratic orbits by the gravitational influence of Neptune's early outward migration. Most scattered disc objects (SDOs) have perihelia within the Kuiper belt but aphelia as far as 150 AU from the Sun. SDOs' orbits are also highly inclined to the ecliptic plane, and are often almost perpendicular to it. Some astronomers consider the scattered disc to be merely another region of the Kuiper belt, and describe scattered disc objects as "scattered Kuiper belt objects." Some astronomers also classify centaurs as inward-scattered Kuiper belt objects along with the outward-scattered residents of the scattered disc.Eris
ErisEris (dwarf planet)
Eris, formal designation 136199 Eris, is the most massive known dwarf planet in the Solar System and the ninth most massive body known to orbit the Sun directly...
(68 AU average) is the largest known scattered disc object, and caused a debate about what constitutes a planet, since it is 25% more massive than Pluto and about the same diameter. It is the most massive of the known dwarf planets. It has one moon, Dysnomia
Dysnomia (moon)
- References :...
. Like Pluto, its orbit is highly eccentric, with a perihelion of 38.2 AU (roughly Pluto's distance from the Sun) and an aphelion of 97.6 AU, and steeply inclined to the ecliptic plane.
Farthest regions
The point at which the Solar System ends and interstellar space begins is not precisely defined, since its outer boundaries are shaped by two separate forces: the solar wind and the Sun's gravity. The outer limit of the solar wind's influence is roughly four times Pluto's distance from the Sun; this heliopause is considered the beginning of the interstellar mediumInterstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
. However, the Sun's Roche sphere
Hill sphere
An astronomical body's Hill sphere is the region in which it dominates the attraction of satellites. To be retained by a planet, a moon must have an orbit that lies within the planet's Hill sphere. That moon would, in turn, have a Hill sphere of its own...
, the effective range of its gravitational dominance, is believed to extend up to a thousand times farther.
Heliopause
The heliosphere is divided into two separate regions. The solar wind travels at roughly 400 km/s until it collides with the interstellar wind; the flow of plasma in the interstellar mediumInterstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
. The collision occurs at the termination shock, which is roughly 80–100 AU from the Sun upwind of the interstellar medium and roughly 200 AU from the Sun downwind. Here the wind slows dramatically, condenses and becomes more turbulent, forming a great oval structure known as the heliosheath. This structure is believed to look and behave very much like a comet's tail, extending outward for a further 40 AU on the upwind side but tailing many times that distance downwind; but evidence from the Cassini and Interstellar Boundary Explorer
Interstellar Boundary Explorer
Interstellar Boundary Explorer is a NASA satellite that will make the first map of the boundary between the Solar System and interstellar space. The mission is part of NASA's Small Explorer program. The IBEX satellite was launched with a Pegasus-XL rocket on October 19, 2008, at 17:47:23 UTC...
spacecraft has suggested that it is in fact forced into a bubble shape by the constraining action of the interstellar magnetic field. Both Voyager 1
Voyager 1
The Voyager 1 spacecraft is a 722-kilogram space probe launched by NASA in 1977, to study the outer Solar System and eventually interstellar space. Operating for as of today , the spacecraft receives routine commands and transmits data back to the Deep Space Network. At a distance of as of...
and Voyager 2
Voyager 2
The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...
are reported to have passed the termination shock and entered the heliosheath, at 94 and 84 AU from the Sun, respectively. The outer boundary of the heliosphere, the heliopause, is the point at which the solar wind finally terminates and is the beginning of interstellar space.
The shape and form of the outer edge of the heliosphere is likely affected by the fluid dynamics
Fluid dynamics
In physics, fluid dynamics is a sub-discipline of fluid mechanics that deals with fluid flow—the natural science of fluids in motion. It has several subdisciplines itself, including aerodynamics and hydrodynamics...
of interactions with the interstellar medium as well as solar magnetic fields prevailing to the south, e.g. it is bluntly shaped with the northern hemisphere extending 9 AU farther than the southern hemisphere. Beyond the heliopause, at around 230 AU, lies the bow shock
Bow shock
A bow shock is the area between a magnetosphere and an ambient medium. For stars, this is typically the boundary between their stellar wind and the interstellar medium....
, a plasma "wake" left by the Sun as it travels through the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
.
No spacecraft have yet passed beyond the heliopause, so it is impossible to know for certain the conditions in local interstellar space. It is expected that NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
's Voyager spacecraft
Voyager program
The Voyager program is a U.S program that launched two unmanned space missions, scientific probes Voyager 1 and Voyager 2. They were launched in 1977 to take advantage of a favorable planetary alignment of the late 1970s...
will pass the heliopause some time in the next decade and transmit valuable data on radiation levels and solar wind back to the Earth. How well the heliosphere shields the Solar System from cosmic rays is poorly understood. A NASA-funded team has developed a concept of a "Vision Mission" dedicated to sending a probe to the heliosphere.
Oort cloud
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...
(LY)), and possibly to as far as 100,000 AU (1.87 LY). It is believed to be composed of comets that were ejected from the inner Solar System by gravitational interactions with the outer planets. Oort cloud objects move very slowly, and can be perturbed by infrequent events such as collisions, the gravitational effects of a passing star, or the galactic tide
Galactic tide
A galactic tide is a tidal force experienced by objects subject to the gravitational field of a galaxy such as the Milky Way. Particular areas of interest concerning galactic tides include galactic collisions, the disruption of dwarf or satellite galaxies, and the Milky Way's tidal effect on the...
, the tidal force
Tidal force
The tidal force is a secondary effect of the force of gravity and is responsible for the tides. It arises because the gravitational force per unit mass exerted on one body by a second body is not constant across its diameter, the side nearest to the second being more attracted by it than the side...
exerted by the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
.
Sedna
90377 Sedna90377 Sedna
90377 Sedna is a trans-Neptunian object discovered in 2003, which was about three times as far from the Sun as Neptune. For most of its orbit it is even further from the Sun, with its aphelion estimated at 960 astronomical units , making it one of the most distant known objects in the Solar System...
(525.86 AU average) is a large, reddish Pluto-like object with a gigantic, highly elliptical orbit that takes it from about 76 AU at perihelion to 928 AU at aphelion and takes 12,050 years to complete. Mike Brown
Michael E. Brown
Michael E. Brown has been a professor of planetary astronomy at the California Institute of Technology since 2003....
, who discovered the object in 2003, asserts that it cannot be part of the scattered disc
Scattered disc
The scattered disc is a distant region of the Solar System that is sparsely populated by icy minor planets, a subset of the broader family of trans-Neptunian objects. The scattered-disc objects have orbital eccentricities ranging as high as 0.8, inclinations as high as 40°, and perihelia greater...
or the Kuiper belt as its perihelion is too distant to have been affected by Neptune's migration. He and other astronomers consider it to be the first in an entirely new population, which also may include the object , which has a perihelion of 45 AU, an aphelion of 415 AU, and an orbital period of 3,420 years. Brown terms this population the "Inner Oort cloud," as it may have formed through a similar process, although it is far closer to the Sun. Sedna is very likely a dwarf planet, though its shape has yet to be determined with certainty.
Boundaries
Much of our Solar System is still unknown. The Sun's gravitational field is estimated to dominate the gravitational forces of surrounding stars out to about two light years (125,000 AU). Lower estimates for the radius of the Oort cloud, by contrast, do not place it farther than 50,000 AU. Despite discoveries such as Sedna, the region between the Kuiper belt and the Oort cloud, an area tens of thousands of AU in radius, is still virtually unmapped. There are also ongoing studies of the region between Mercury and the Sun. Objects may yet be discovered in the Solar System's uncharted regions.Galactic context
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
galaxy
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
, a barred spiral galaxy
Barred spiral galaxy
A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars. Bars are found in approximately two-thirds of all spiral galaxies...
with a diameter of about 100,000 light-year
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...
s containing about 200 billion stars. Our Sun resides in one of the Milky Way's outer spiral arms, known as the Orion Arm
Orion Arm
The Orion–Cygnus Arm is a minor spiral arm of the Milky Way galaxy some 3,500 light years across and approximately 10,000 light years in length. The Solar System is within the Orion–Cygnus Arm...
or Local Spur. The Sun lies between 25,000 and 28,000 light years from the Galactic Centre, and its speed within the galaxy is about 220 kilometres per second
Metre per second
Metre per second is an SI derived unit of both speed and velocity , defined by distance in metres divided by time in seconds....
, so that it completes one revolution every 225–250 million years. This revolution is known as the Solar System's galactic year
Galactic year
The galactic year, also known as a cosmic year, is the duration of time required for the Solar System to orbit once around the center of the Milky Way Galaxy. Estimates of the length of one orbit range from 225 to 250 million "terrestrial" years....
. The solar apex
Solar apex
The solar apex is the direction that the Sun travels with respect to the Local Standard of Rest. In lay terms, it's the "target" within the Milky Way that the Sun appears to be "chasing" as it orbits the galaxy...
, the direction of the Sun's path through interstellar space, is near the constellation of Hercules
Hercules (constellation)
Hercules is a constellation named after Hercules, the Roman mythological hero adapted from the Greek hero Heracles. Hercules was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and it remains one of the 88 modern constellations today...
in the direction of the current location of the bright star Vega
Vega
Vega is the brightest star in the constellation Lyra, the fifth brightest star in the night sky and the second brightest star in the northern celestial hemisphere, after Arcturus...
. The plane of the ecliptic lies at an angle of about 60° to the galactic plane
Galactic plane
The galactic plane is the plane in which the majority of a disk-shaped galaxy's mass lies. The directions perpendicular to the galactic plane point to the galactic poles...
.
The Solar System's location in the galaxy is very likely a factor in the evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
of life
Life
Life is a characteristic that distinguishes objects that have signaling and self-sustaining processes from those that do not, either because such functions have ceased , or else because they lack such functions and are classified as inanimate...
on Earth. Its orbit is close to being circular and is at roughly the same speed as that of the spiral arms, which means it passes through them only rarely. Since spiral arms are home to a far larger concentration of potentially dangerous supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
e, this has given Earth long periods of interstellar stability for life to evolve. The Solar System also lies well outside the star-crowded environs of the galactic centre. Near the centre, gravitational tugs from nearby stars could perturb bodies in the Oort Cloud and send many comets into the inner Solar System, producing collisions with potentially catastrophic implications for life on Earth. The intense radiation of the galactic centre could also interfere with the development of complex life. Even at the Solar System's current location, some scientists have hypothesised that recent supernovae may have adversely affected life in the last 35,000 years by flinging pieces of expelled stellar core towards the Sun as radioactive dust grains and larger, comet-like bodies.
Neighbourhood
The immediate galactic neighbourhood of the Solar System is known as the Local Interstellar CloudLocal Interstellar Cloud
The Local Interstellar Cloud is the interstellar cloud roughly 30 light years across through which the Earth's solar system is currently moving. The Solar System is thought to have entered the Local Interstellar Cloud at some time between 44,000 and 150,000 years ago and is expected to remain...
or Local Fluff, an area of denser cloud in an otherwise sparse region known as the Local Bubble
Local Bubble
The Local Bubble is a cavity in the interstellar medium of the Orion Arm of the Milky Way. It is at least 300 light years across and has a neutral hydrogen density of about 0.05 atoms per cubic centimetre, or approximately one tenth of the average for the ISM in the Milky Way , and half that for...
, an hourglass-shaped cavity in the interstellar medium
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
roughly 300 light years across. The bubble is suffused with high-temperature plasma that suggests it is the product of several recent supernovae.
There are relatively few stars within ten light years (95 trillion km) of the Sun. The closest is the triple star system Alpha Centauri
Alpha Centauri
Alpha Centauri is the brightest star in the southern constellation of Centaurus...
, which is about 4.4 light years away. Alpha Centauri A and B are a closely tied pair of Sun-like stars, while the small red dwarf
Red dwarf
According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type....
Alpha Centauri C (also known as Proxima Centauri
Proxima Centauri
Proxima Centauri is a red dwarf star about 4.2 light-years distant in the constellation of Centaurus. It was discovered in 1915 by Robert Innes, the Director of the Union Observatory in South Africa, and is the nearest known star to the Sun, although it is too faint to be seen with the naked eye...
) orbits the pair at a distance of 0.2 light years. The stars next closest to the Sun are the red dwarfs Barnard's Star
Barnard's star
Barnard's Star, also known occasionally as Barnard's "Runaway" Star, is a very low-mass red dwarf star approximately six light-years away from Earth in the constellation of Ophiuchus . In 1916, the American astronomer E.E...
(at 5.9 light years), Wolf 359 (7.8 light years) and Lalande 21185
Lalande 21185
Lalande 21185 is a red dwarf star in the constellation of Ursa Major. Although relatively close by, it is only magnitude 7 in visible light and thus is too dim to see with the unaided eye...
(8.3 light years). The largest star within ten light years is Sirius
Sirius
Sirius is the brightest star in the night sky. With a visual apparent magnitude of −1.46, it is almost twice as bright as Canopus, the next brightest star. The name "Sirius" is derived from the Ancient Greek: Seirios . The star has the Bayer designation Alpha Canis Majoris...
, a bright main-sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...
star roughly twice the Sun's mass and orbited by a white dwarf
White dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...
called Sirius B. It lies 8.6 light years away. The remaining systems within ten light years are the binary red dwarf system Luyten 726-8
Luyten 726-8
Luyten 726-8 is a binary star system that is one of Earth's nearest neighbors, at about 8.7 light years from Earth in the constellation Cetus...
(8.7 light years) and the solitary red dwarf Ross 154
Ross 154
Ross 154 is a red dwarf star approximately 3.0 pc or 9.68 light years from the Sun. It is the nearest star in the southern constellation Sagittarius, and one of the closest to the Sun...
(9.7 light years). Our closest solitary sun-like star is Tau Ceti
Tau Ceti
Tau Ceti is a star in the constellation Cetus that is spectrally similar to the Sun, although it has only about 78% of the Sun's mass. At a distance of just under 12 light-years from the Solar System, it is a relatively close star. Tau Ceti is metal-deficient and so is thought to be less likely to...
, which lies 11.9 light years away. It has roughly 80 percent the Sun's mass, but only 60 percent its luminosity. The closest known extrasolar planet
Extrasolar planet
An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of such planets have been identified as of . It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars...
to the Sun lies around the star Epsilon Eridani
Epsilon Eridani
Epsilon Eridani is a star in the southern constellation Eridanus, along a declination 9.46° south of the celestial equator. This allows the star to be viewed from most of the Earth's surface. At a distance of 10.5 light years , it has an apparent magnitude of 3.73...
, a star slightly dimmer and redder than the Sun, which lies 10.5 light years away. Its one confirmed planet, Epsilon Eridani b
Epsilon Eridani b
Epsilon Eridani b is an unconfirmed extrasolar planet approximately 10 light-years away orbiting the star Epsilon Eridani, in the constellation of Eridanus .- Discovery :...
, is roughly 1.5 times Jupiter's mass and orbits its star every 6.9 years.
Formation and evolution
The Solar System formed from the gravitational collapse of a giant molecular cloudMolecular cloud
A molecular cloud, sometimes called a stellar nursery if star formation is occurring within, is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen ....
4.568 billion years ago. This initial cloud was likely several light-years across and probably birthed several stars.
As the region that would become the Solar System, known as the pre-solar nebula
Solar nebula
In cosmogony, the nebular hypothesis is the most widely accepted model explaining the formation and evolution of the Solar System. There is evidence that it was first proposed in 1734 by Emanuel Swedenborg. Originally applied only to our own Solar System, this method of planetary system formation...
, collapsed, conservation of angular momentum
Angular momentum
In physics, angular momentum, moment of momentum, or rotational momentum is a conserved vector quantity that can be used to describe the overall state of a physical system...
made it rotate faster. The centre, where most of the mass collected, became increasingly hotter than the surrounding disc. As the contracting nebula rotated, it began to flatten into a spinning protoplanetary disc with a diameter of roughly 200 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
and a hot, dense protostar
Protostar
A protostar is a large mass that forms by contraction out of the gas of a giant molecular cloud in the interstellar medium. The protostellar phase is an early stage in the process of star formation. For a one solar-mass star it lasts about 100,000 years...
at the centre. At this point in its evolution
Stellar evolution
Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years to trillions of years .Stellar evolution is not studied by observing the life of a single...
, the Sun is believed to have been a T Tauri star
T Tauri star
T Tauri stars are a class of variable stars named after their prototype – T Tauri. They are found near molecular clouds and identified by their optical variability and strong chromospheric lines.-Characteristics:...
. Studies of T Tauri stars show that they are often accompanied by discs of pre-planetary matter with masses of 0.001–0.1 solar mass
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
es, with the vast majority of the mass of the nebula in the star itself. The planets formed by accretion
Accretion (astrophysics)
In astrophysics, the term accretion is used for at least two distinct processes.The first and most common is the growth of a massive object by gravitationally attracting more matter, typically gaseous matter in an accretion disc. Accretion discs are common around smaller stars or stellar remnants...
from this disk.
Within 50 million years, the pressure and density of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
in the centre of the protostar became great enough for it to begin thermonuclear fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
. The temperature, reaction rate, pressure, and density increased until hydrostatic equilibrium
Hydrostatic equilibrium
Hydrostatic equilibrium or hydrostatic balance is the condition in fluid mechanics where a volume of a fluid is at rest or at constant velocity. This occurs when compression due to gravity is balanced by a pressure gradient force...
was achieved, with the thermal energy countering the force of gravitational contraction. At this point the Sun became a full-fledged main-sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...
star.
The Solar System as we know it today will last until the Sun begins its evolution off the main sequence of the Hertzsprung–Russell diagram
Hertzsprung–Russell diagram
The Hertzsprung–Russell diagram is a scatter graph of stars showing the relationship between the stars' absolute magnitudes or luminosities versus their spectral types or classifications and effective temperatures. Hertzsprung–Russell diagrams are not pictures or maps of the locations of the stars...
. As the Sun burns through its supply of hydrogen fuel, the energy output supporting the core tends to decrease, causing it to collapse in on itself. This increase in pressure heats the core, so it burns even faster. As a result, the Sun is growing brighter at a rate of roughly ten percent every 1.1 billion years.
Around 5.4 billion years from now, the hydrogen in the core of the Sun will have been entirely converted to helium, ending the main-sequence phase. As the hydrogen reactions shut down, the core will contract further, increasing pressure and temperature, causing fusion to commence via the helium process. Helium in the core burns at a much hotter temperature, and the energy output will be much greater than during the hydrogen process. At this time, the outer layers of the Sun will expand to roughly up to 260 times its current diameter; the Sun will become a red giant
Red giant
A red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower...
. Because of its vastly increased surface area, the surface of the Sun will be considerably cooler than it is on the main sequence (2600 K at the coolest).
Eventually, helium in the core will exhaust itself at a much faster rate than the hydrogen, and the Sun's helium burning phase will be but a fraction of the time compared to the hydrogen burning phase. The Sun is not massive enough to commence fusion of heavier elements, and nuclear reactions in the core will dwindle. Its outer layers will fall away into space, leaving a white dwarf
White dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...
, an extraordinarily dense object, half the original mass of the Sun but only the size of the Earth. The ejected outer layers will form what is known as a planetary nebula
Planetary nebula
A planetary nebula is an emission nebula consisting of an expanding glowing shell of ionized gas ejected during the asymptotic giant branch phase of certain types of stars late in their life...
, returning some of the material that formed the Sun to the interstellar medium.
Visual summary
A sampling of closely imaged Solar System bodies, selected for size and detail. The Sun is approximately 10,000 times larger than, and 41 trillion times the volume of, the smallest object shown (Prometheus). See also List of Solar System objects by size, List of natural satellites, List of minor planets, and Lists of comets. Images here are not an endorsement of natural color in visible light. _large.jpg) |
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| Sun Sun The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields... |
Jupiter Jupiter Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,... |
Saturn Saturn Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,... |
Uranus Uranus Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus... |
Neptune Neptune Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times... |
Earth Earth Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets... |
Venus Venus Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows... |
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| Mars Mars Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance... |
Ganymede Ganymede (moon) Ganymede is a satellite of Jupiter and the largest moon in the Solar System. It is the seventh moon and third Galilean satellite outward from Jupiter. Completing an orbit in roughly seven days, Ganymede participates in a 1:2:4 orbital resonance with the moons Europa and Io, respectively... |
Titan Titan (moon) Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found.... |
Mercury Mercury (planet) Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits... |
Callisto Callisto (moon) Callisto named after the Greek mythological figure of Callisto) is a moon of the planet Jupiter. It was discovered in 1610 by Galileo Galilei. It is the third-largest moon in the Solar System and the second largest in the Jovian system, after Ganymede. Callisto has about 99% the diameter of the... |
Io Io (moon) Io ) is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of , the fourth-largest moon in the Solar System. It was named after the mythological character of Io, a priestess of Hera who became one of the lovers of Zeus.... |
Moon Moon The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more... |
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| Europa Europa (moon) Europa Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock and probably has an iron core. It has a tenuous atmosphere composed primarily of oxygen. Its surface is composed of ice and is one of the smoothest in the Solar System. This surface is striated by cracks and... |
Triton Triton (moon) Triton is the largest moon of the planet Neptune, discovered on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2,700 km in diameter, it is... |
Titania Titania (moon) Titania is the largest of the moons of Uranus and the eighth largest moon in the Solar System at a diameter of 1578 km. Discovered by William Herschel in 1787, Titania is named after the queen of the fairies in Shakespeare's A Midsummer Night's Dream... |
Rhea Rhea (moon) Rhea is the second-largest moon of Saturn and the ninth largest moon in the Solar System. It was discovered in 1672 by Giovanni Domenico Cassini.-Name:Rhea is named after the Titan Rhea of Greek mythology, "mother of the gods"... |
Oberon Oberon (moon) Oberon , also designated ', is the outermost major moon of the planet Uranus. It is the second largest and second most massive of the Uranian moons, and the ninth most massive moon in the Solar System. Discovered by William Herschel in 1787, Oberon is named after the mythical king of the fairies... |
Iapetus Iapetus (moon) Iapetus ), occasionally Japetus , is the third-largest moon of Saturn, and eleventh in the Solar System. It was discovered by Giovanni Domenico Cassini in 1671... |
Umbriel Umbriel (moon) Umbriel is a moon of Uranus discovered on October 24, 1851, by William Lassell. It was discovered at the same time as Ariel and named after a character in Alexander Pope's poem The Rape of the Lock. Umbriel consists mainly of ice with a substantial fraction of rock, and may be differentiated into a... |
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| Ariel Ariel (moon) Ariel is the brightest and fourth-largest of the 27 known moons of Uranus. Ariel orbits and rotates in the equatorial plane of Uranus, which is almost perpendicular to the orbit of Uranus, and so has an extreme seasonal cycle.... |
Dione Dione (moon) Dione is a moon of Saturn discovered by Cassini in 1684. It is named after the titan Dione of Greek mythology. It is also designated Saturn IV.- Name :... |
Tethys Tethys (moon) Tethys or Saturn III is a mid-sized moon of Saturn about across. It was discovered by G. D. Cassini in 1684 and is named after titan Tethys of Greek mythology. Tethys is pronounced |Odysseus]] is about 400 km in diameter, while the largest graben—Ithaca Chasma is about 100 km wide and... |
Vesta 4 Vesta Vesta, formally designated 4 Vesta, is one of the largest asteroids, with a mean diameter of about . It was discovered by Heinrich Wilhelm Olbers on March 29, 1807, and is named after the Roman virgin goddess of home and hearth, Vesta.... |
Enceladus Enceladus (moon) Enceladus is the sixth-largest of the moons of Saturn. It was discovered in 1789 by William Herschel. Until the two Voyager spacecraft passed near it in the early 1980s very little was known about this small moon besides the identification of water ice on its surface... |
Miranda Miranda (moon) -External links:* at * at The Nine8 Planets* at Views of the Solar System* * from the... |
Proteus Proteus (moon) Proteus , also known as Neptune VIII, is the second largest Neptunian moon, and Neptune's largest inner satellite. Discovered by Voyager 2 spacecraft in 1989, it is named after Proteus, the shape-changing sea god of Greek mythology... |
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| Mimas Mimas (moon) Mimas is a moon of Saturn which was discovered in 1789 by William Herschel. It is named after Mimas, a son of Gaia in Greek mythology, and is also designated Saturn I.... |
Hyperion Hyperion (moon) Hyperion , also known as Saturn VII, is a moon of Saturn discovered by William Cranch Bond, George Phillips Bond and William Lassell in 1848. It is distinguished by its irregular shape, its chaotic rotation, and its unexplained sponge-like appearance... |
Phoebe Phoebe (moon) Phoebe is an irregular satellite of Saturn. It was discovered by William Henry Pickering on 17 March 1899 from photographic plates that had been taken starting on 16 August 1898 at the Boyden Observatory near Arequipa, Peru, by DeLisle Stewart... |
Janus Janus (moon) Janus is an inner satellite of Saturn. It is also known as Saturn X . It is named after the mythological Janus.-Discovery and orbit:Janus occupies practically the same orbit as the moon Epimetheus... |
Amalthea Amalthea (moon) Amalthea is the third moon of Jupiter in order of distance from the planet. It was discovered on September 9, 1892, by Edward Emerson Barnard and named after Amalthea, a nymph in Greek mythology. It is also known as '.... |
Epimetheus Epimetheus (moon) Epimetheus is an inner satellite of Saturn. It is also known as Saturn XI. It is named after the mythological Epimetheus, brother of Prometheus.-Discovery:Epimetheus occupies essentially the same orbit as the moon Janus... |
Prometheus Prometheus (moon) -Animations:-External links:* at ** anaglyph view of Prometheus... |
See also
- Astronomical symbolsAstronomical symbolsAstronomical symbols are symbols used to represent various celestial objects, theoretical constructs and observational events in astronomy. The earliest forms of these symbols appear in Greek papyri of late antiquity. The Byzantine codices in which the Greek papyri were preserved continued and...
- List of geological features of the Solar System
- Numerical model of the Solar System
- OrreryOrreryAn orrery is a mechanical device that illustrates the relative positions and motions of the planets and moons in the Solar System in a heliocentric model. Though the Greeks had working planetaria, the first orrery that was a planetarium of the modern era was produced in 1704, and one was presented...
, mechanical models of Solar System - Planetary mnemonic
- Solar System in fictionSolar system in fictionThe Solar System and its various bodies were the earliest objects to be treated as fictional locations in works of science fiction. Among these, imaginary voyages to and explorations of Earth's Moon are found in seventeenth century literature...
- Solar System modelSolar system modelSolar System models, especially mechanical models, called orreries, that illustrate the relative positions and motions of the planets and moons in the Solar System have been built for centuries. While they often showed relative sizes, these models were usually not built to scale...