Trojan asteroid
Encyclopedia
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,...
around the Sun. Relative to Jupiter, each Trojan librates
Libration
In astronomy, libration is an oscillating motion of orbiting bodies relative to each other, notably including the motion of the Moon relative to Earth, or of Trojan asteroids relative to planets.-Lunar libration:...
around one of the planet's two Lagrangian point
Lagrangian point
The Lagrangian points are the five positions in an orbital configuration where a small object affected only by gravity can theoretically be stationary relative to two larger objects...
s of stability, and , that respectively lie 60° ahead of and behind the planet in its orbit. Trojan asteroids are distributed in two elongated, curved regions around these Lagrangian points with an average semi-major axis
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...
of about 5.2 AU.
The first Trojan discovered, 588 Achilles
588 Achilles
588 Achilles is an asteroid discovered on February 22, 1906, by the German astronomer Max Wolf. It was the first of the trojan asteroids to be discovered, and is named after Achilles, the fictional hero from the Iliad. It orbits in the Lagrangian point of the Sun-Jupiter system...
, was spotted in 1906 by the German astronomer Max Wolf
Max Wolf
Maximilian Franz Joseph Cornelius Wolf was a German astronomer and a pioneer in the field of astrophotography...
. A total of 4,076 Jupiter Trojans have been found . The term "Trojan" derives from the fact that, by convention, they are each named after a mythological figure from the Trojan War
Trojan War
In Greek mythology, the Trojan War was waged against the city of Troy by the Achaeans after Paris of Troy took Helen from her husband Menelaus, the king of Sparta. The war is among the most important events in Greek mythology and was narrated in many works of Greek literature, including the Iliad...
. The total number of Jupiter Trojans larger than 1 km in diameter is believed to be about , approximately equal to the number of asteroids larger than 1 km in 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...
. Like main-belt asteroids, Trojans form families
Asteroid family
An asteroid family is a population of asteroids that share similar proper orbital elements, such as semimajor axis, eccentricity, and orbital inclination. The members of the families are thought to be fragments of past asteroid collisions...
.
Jupiter Trojans are dark bodies with reddish, featureless spectra
Spectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...
. No firm evidence of the presence of water, organic matter
Organic matter
Organic matter is matter that has come from a once-living organism; is capable of decay, or the product of decay; or is composed of organic compounds...
or other chemical compounds on their surfaces has been obtained. The Trojans' densities (as measured by studying binaries
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...
or rotational lightcurves) vary from 0.8 to 2.5 g·cm−3. Trojans are thought to have been captured into their orbits during the early stages of the Solar System's formation or slightly later, during the migration
Planetary migration
Planetary migration occurs when a planet or other stellar satellite interacts with a disk of gas or planetesimals, resulting in the alteration of the satellite's orbital parameters, especially its semi-major axis...
of giant planets.
The term "Trojan
Trojan (astronomy)
In astronomy, a Trojan is a minor planet or natural satellite that shares an orbit with a larger planet or moon, but does not collide with it because it orbits around one of the two Lagrangian points of stability , and , which lie approximately 60° ahead of and behind the larger body,...
" has come to be used more generally to refer to other small Solar System 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:...
with similar relationships to larger bodies: for example, there are both Mars Trojans and Neptune Trojan
Neptune Trojan
Neptune 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, and Saturn has Trojan moons. NASA has announced the discovery of an Earth Trojan. The term "Trojan asteroid" is normally understood to specifically mean the Jupiter Trojans since the first Trojans were discovered near Jupiter's orbit and Jupiter currently has by far the most known Trojans.
Observational history
In 1772, Italian-born mathematician Joseph-Louis Lagrange, in studying the restricted three-body problem, predicted that a small body sharing an orbit with a planet but lying 60° ahead or behind it will be trapped near these points. The trapped body will librateLibration
In astronomy, libration is an oscillating motion of orbiting bodies relative to each other, notably including the motion of the Moon relative to Earth, or of Trojan asteroids relative to planets.-Lunar libration:...
slowly around the point of equilibrium in a tadpole or horseshoe orbit
Horseshoe orbit
A horseshoe orbit is a type of co-orbital motion of a small orbiting body relative to a larger orbiting body . The orbital period of the smaller body is very nearly the same as for the larger body, and its path appears to have a horseshoe shape in a rotating reference frame as viewed from the...
. These leading and trailing points are called the and Lagrange points. However, no asteroids trapped in Lagrange points were observed until more than a century after Lagrange's hypothesis. Those around Jupiter were the first to be discovered.
E. E. Barnard made the first recorded observation of a Trojan asteroid, , in 1904, but neither he nor others appreciated its significance at the time. Barnard believed he saw the recently discovered Saturnian satellite 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...
, which was only two arc-minutes away in the sky at the time, or possibly a 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...
. The object's identity was not realized until its orbit was constructed in 1999.
The first recognized discovery of a Trojan occurred in February 1906, when astronomer Max Wolf
Max Wolf
Maximilian Franz Joseph Cornelius Wolf was a German astronomer and a pioneer in the field of astrophotography...
of Heidelberg-Königstuhl State Observatory discovered an asteroid
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...
at the Lagrangian point
Lagrangian point
The Lagrangian points are the five positions in an orbital configuration where a small object affected only by gravity can theoretically be stationary relative to two larger objects...
of the 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,...
system, later named 588 Achilles
588 Achilles
588 Achilles is an asteroid discovered on February 22, 1906, by the German astronomer Max Wolf. It was the first of the trojan asteroids to be discovered, and is named after Achilles, the fictional hero from the Iliad. It orbits in the Lagrangian point of the Sun-Jupiter system...
. In 1906–1907 two more Jupiter Trojans were found by fellow German astronomer August Kopff
August Kopff
August Kopff was a German astronomer who discovered several comets and asteroids. He worked in Heidelberg, then joined the Humboldt University of Berlin where he became the Director of the Institute for Astronomical Calculation.He discovered some comets, including periodic comet 22P/Kopff and the...
(624 Hektor
624 Hektor
624 Hektor is the largest Jupiter Trojan. It was discovered in 1907 by August Kopff.Hektor is a D-type asteroid, dark and reddish in colour. It lies in Jupiter's leading Lagrangian point, , called the 'Greek' node after one of the two sides in the legendary Trojan War...
and 617 Patroclus
617 Patroclus
617 Patroclus is a binary minor planet made up of two similarly-sized objects orbiting their common centre of gravity. It is a Trojan asteroid, sharing an orbit with Jupiter. It was discovered in 1906 by August Kopff, and was the second trojan to be discovered...
). Hektor, like Achilles, belonged to the swarm ("ahead" of the planet in its orbit), while Patroclus was the first asteroid known to reside at the Lagrangian point ("behind" the planet). By 1938, 11 Trojans had been detected. This number increased to 14 only in 1961. As instruments improved, the rate of discovery grew rapidly: by January 2000, a total of 257 had been discovered; by May 2003, the number had grown to 1,600. there are 2,603 known Trojan asteroids at and 1,473 at ,
Nomenclature
The custom of naming all asteroids in Jupiter's and points after famous heroes of the Trojan War was suggested by Johann PalisaJohann Palisa
Johann Palisa was an Austrian astronomer, born in Opava in Austrian Silesia .He was a prolific discoverer of asteroids, discovering 122 in all, from 136 Austria in 1874 to 1073 Gellivara in 1923...
of Vienna
Vienna
Vienna is the capital and largest city of the Republic of Austria and one of the nine states of Austria. Vienna is Austria's primary city, with a population of about 1.723 million , and is by far the largest city in Austria, as well as its cultural, economic, and political centre...
, who was the first to accurately calculate their orbits. Asteroids in the group are named after Greek
Greece
Greece , officially the Hellenic Republic , and historically Hellas or the Republic of Greece in English, is a country in southeastern Europe....
heroes (the "Greek node or camp" or "Achilles group"), and those at the point are named after the heroes of Troy
Troy
Troy was a city, both factual and legendary, located in northwest Anatolia in what is now Turkey, southeast of the Dardanelles and beside Mount Ida...
(the "Trojan node or camp"). Confusingly, 617 Patroclus was named before the Greece/Troy rule was devised, and a Greek name thus appears in the Trojan node; the Greek node also has one "misplaced" asteroid, 624 Hektor, named after a Trojan hero.
Numbers and mass
Two more recent studies indicate, however, that the above numbers may overestimate the number of Trojans by several-fold. This overestimate is caused by (1) the assumption that all Trojans have a low albedo of about 0.04, whereas small bodies may actually have an average albedo as high as 0.12; (2) an incorrect assumption about the distribution of Trojans in the sky. According to the new estimates, the total number of Trojan asteroids with a diameter larger than 2 km is and in the and swarms, respectively. These numbers would be reduced by a factor of 2 if small Trojans are more reflective than large ones.
The number of Trojans observed in the swarm is slightly larger than that observed in ; however, since the brightest Trojans show little variation in numbers between the two populations, this disparity is probably due to observational bias. However, some models indicate that the swarm may be slightly more stable than the swarm.
The largest of the Trojans is 624 Hektor
624 Hektor
624 Hektor is the largest Jupiter Trojan. It was discovered in 1907 by August Kopff.Hektor is a D-type asteroid, dark and reddish in colour. It lies in Jupiter's leading Lagrangian point, , called the 'Greek' node after one of the two sides in the legendary Trojan War...
, which has an average diameter of 203 ± 3.6 km. There are few large Trojans in comparison to the overall population. With decreasing size, the number of Trojans grows very quickly down to 84 km, much more so than in the main asteroid belt. A diameter of 84 km corresponds to an absolute magnitude of 9.5, assuming an albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
of 0.04. Within the 4.4–40 km range the Trojans' size distribution resembles that of the main belt asteroids. An absence of data means that nothing is known about the masses of the smaller Trojans. The size distribution suggests that the smaller Trojans are the products of collisions by larger Trojans.
Orbits
Trojans do not maintain a fixed separation from Jupiter. They slowly librate around their respective equilibrium points, periodically moving closer to Jupiter or further from it. Trojans generally follow paths called tadpole orbits around the Lagrangian points; the average period of their libration is about 150 years. The amplitude of the libration (along the Jovian orbit) varies from 0.6° to 88°, with the average being about 33°. Simulations show that Trojans can follow even more complicated trajectories when moving from one Lagrangian point to another—these are called horseshoe orbit
Horseshoe orbit
A horseshoe orbit is a type of co-orbital motion of a small orbiting body relative to a larger orbiting body . The orbital period of the smaller body is very nearly the same as for the larger body, and its path appears to have a horseshoe shape in a rotating reference frame as viewed from the...
s (currently no Jupiter Trojan with such an orbit is known).
Dynamical families and binaries
Discerning dynamical familiesCollisional family
In astronomy, a collisional family is a group of objects that are thought to have a common origin in an impact . They have similar compositions, and most share similar orbital elements....
within the Trojan population is more difficult than it is in the main asteroid belt, because the Trojans are locked within a far narrower range of possible positions. This means that clusters tend to overlap and merge with the overall swarm. However, as of 2003 roughly a dozen dynamical families have been identified within the Trojans. Trojan families are much smaller in size than families in the main belt; the largest identified family, the Menelaus group, consists of only eight members.
In 2001, 617 Patroclus
617 Patroclus
617 Patroclus is a binary minor planet made up of two similarly-sized objects orbiting their common centre of gravity. It is a Trojan asteroid, sharing an orbit with Jupiter. It was discovered in 1906 by August Kopff, and was the second trojan to be discovered...
was the first Trojan to be identified as a binary asteroid. The binary's orbit is extremely close, at 650 km, compared to 35,000 km for the primary's Hill 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 largest Trojan asteroid—624 Hektor—likely is a contact binary
Contact binary (asteroid)
In the study of asteroids, a contact binary is caused when two asteroids gravitate toward each other until they touch, forming an oddly-shaped single body. Asteroids suspected of being contact binaries include the unusually elongated 624 Hektor and the bilobated 216 Kleopatra and 4769 Castalia...
with a moonlet.
Physical properties
Jupiter Trojans are dark bodies of irregular shape. Their geometric albedoGeometric albedo
The geometric albedo of an astronomical body is the ratio of its actual brightness at zero phase angle to that of an idealized flat, fully reflecting, diffusively scattering disk with the same cross-section....
s generally vary between 3 and 10%. The average value is 0.056 ± 0.003 for the objects larger than 57 km, and 0.121 ± 0.003 (R-band) for those smaller than 25 km. The asteroid 4709 Ennomos
4709 Ennomos
4709 Ennomos is a Jupiter Trojan discovered on October 12, 1988 by Shoemaker, C. at Palomar. It is named after Ennomos, a Trojan hero in the Iliad.- External links :*...
has the highest albedo (0.18) of all known Trojans. Little is known about the masses, chemical composition, rotation or other physical properties of the Trojans.
Rotation
The rotational properties of Trojans are not well known. Analysis of the rotational light curveLight curve
In astronomy, a light curve is a graph of light intensity of a celestial object or region, as a function of time. The light is usually in a particular frequency interval or band...
s of 72 Trojan asteroids gave an average rotational period of about 11.2 hours, whereas the average period of the control sample of the main belt asteroids was 10.6 hours. The distribution of the rotational periods of Trojans appeared to be well approximated by a Maxwellian function,The Maxwellian function is
, where 
is the average rotational period, 
is the dispersionStatistical dispersion
In statistics, statistical dispersion is variability or spread in a variable or a probability distribution...
of periods. whereas the distribution for main belt asteroids was found to be non-Maxwellian, with a deficit of periods in the range 8–10 hours. The Maxwellian distribution of the rotational periods of Trojans may indicate that they have undergone a stronger collisional evolution compared to the main belt.
However, in 2008 a team from Calvin College
Calvin College
Calvin College is a comprehensive liberal arts college located in Grand Rapids, Michigan. Founded in 1876, Calvin College is an educational institution of the Christian Reformed Church and stands in the Reformed tradition of Protestantism...
analyzed the light curve
Light curve
In astronomy, a light curve is a graph of light intensity of a celestial object or region, as a function of time. The light is usually in a particular frequency interval or band...
s of a debiased sample of ten Trojans, and found a median
Median
In probability theory and statistics, a median is described as the numerical value separating the higher half of a sample, a population, or a probability distribution, from the lower half. The median of a finite list of numbers can be found by arranging all the observations from lowest value to...
spin period of 18.9 hours. This value was significantly higher than that for main belt asteroids of similar size (11.5 hours). The difference could mean that the Trojans possess a lower average density, which may imply that they formed in 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...
(see below).
Composition
SpectroscopicallySpectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
, the Jupiter Trojans mostly are D-type asteroid
D-type asteroid
D-type asteroids have a very low albedo and a featureless reddish electromagnetic spectrum. It has been suggested that they have a composition of organic rich silicates, carbon and anhydrous silicates, possibly with water ice in their interiors...
s, which predominate in the outer regions of the main belt. A small number are classified as P
P-type asteroid
P-type asteroids have low albedo and a featureless reddish electromagnetic spectrum. It has been suggested that they have a composition of organic rich silicates, carbon and anhydrous silicates, possibly with water ice in their interior...
or C-type asteroid
C-type asteroid
C-type asteroids are carbonaceous asteroids. They are the most common variety, forming around 75% of known asteroids, and an even higher percentage in the outer part of the asteroid belt beyond 2.7 AU, which is dominated by this asteroid type...
s. Their spectra are red (meaning that they reflect more light at longer wavelengths) or neutral and featureless. No firm evidence of water, organics or other chemical compounds has been obtained . However, 4709 Ennomos
4709 Ennomos
4709 Ennomos is a Jupiter Trojan discovered on October 12, 1988 by Shoemaker, C. at Palomar. It is named after Ennomos, a Trojan hero in the Iliad.- External links :*...
has an albedo slightly higher than the Trojan average, which may indicate the presence of water ice. In addition, a number of other Trojans, such as 911 Agamemnon
911 Agamemnon
911 Agamemnon is a trojan asteroid that orbits the Sun at the same distance as the planet Jupiter. It is located in the leading Lagrangian point ....
and 617 Patroclus
617 Patroclus
617 Patroclus is a binary minor planet made up of two similarly-sized objects orbiting their common centre of gravity. It is a Trojan asteroid, sharing an orbit with Jupiter. It was discovered in 1906 by August Kopff, and was the second trojan to be discovered...
, have shown very weak absorptions at 1.7 and 2.3 μm, which might indicate the presence of organics. The Trojans' spectra are similar to those of the irregular moons of Jupiter and, to certain extent, comet nuclei, though Trojans are spectrally very different from the redder Kuiper belt objects. A Trojan's spectrum can be matched to a mixture of water ice, a large amount of carbon-rich material (charcoal
Charcoal
Charcoal is the dark grey residue consisting of carbon, and any remaining ash, obtained by removing water and other volatile constituents from animal and vegetation substances. Charcoal is usually produced by slow pyrolysis, the heating of wood or other substances in the absence of oxygen...
), and possibly magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
-rich 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. The composition of the Trojan population appears to be markedly uniform, with little or no differentiation between the two swarms.
A team from the Keck Observatory in Hawaii announced in 2006 that it had measured the density of the binary Trojan asteroid 617 Patroclus
617 Patroclus
617 Patroclus is a binary minor planet made up of two similarly-sized objects orbiting their common centre of gravity. It is a Trojan asteroid, sharing an orbit with Jupiter. It was discovered in 1906 by August Kopff, and was the second trojan to be discovered...
as being less than that of water ice (0.8 g/cm3), suggesting that the pair, and possibly many other Trojan objects, more closely resemble 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 or 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 in composition—water ice with a layer of dust—than they do the main belt asteroids. Countering this argument, the density of Hektor as determined from its rotational lightcurve (2.480 g/cm3) is significantly higher than that of 617 Patroclus. Such a difference in densities is puzzling and indicates that density may not be a good indicator of asteroid origin.
Origin and evolution
Two main theories have emerged to explain the formation and evolution of the Trojans. The first suggests that the Trojans formed in the same part of the Solar SystemSolar System
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 is in the Sun...
as Jupiter and entered their orbits while the planet was forming. The last stage of Jupiter's formation involved runaway growth of its mass through the accretion of large amounts 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...
and 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...
from the protoplanetary disk
Protoplanetary disk
A protoplanetary disk is a rotating circumstellar disk of dense gas surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star...
; during this growth, which lasted for only about 10,000 years, the mass of Jupiter increased by a factor of ten. The planetesimal
Planetesimal
Planetesimals are solid objects thought to exist in protoplanetary disks and in debris disks.A widely accepted theory of planet formation, the so-called planetesimal hypothesis of Viktor Safronov, states that planets form out of cosmic dust grains that collide and stick to form larger and larger...
s that had approximately the same orbits as Jupiter were caught by the increased gravity of the planet. The capture mechanism was very efficient—about 50% of all remaining planetesimals were trapped. This hypothesis has two major problems: the number of trapped bodies exceeds the observed population of Trojans by four orders of magnitude
Order of magnitude
An order of magnitude is the class of scale or magnitude of any amount, where each class contains values of a fixed ratio to the class preceding it. In its most common usage, the amount being scaled is 10 and the scale is the exponent being applied to this amount...
, and the present Trojan asteroids have larger orbital inclinations than are predicted by the capture model. However, simulations of this scenario show that such a mode of formation also would inhibit the creation of similar Trojans around 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,...
, and this has been borne out by observation: to date no Trojans have been found near Saturn.
The second theory, part of the Nice model
Nice model
The Nice model is a scenario for the dynamical evolution of the Solar System. It is named for the location of the Observatoire de la Côte d'Azur, where it was initially developed, in Nice, France. It proposes the migration of the giant planets from an initial compact configuration into their...
, proposes that the Trojans were captured during planetary migration
Planetary migration
Planetary migration occurs when a planet or other stellar satellite interacts with a disk of gas or planetesimals, resulting in the alteration of the satellite's orbital parameters, especially its semi-major axis...
, which happened about years after the Solar System's formation. The migration was triggered by the passage of Jupiter and Saturn through the 1:2 mean motion 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...
. During this period 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...
and to some extent Saturn moved outward, while Jupiter moved slightly inward. Migrating giant planets destabilized the primordial 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...
, throwing millions of objects into the inner Solar System. In addition, their combined gravitational influence would have quickly disturbed any pre-existing Trojans. Under this theory, the present Trojan population eventually accumulated from runaway Kuiper belt objects as Jupiter and Saturn moved away from the resonance.
The long-term future of the Trojans is open to question, as multiple weak resonances with Jupiter and Saturn cause them to behave chaotically over time. In addition, collisional shattering slowly depletes the Trojan population as fragments are ejected. Ejected Trojans could become temporary satellites of Jupiter or Jupiter family comets. Simulations show that the orbits of up to 17% of Jupiter's Trojans are unstable over the age of the Solar System. Levison et al. believe that roughly 200 ejected Trojans greater than 1 km in diameter might be traveling the Solar System, with a few possibly on Earth-crossing orbits. Some of the escaped Trojans may become Jupiter family comets as they approach the Sun and their surface ice begins evaporating.
See also
- List of Trojan asteroids (Greek camp)
- List of Trojan asteroids (Trojan camp)
- Pronunciation of Trojan asteroid names
- List of objects at Lagrangian points
- List of Jupiter-crossing minor planets
- Comet Shoemaker-Levy 9Comet Shoemaker-Levy 9Comet Shoemaker–Levy 9 was a comet that broke apart and collided with Jupiter in July 1994, providing the first direct observation of an extraterrestrial collision of solar system objects. This generated a large amount of coverage in the popular media, and the comet was closely observed by...