Gliese 876
Encyclopedia
Gliese 876 is a red dwarf
star
approximately 15 light-year
s away from Earth
in the constellation
of Aquarius
. As of 2011, it has been confirmed that four extrasolar planet
s orbit the star. Two of the middle planets
are similar to Jupiter
, while the closest planet is thought to be similar to a small Neptune
or a large terrestrial planet
, and the outer planet has mass similar to Uranus
. The orbits of all but the closest planet are locked in a rare three-body Laplace resonance.
. According to astrometric
measurements made by the Hipparcos
satellite
, the star shows a parallax
of 213.28 milliarcsecond
s, which corresponds to a distance of 4.69 pc, currently making it the second closest known star with orbiting planets, after Epsilon Eridani
. Despite being located so close to Earth, the star is so faint that it is invisible to the naked eye
and can only be seen using a telescope
.
of Gliese 876 is cooler than our Sun and the star has a smaller radius. These factors combine to make the star only 1.24% as luminous
as the Sun, and most of this is at infrared
wavelength
s.
Estimating the age and metallicity of cool stars is difficult due to the formation of diatomic molecules in their atmosphere
s, which makes the spectrum
extremely complex. By fitting the observed spectrum to model spectra, it is estimated that Gliese 876 has a slightly lower abundance of heavy elements compared to the Sun (around 75% the solar abundance of iron
). Based on chromospheric
activity the star is likely to be around 6.5 to 9.9 billion years old, depending on the theoretical model used. However, the low rotational period of the star as well as its membership among the young disk population suggest that the star is between 0.1–5 billion years old.
Like many low-mass stars, Gliese 876 is a variable star
. Its variable star designation
is IL Aquarii and it is classified as a BY Draconis variable
. Its brightness fluctuates by around 0.04 magnitudes
. This type of variability is thought to be caused by large starspot
s moving in and out of view as the star rotates. Gliese 876 emits X-rays.
around Gliese 876 by two independent teams led by Geoffrey Marcy
and Xavier Delfosse. The planet was designated Gliese 876 b
and was detected by making measurements of the star's radial velocity
as the planet's gravity pulled it around. The planet, around twice the mass of Jupiter
, revolves around its star in an orbit taking approximately 61 day
s to complete, at a distance of only 0.208 AU
, less than the distance from the Sun to Mercury
.
On April 4, 2001, a second planet was detected in the system, inside the orbit of the previously-discovered planet. The 0.62 Jupiter-mass planet, designated Gliese 876 c
is in a 1:2 orbital resonance
with the outer planet, taking 30.340 days to orbit the star. This relationship between the orbital period
s initially disguised the planet's radial velocity signature as an increased orbital eccentricity of the outer planet. Eugenio Rivera and J. Lissauer found that the two planets undergo strong gravitational interactions as they orbit the star, causing the orbital elements
to change rapidly.
Both of the system's Jupiter-mass planets are located in the 'traditional' habitable zone
(HZ) of Gliese 876, which extends between 0.116 to 0.227 AU from the star.
On June 13, 2005, further observations by a team led by Rivera revealed a third planet in the system, inside the orbits of the two Jupiter-size planets. The planet, designated Gliese 876 d
, was estimated to have a minimum mass only 5.88 times that of the Earth
and may be a terrestrial planet
.
In 2008, the system was used as a test case for the migration of 5 Earth-mass planets which had formed inside the orbit of the innermost gas giant of the system. If it formed at (in this test) 0.07 AU from the star, b's gravity would have pulled d into an eccentric orbit. That orbit then would have restabilised to its current location.
In January 2009, the mutual inclination between planets b and c was determined using a combination of radial velocity and astrometric
measurements. The planets were found to be almost coplanar, with an angle of only 5.0° between their orbital planes. It is the first planetary system around a normal star to have mutual inclination between planets measured (previously the mutual inclination of the planets orbiting the pulsar
PSR B1257+12
had been determined by measuring their gravitational interactions). Later measurements reduced the value of the mutual inclination, and in the latest four-planet models the incorporation mutual inclinations does not result in significant improvements relative to coplanar solutions.
On June 23, 2010, astronomers announced the discovery of a fourth planet (designated Gliese 876 e
) in a 1:2:4 Laplace resonance with Gliese 876 b and c. This is the second known example of a Laplace resonance, the first being Jupiter's moons Ganymede
, Europa
and Io
. This discovery better constrains the mass and orbital properties of the other three planets, including the high eccentricity of the innermost planet. Numerical integration indicates that the coplanar, four-planet system is stable for at least another billion years. This planetary system comes close to a triple conjunction
between the three outer planets once per orbit of the outermost planet.
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....
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...
approximately 15 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 away from 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...
in the constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....
of Aquarius
Aquarius (constellation)
Aquarius is a constellation of the zodiac, situated between Capricornus and Pisces. Its name is Latin for "water-bearer" or "cup-bearer", and its symbol is , a representation of water....
. As of 2011, it has been confirmed that four extrasolar planet
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...
s orbit the star. Two of the middle planets
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...
are similar to 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,...
, while the closest planet is thought to be similar to a small 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...
or a large terrestrial planet
Terrestrial planet
A terrestrial planet, telluric planet or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun...
, and the outer planet has mass similar to 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...
. The orbits of all but the closest planet are locked in a rare three-body Laplace resonance.
Distance and visibility
Gliese 876 is located fairly close to our 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...
. According to astrometric
Astrometry
Astrometry is the branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. The information obtained by astrometric measurements provides information on the kinematics and physical origin of our Solar System and our Galaxy, the Milky...
measurements made by the Hipparcos
Hipparcos
Hipparcos was a scientific mission of the European Space Agency , launched in 1989 and operated between 1989 and 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial objects on the sky...
satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
, the star shows a parallax
Parallax
Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek παράλλαξις , meaning "alteration"...
of 213.28 milliarcsecond
Minute of arc
A minute of arc, arcminute, or minute of angle , is a unit of angular measurement equal to one sixtieth of one degree. In turn, a second of arc or arcsecond is one sixtieth of one minute of arc....
s, which corresponds to a distance of 4.69 pc, currently making it the second closest known star with orbiting planets, after 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...
. Despite being located so close to Earth, the star is so faint that it is invisible to the naked eye
Naked eye
The naked eye is a figure of speech referring to human visual perception unaided by a magnifying or light-collecting optical device, such as a telescope or microscope. Vision corrected to normal acuity using corrective lenses is considered "naked"...
and can only be seen using a telescope
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...
.
Stellar characteristics
As a red dwarf star, Gliese 876 is much less massive than our Sun: estimates suggest it has only 32% of the mass of the Sun. The surface temperatureTemperature
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...
of Gliese 876 is cooler than our Sun and the star has a smaller radius. These factors combine to make the star only 1.24% as luminous
Luminosity
Luminosity is a measurement of brightness.-In photometry and color imaging:In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.The luminosity function...
as the Sun, and most of this is at infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
s.
Estimating the age and metallicity of cool stars is difficult due to the formation of diatomic molecules in their 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, which makes the spectrum
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...
extremely complex. By fitting the observed spectrum to model spectra, it is estimated that Gliese 876 has a slightly lower abundance of heavy elements compared to the Sun (around 75% the solar abundance of 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...
). Based on chromospheric
Chromosphere
The chromosphere is a thin layer of the Sun's atmosphere just above the photosphere, roughly 2,000 kilometers deep....
activity the star is likely to be around 6.5 to 9.9 billion years old, depending on the theoretical model used. However, the low rotational period of the star as well as its membership among the young disk population suggest that the star is between 0.1–5 billion years old.
Like many low-mass stars, Gliese 876 is a variable star
Variable star
A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...
. Its variable star designation
Variable star designation
Variable stars are named using a variation on the Bayer designation format of an identifying label combined with the Latin genitive of the name of the constellation in which the star lies...
is IL Aquarii and it is classified as a BY Draconis variable
BY Draconis variable
BY Draconis variables are main sequence variable stars of late spectral types, usually K or M. The name comes from the archetype for this category of variable star system, BY Draconis. They exhibit variations in their luminosity due to rotation of the star coupled with star spots, and other...
. Its brightness fluctuates by around 0.04 magnitudes
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
. This type of variability is thought to be caused by large starspot
Starspot
Starspots are equivalent to sunspots but located on other stars. Spots the size of sunspots are very hard to detect since they are too small to cause fluctuations in brightness...
s moving in and out of view as the star rotates. Gliese 876 emits X-rays.
Planetary system
On June 23, 1998, an extrasolar planet was announced in orbitOrbit
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 Gliese 876 by two independent teams led by Geoffrey Marcy
Geoffrey Marcy
Geoffrey W. Marcy is an American astronomer, who is currently Professor of Astronomy at the University of California, Berkeley, famous for discovering more extrasolar planets than anyone else, 70 out of the first 100 to be discovered, along with R...
and Xavier Delfosse. The planet was designated Gliese 876 b
Gliese 876 b
Gliese 876 b is an extrasolar planet orbiting the red dwarf star Gliese 876. It completes one orbit in approximately 61 days. Discovered in June 1998, Gliese 876 b was the first planet to be discovered orbiting a red dwarf star.- Discovery :...
and was detected by making measurements of the star's radial velocity
Radial velocity
Radial velocity is the velocity of an object in the direction of the line of sight . In astronomy, radial velocity most commonly refers to the spectroscopic radial velocity...
as the planet's gravity pulled it around. The planet, around twice the mass of 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,...
, revolves around its star in an orbit taking approximately 61 day
Day
A day is a unit of time, commonly defined as an interval equal to 24 hours. It also can mean that portion of the full day during which a location is illuminated by the light of the sun...
s to complete, at a distance of only 0.208 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
, less than the distance from the Sun to 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...
.
Gliese 876 c
Gliese 876 c is an extrasolar planet orbiting the red dwarf star Gliese 876, taking about 30 days to complete an orbit. The planet was discovered in April 2001 and is the second planet in order of increasing distance from its star.- Discovery :...
is in a 1:2 orbital 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 the outer planet, taking 30.340 days to orbit the star. This relationship between the orbital period
Orbital period
The orbital period is the time taken for a given object to make one complete orbit about another object.When mentioned without further qualification in astronomy this refers to the sidereal period of an astronomical object, which is calculated with respect to the stars.There are several kinds of...
s initially disguised the planet's radial velocity signature as an increased orbital eccentricity of the outer planet. Eugenio Rivera and J. Lissauer found that the two planets undergo strong gravitational interactions as they orbit the star, causing the orbital elements
Orbital elements
Orbital elements are the parameters required to uniquely identify a specific orbit. In celestial mechanics these elements are generally considered in classical two-body systems, where a Kepler orbit is used...
to change rapidly.
Both of the system's Jupiter-mass planets are located in the 'traditional' habitable zone
Habitable zone
In astronomy and astrobiology, a habitable zone is an umbrella term for regions that are considered favourable to life. The concept is inferred from the empirical study of conditions favourable for Life on Earth...
(HZ) of Gliese 876, which extends between 0.116 to 0.227 AU from the star.
On June 13, 2005, further observations by a team led by Rivera revealed a third planet in the system, inside the orbits of the two Jupiter-size planets. The planet, designated Gliese 876 d
Gliese 876 d
Gliese 876 d is an extrasolar planet approximately 15 light-years away in the constellation of Aquarius . The planet was the third planet discovered orbiting the red dwarf star Gliese 876. At the time of its discovery, the planet had the lowest mass of any known extrasolar planet apart from the...
, was estimated to have a minimum mass only 5.88 times that of the 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 may be a terrestrial planet
Terrestrial planet
A terrestrial planet, telluric planet or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun...
.
In 2008, the system was used as a test case for the migration of 5 Earth-mass planets which had formed inside the orbit of the innermost gas giant of the system. If it formed at (in this test) 0.07 AU from the star, b's gravity would have pulled d into an eccentric orbit. That orbit then would have restabilised to its current location.
In January 2009, the mutual inclination between planets b and c was determined using a combination of radial velocity and astrometric
Astrometry
Astrometry is the branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. The information obtained by astrometric measurements provides information on the kinematics and physical origin of our Solar System and our Galaxy, the Milky...
measurements. The planets were found to be almost coplanar, with an angle of only 5.0° between their orbital planes. It is the first planetary system around a normal star to have mutual inclination between planets measured (previously the mutual inclination of the planets orbiting the pulsar
Pulsar
A pulsar is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name...
PSR B1257+12
PSR B1257+12
PSR B1257+12, sometimes abbreviated as PSR 1257+12, is a pulsar located roughly 2000 light-years from the Sun. In 2007, it was confirmed that three extrasolar planets orbit the pulsar.- Pulsar :...
had been determined by measuring their gravitational interactions). Later measurements reduced the value of the mutual inclination, and in the latest four-planet models the incorporation mutual inclinations does not result in significant improvements relative to coplanar solutions.
On June 23, 2010, astronomers announced the discovery of a fourth planet (designated Gliese 876 e
Gliese 876 e
Gliese 876 e is an extrasolar planet orbiting the star Gliese 876 in the constellation of Aquarius. It is in a 1:2:4 Laplace resonance with the planets Gliese 876 c and Gliese 876 b: for each orbit of planet e, planet b completes two orbits and planet c completes four...
) in a 1:2:4 Laplace resonance with Gliese 876 b and c. This is the second known example of a Laplace resonance, the first being Jupiter's moons 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...
, 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...
and 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....
. This discovery better constrains the mass and orbital properties of the other three planets, including the high eccentricity of the innermost planet. Numerical integration indicates that the coplanar, four-planet system is stable for at least another billion years. This planetary system comes close to a triple conjunction
Conjunction
Conjunction can refer to:* Conjunction , an astronomical phenomenon* Astrological aspect, an aspect in horoscopic astrology* Conjunction , a part of speech** Conjunctive mood , same as subjunctive mood...
between the three outer planets once per orbit of the outermost planet.
See also
- Gliese 876 in fiction
- List of nearest stars
- List of extrasolar planets
External links
- Image Gliese 876
- Extrasolar Planet Interactions by Rory Barnes & Richard Greenberg, Lunar and Planetary Lab, University of Arizona
- Sky map