Thursday, August 9, 2007

Gliese 876

Star type
g M3.5 V

Distance from Earth
g 15.34 ly

Star Service No.
g NA

g Around 6.520 or 9.900 billion years old, depending on the theoretical model used

Diameter/Mass/Temp (xSol)
g 39%; 32%; 3,480 ± 50 K

Brightness (xSol)
g 1.24%

g Around 75% the solar abundance of iron

Comparison to Sol
g See illustration (red dwarf is net to Sol)

Picture of star
g See picture

Star system features
g Past: The observance of a 2:1 resonance implies that the two of the star system’s planets originally had different orbital periods. However, as one planet moved from its original location, it gravitationally pulled the other planet into the 2:1 orbital synchrony (see Geoffrey Bryden's illustrations and explanation of this process). This orbital adjustment also occurred in our Solar System, leading the planets Neptune and Pluto into an orbital resonance of 3:2 (as Neptune moved to shepherd Pluto).

Known planets
g Planet Gliese 876 d: inside the orbits of the two Jupiter-size planets. The planet has a minimum mass only 5.88 times that of the Earth and may be a terrestrial planet. Based on the radial velocity measurements and modelling the interactions between the two giant planets, the system's inclination is estimated to be around 50° to the plane of the sky. If this is the case and the system is assumed to be coplanar, the planetary masses are around 30% greater than the lower limits established by the radial velocity method. This would make the inner planet have a true mass around 7.5 times that of Earth (or a mass less than half that of Neptune) Due to its close orbit, the planet is presumed to be tidally locked with one surface eternally facing its host star, and so may have a bright-side temperature around 157-377° C but be extremely cold on the dark side. "The escape velocity from the surface of the planet is estimated to be slightly more than twice that of Earth, so that the planet may well have retained a substantial atmosphere and may thus have a larger radius ... [but] it is unlikely that this planet has a moon" due to a Hill sphere radius of less than five planetary radii. Receives only 30% of sunlight that Earth does, but atmospheric gasses could exist in large enough quantities to keep in heat. See artist’s interpretation of planet.
g Planet Gliese 876 c: 0.62 Jupiter-mass planet is in a 1:2 orbital resonance with the outer planet, taking 30.340 days to orbit the star. This relationship between the orbital periods initially disguised the planet's radial velocity signature as an increased orbital eccentricity of the outer planet. The two planets undergo strong gravitational interactions as they orbit the star, causing the orbital elements to change See artist’rapidly
g Planet Gliese 876b: around twice the mass of Jupiter, revolves around its star in an orbit taking approximately 61 days to complete, at a distance of only 0.208 AU, less than the distance from the Sun to Mercury; Despite its close orbital distance, Gliese 876 is so dim that the presumed gaseous surface of the planet has a temperature around -103° F, however, water could exist as liquid droplets in warmer layers not far below the surface of the planet. See artist's interpretation of planet.
g No Kuiper Belt

Habitable zone
g Both of the system's Jupiter-mass planets are located in the 'traditional' habitable zone of Gliese 876, which extends between 0.116 to 0.227 AU from the star. This leaves little room for an additional habitable Earth-size planet in that part of the systeml at that orbital distance, such a planet would have an orbital period of only around about 24 days. Large moons of the gas giants, if they exist, may be able to support life. Furthermore, the habitable zone for planets whose rotation is synchronous with their orbital motion may be wider than the traditional view, which may enable the existence of habitable planets elsewhere in the system.

Orbital map
g See map

View from star
g See sky map

Nearby stars
(Star systems with 10 light years)
g Luyten 789-6
g Gliese 1002
g Gliese 1005
g Lacaille 9352
g Gliese 908
g Gliese 1
g Fomalhaut (α PsA)
g Gliese 825
g van Maanen 2
g Tau Ceti
g Luyten 726-8

Map locating star system
g See stellar map (listed as “Ross 780”)

Location in Earth sky
g In the southeastern part of Constellation Aquarius, the Water Bearer, north of Skat (Delta Aquarii), east of Tau Aquarii, and south of Lambda Aquarii; so faint that it is invisible to the naked eye and can only be seen using a telescope

Other names
g Ross 780
g BD-15°6290
g IL Aquarii
g G 156-057
g GCTP 5546.00
g LHS 530
g Vys 337
g HIP 113020
g LTT 9244
g LPM 841
g LFT 1745

Sci-fi mentions
g NA

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