Gliese 876 b

Gliese 876 b is an exoplanet orbiting the red dwarf 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.

Discovery

Gliese 876 b was initially announced by

Given the planet's high mass, it is likely that Gliese 876 b is a gas giant with no solid surface. Since the planet has only been detected indirectly through its gravitational effects on the star, properties such as its radius, composition, and temperature are unknown. Assuming a composition similar to Jupiter and an environment close to chemical equilibrium, it is predicted that the atmosphere of Gliese 876 b is cloudless, though cooler regions of the planet may be able to form water clouds.^[10]

A limitation of the radial velocity method used to detect Gliese 876 b is that only a lower limit on the planet's

This planet, like c and e, has likely migrated inward.[12]

Host star

The planet orbits a (M-type) star named Gliese 876. The star has a mass of 0.33 M_☉ and a radius of around 0.36 R_☉. It has a surface temperature of 3350 K and is 2.55 billion years old. In comparison, the Sun is about 4.6 billion years old^[13] and has a surface temperature of 5778 K.^[14]

Orbit

Gliese 876 b is in a 1:2:4

Gliese 876 b currently lies beyond the outer edge of the habitable zone but because Gliese 876 is a slowly evolving main-sequence red dwarf its habitable zone is very slowly moving outwards and will continue to do so for trillions of years. Therefore, Gliese 876 b will, in trillions of years time, lie inside Gliese 876's habitable zone, as defined by the ability of an Earth-mass planet to retain liquid water at its surface, and remain there for at least 4.6 billion years.^[17] While the prospects for life on a gas giant are unknown, large moons may be able to support a habitable environment. Models of tidal interactions between a hypothetical moon, the planet and the star suggest that large moons should be able to survive in orbit around Gliese 876 b for the lifetime of the system.^[18] On the other hand, it is unclear whether such moons could form in the first place.^[19] However, the large mass of the gas giant may make it more likely for larger moons to form.^{[citation needed]}

For a stable orbit the ratio between the moon's orbital period P_s around its primary and that of the primary around its star P_p must be < 1/9, e.g. if a planet takes 90 days to orbit its star, the maximum stable orbit for a moon of that planet is less than 10 days.^[20][21] Simulations suggest that a moon with an orbital period less than about 45 to 60 days will remain safely bound to a massive giant planet or brown dwarf that orbits 1 AU from a Sun-like star.^[22] In the case of Gliese 876 b, the orbital period would have to be no greater than a week (7 days) in order to have a stable orbit.

Tidal effects could also allow the moon to sustain plate tectonics, which would cause volcanic activity to regulate the moon's temperature^[23][24] and create a geodynamo effect which would give the satellite a strong magnetic field.^[25]

To support an Earth-like atmosphere for about 4.6 billion years (the age of the Earth), the moon would have to have a Mars-like density and at least a mass of 0.07 M_E.^[26] One way to decrease loss from sputtering is for the moon to have a strong magnetic field that can deflect stellar wind and radiation belts. NASA's Galileo's measurements hints large moons can have magnetic fields; it found that Jupiter's moon Ganymede has its own magnetosphere, even though its mass is only 0.025 M_E.^[22]

See also

• Appearance of extrasolar planets
• List of exoplanets discovered before 2000

Notes

References