Gliese 876 b

Extrasolar planet orbiting Gliese 876

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.

Gliese 876 b was initially announced by Geoffrey Marcy on June 22, 1998 at a symposium of the International Astronomical Union in Victoria, British Columbia, Canada. The discovery was made using data from the Keck and Lick observatories.

The planet orbits a (M type) star named Gliese 876. The star has a mass of 0.33 and a radius of around 0.36 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 and has a surface temperature of 5778 K.

Gliese 876 b is in a 1:2:4 Laplace resonance with the inner planet Gliese 876 c and the outer planet Gliese 876 e: in the time it takes planet e to complete one orbit, planet b completes two and planet c completes four. This is the second known example of a Laplace resonance, the first being Jupiter's moons Io, Europa and Ganymede. 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. 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 and create a geodynamo effect which would give the satellite a strong magnetic field. 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 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