Adrestia (or 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.

Adrestia is in a 1:2:4 Laplace resonance with the inner planet Damocles and the outer planet Haren: 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. As a result, the orbital elements of the planets change fairly rapidly as they dynamically interact with one another. The planet’s orbit has a low eccentricity, similar to the planets in our solar system. The semimajor axis of the orbit is only 0.208 AU, less than that of Mercury in our solar system. However Gliese 876 is such a faint star that this puts it in the outer part of the habitable zone.

The planet’s mass is around 2.2756 times the mass of Jupiter. Adrestia is a gas giant with no solid surface. The atmosphere of Adrestia is cloudless, though cooler regions of the planet are able to form water clouds. This planet, like c and e, has likely migrated inward.

Adrestia 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, Adrestia will, in trillions of years time, lie optimally 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.

Adrestia has several moons, two of which are roughly terrestrial and are vaguely habitable, though no current life exists on them.

Adrestia is used to provide hydrogen fuel for use in many of the drive systems used by the SA. It is perfect for refueling ships as the head into or out of this system.


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