Nyx

The “Ninth Planet” discovered in the mid Twenty First century, Nyx has 10 times the mass of Earth and orbits between 200 and 1200 AU over approximately 17,000 years. It is considered a Super-Earth terrestrial world with a diameter nearly 3 times that of Earth. At the current time, Nyx is approximately 997.5 AU from the Sun.

Nyx, named after the Greek Goddess of Night, is perpetually cold and dark. The Sun only appears as one of the brighter stars in the sky. Whatever atmosphere it once had is now frozen and lying on the surface which is 70% water ice.

Scientists have speculated that the planet would be good for a colony since water ice is plentiful and its Mass to Diameter ratio means that the surface gravity is close to 1.11g. Also, it’s distance from the Sun means it does not receive very much solar radiation. With fusion reactors and air & water generated from melted ice the world could be quite comfortable for human colonists in domed cities.