It is located in the Andromeda Galaxy, and seems to be slightly larger when compared to Earth. This idea is supported by the "prawns" growing up to two feet taller than most humans. The atmosphere apparently consists of an oxygen/nitrogen mix, fairly similar to that of Earth, or the non-humans would not be able to so easily adapt to Earth's air and gravity. The geography of the planet is also very similar to Earth's with large landmasses and oceans.
Non-human technology is highly advanced, leading some to theorize that their civilization may be a few hundred years, perhaps a millenium more developed than humanity.
It is far enough away from Earth that it will take three years for Christopher to travel there and back with a rescue mission.