Pluto

Pluto is the fourth largest dwarf planet in the solar system and is the second closest. After it's discovery Pluto was initially classified as a planet, but in 2006 it was downgraded to a dwarf planet. Pluto was first colonized in 2130 and now is home to 6 million humans and an additional 20 million in orbit. The entire Kuiper belt (where Pluto resides) has a population of 600 million.