Mars

Mars is the most habitable planet in Sol. Despite its cold and unforgiving natural climate as well as infertile soil, human structures with climate-controlled interiors and topsoil from Earth have allowed Mars to be inhabited relatively comfortably.

Humans have been colonizing Mars for under 100 years, so the planet is not entirely colonized and population is still very low.

There is no living life native to Mars, but fossils of ancient Martians have been found.