The Traveler made it a GARDEN world, and had capabilities of giving each and every celestial body within the Sol system an atmosphere of its own. It had the powers to make jungles on VENUS, which currently in real life is a gas, volcanic deathtrap. Originally you would be right, as Even going near Mercury would burn a human lifeform. But in the sense of the story and the power of the Traveler, anything is possible.