Sometimes Science Fiction is confused with Fantasy, but as far as it means "fiction about things that are scientifically possible but don't exist" I'd say traveling to other planets is SF. Traveling to other worlds (ie. interstellar) is fantasy. And living on other planets (terraforming) or outer space is somewhere in the middle: technically not impossible but very undesirable without major future tech.

Fans of space colonization are encouraged to read: a City on Mars by Weinersmiths, https://www.acityonmars.com/