I would consider planetary sciences on topic. The site is intended for the study of the earth, that is OUR earth. But the study of other planets as "alternative" earths will help our own.
A possible exception to the rule is the study of other planets within constellations in "space." That becomes an "astronomy" issue that might not relate to earth science as we understand it.
Then again it might. The Astronomy SE site has a relatively weak following, and a lot questions on earth sciences aren't getting answered, and the solution might be a combined "Earth and Planetary" Science site.
I am writing this with an unusual background; an American liberal arts major whose exposure in school to science was in "general science." That's mainly earth science and astronomy, basically anything that wasn't biology, chemistry, or physics.