First off, remember that tags should be useful for grouping questions based on subject matter. Someone who is an expert in a particular area should be able to follow a particular tag and be notified of questions to which they are likely to be able to provide good answers, or be notified of questions that they are likely to be interested in. Too much segregation between tags goes against that goal. A tag should also provide meaningful information when used on its own on a question.
In that venue, I feel planets and planetoids are similar enough that separating them doesn't make much sense. Someone who is familiar with one is likely to be familiar with the other.
space when referring to the part of the universe outside of a planet's atmosphere appears overly broad. It can probably be replaced with some other tag where it is used. Maybe astronomy would be useful on questions that perhaps otherwise would be tagged just space. I do think that we should have a tag space-travel that deals with questions pertaining to travel through the universe. A related tag could be travel or terrestrial-travel for travelling on or near a planet's surface (for example: on foot, by horseback, dog sled or airplane).
astronomy and meteorology seem like good tags to me, assuming that the subjects themselves are to be considered on topic.