If we could all agree on that tags being synonomously used, could somebody with the appropriate rights define the synonym, please?
Hmm, planetary-science covers a topic space that is not applicable to all planet questions, so I don't think a synonym is appropriate. planetary-science was originally conceived to capture questions about atmospheric or geological properties of planets, rather than purely astronomical observations. I see that the term "planetary science" may have broadened since we created the tag--or perhaps we were unaware of the broader use when we conceived it.
In any case, I could see justification for renaming it to better capture the subject area, but I don't think it would be helpful to just blob all of the subject matter together into the massive planet tag. Tags are supposed to help us find information about certain subjects easier, and the topic of planet in general is so broad it is not very helpful in categorizing alone.