Very broadly speaking, tags should be used to categorize the type of domain knowledge that answers need to draw on.
When adding tags, ask yourself the question "is an expert in abc-xyz the most suitable individual to answer my question?". If the answer to this is a resounding yes, then the tag probably makes sense; if the answer is anything less than that, then consider some other tag, or leave it out entirely.
In other words, if you are asking about the planet itself, then tagging the question planets makes sense.
If you are asking about effects on the environment of some specific condition of the planet, then environment is probably more appropriate.
If you are asking about effects on the society of that condition of the planet then you could usefully tag the question simply society.
If your question is about the effects of commonplace space travel on the society of a planet, then a tag combination like society space-travel probably makes sense to categorize the question. Note that such a question is not about planets and should not be tagged as such.
If your question is about the effects of a specific level of surface gravity on the development of airships, then a combination like airship gravity probably makes sense, possibly alongside technological-development and/or history depending on the specific focus of the question.
And so on.
Just because the text of a question starts out with, say, "I have a civilization living on a planet where..." this does not necessarily make it a question about planets. The major exception to this is if knowledge about planets (that is, naturally formed celestial objects orbiting a central star) is specifically useful in answering the actual question being asked.