We seem to have a fair few Rocheworld style questions accumulating over the years. Is it worth having a tag for them? Would enough people even recognise the tag for it to be meaningful?
Is that a popular book?
I read a lot, and I had never heard of Rocheworld, so I did a quick search and figured out it's a book about laser driven light sail propulsion to the double planet that orbits Barnard's Star.
Having never read it, even with the wikipedia entry to give me hints, I have no idea what actually makes up a Rocheworld question.
I suggest that instead of having a tag like rocheworld, I think it would be a lot less confusing to have a tag like laser-driven-light-sail, or double-planet, or extra-stellar-spaceflight, or double-planets-orbiting-barnards-star-that-you-get-to-using-a-laser-driven-light-sail, because then it's not ambiguous as to what it means.
What would this tag be about? Forward's light sail propulsion system? Flouwen? Rocheworld double planet shape? If it is not clear from the tag name, then it is a very bad tag name. I might agree that propulsion systems and planets with shared atmosphere might, possibly, deserve own tags if there are none already. But I'm sure that Rocheworld is not a name for tag.
What I've seen is a not uncommon pattern of several questions on a related topic (in this case "rocheworld") coming close together, where one or sometimes two people are producing these questions for a while.
Then it dies down, like a fad.
I'd be afraid that if we make tags for all these subject-of-the-moment type things we'd be knee deep in tags that are hardly used. We already have useful tags for this type of question - like gravity, planets. Maybe we should not seek to generate tags unless there's a consistent subject or category that crops up e.g. at least once a week, maybe once every two weeks.
tl dr: Darn Straight - Add It
What is the purpose of a tag? In my approximation it is metadata. What is metadata used for? It's used so people can find subjects for which they are looking. You can have five tags per question. If having an extra tag which says rocheworld brings people from away from Stack Exchange here (through a Google search or what have you), helps others who are asking questions as such (so they don't create duplicates and therefore make more work for others), and makes general sense (can be related to the subject without too much consternation), then why not? IMHO, the only reason to not add it is if there's already something which could be used as a synonym. Just because some people don't know what it is doesn't mean the tag shouldn't be added. That's a very short sighted and snobbish attitude.
To me, as long as there is a good explanation in the Tag Wiki as far as what the meaning of the tag is, then add it. It will pay dividends going forward.