FreeOrion

I came across a rogue planet that doesn't orbit any star. Is this intentional?

Yes, it is intended. Starless systems have a small chance of having planets. They also have a preference for gas giants. If you don't like it you can edit your universe_tables.txt.

Must be pretty rare. I don't recall ever finding one.

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The chance of generating a no-star system was reduced, making no-star with-planet systems correspondingly rarer. There's actually a good chance to find an empire starting in a no-star system if there are a larger number of no-star systems, as I think that the code that places empires and generates there planets doesn't prefer systems with stars.

Shouldn't it be preferred then?

Ideally it should pick a suitable existing planet, make that a homeworld, and only add planets if it can't find any suitable ones.
I suspect however that implementing that has a low priority since it hardly affects gameplay.

We're not tying to make a galaxy simulator where the heat of the star and distance to the planet determines the temperature of a star and thus weather and what kind of life it hosts.

In FO, stars are pretty much for appearances sake, so a homeworld in a system without a star is at no real disadvantage. (some advanced techs require certain types of stars, but by that point you'll have multiple colonies to choose from.)

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