When one thinks of planets, one usually thinks of them within solar systems, orbiting a star. Until fairly recently, it was thought that planets drifting through interstellar space were fairly rare.
A study of these “nomad” planets by astrophysicist Louis Strigari, with Stanford University’s Kavli Institute, however, suggests that these planets may be very, very common. There may possibly be billions of them. And some of them may generate enough heat by themselves to maintain liquid water.
So, what can you do with this? What would the people in your fictional universe use these nomads planets for?
Secret military bases, rebel fortresses, fallback positions for royalty?
Scientific facilities for dangerous, or forbidden, research?
A (supposedly) unescapable prison?
Build monuments on them, to be seen by the inhabitants of the next world it passes in a million years?
Colonize them with “undesirables” who will be forever exiled from our system?
What might people find on this wandering planet?
Ruins of previous inhabitants?
Exotic life forms or mechanical intelligences?
Something “impossible”, like an ancient Earth dinosaur’s skeleton?
I think that these nomad planets are a very underutilized setting for science fiction stories. They take the void of interstellar space, and allow one to fill it in with a multitude of exotic settings. As our nearby interstellar solar systems become more and more understood, they give a writer an opportunity to have a nearby planet, even if one doesn’t exist in the real world. Check out this article from Centauri Dreams for more information on the possibility of life on these planet.