There's another interesting possibility: a rogue planet. https://en.m.wikipedia.org/wiki/Rogue_planet https://en.m.wikipedia.org/wiki/Rogue_planet Under speci

There's another interesting possibility: a rogue planet. https://en.m.wikipedia.org/wiki/Rogue_planet https://en.m.wikipedia.org/wiki/Rogue_planet Under specific conditions, such a planet may even retain an atmosphere, have moderate surface temperatures (and thus, perhaps even liquid water), and life could exist on it if chemically-powered rather than using photosynthesis. Such a planet would be an excellent vehicle to carry life from one star system to another. And another plus: out of a star system's influence sphere (while free-wandering through space), it could have very stable conditions over long periods of time. Which doesn't hurt in the formation or evolution of life. True, there's a lot of (big) if's there. But the universe is biiiiiigggg so I wouldn't rule it out.