Not all planets orbit a star.
Theory suggests that planets could be ripped away from their host stars by gravitational interactions with other planets in their system or stars that fly by. It's also possible for planet-mass objects to form directly from gas clouds like stars do, but their low mass keeps them from fusing hydrogen into helium. Instead of orbiting stars, these objects directly orbit the center of the Milky Way.
Astronomers estimate that there could be as many as a billion of these rogue planets moving through the Milky Way, but they're really hard to find since they don't emit their own light. In fact, the only way astronomers have been able to find Earth-mass rogue planets is with gravitational lensing. That means we have to wait for one of these objects to pass in front of a lightsource (a star or distant galaxy) and bend that source's light with its own gravity. This is an incredible rare type of observation because everything has to line up just right and we have to be looking at just the right time. But using this method, astronomers have been able to find about 20 rogue planets.
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