A rogue planet is a celestial body that is not tethered by gravity to a star, but rather roams through space on its own. It may have once orbited a star, but was knocked off their trajectory by another object, or pulled away by another star, or may possibly be the remnant of a star system that exploded. It may be even possible that planets can form without ever being in orbit. We don't know much about them because astronomers have only detected rogue planets in the 21st century, and then only indirectly.
But more recent data suggest that rogue planets are way more common than previously thought. Current estimates say that there are an average of seven free-floating planets for every star in the Milky Way galaxy! We just can't see them because they emit no light and only rarely cause a shadow. But they are out there, moving between star systems, colliding with other bodies, and sneaking through the darkness. Our newer, more powerful telescopes are expected to shed light, so to speak, on these so-far invisible planets. Read about the search for rogue planets at IEEE Spectrum. -via Metafilter
(Image credit: Chris Gunn/NASA)