Back in 1998, astronomer David J. Stevenson of Caltech theorized that there are planets that roam the vast expanses of cold interstellar space. (The technical term here is actually "planetary-mass objects", since "planet" has strict definition by the International Astronomical Union - just ask Pluto - but for laymen, planets will do.) Such planets do not revolve around any star.
Astronomers using the Canada-France-Hawaii Telescope in Hawaii and the Very Large Telescope in Chile have spotted one such rogue planet, giving credence to the hypothesis that rogue planets may be common:
"This object was discovered during a scan that covered the equivalent of 1,000 times the [area] of the full moon," said study co-author Etienne Artigau of the University of Montreal. [...]
The team believe it has a temperature of about 400C and a mass between four and seven times that of Jupiter - well short of the mass limit that would make it a likely brown dwarf.
What remains unclear is just how the planet came to be - the tiny beginnings of a star, or planet launched from its home? Study co-author Philippe Delorme of the Institute of Planetology and Astrophysics of Grenoble, said that the latter implied a great many planets like it.
"If this little object is a planet that has been ejected from its native system, it conjures up the striking image of orphaned worlds, drifting in the emptiness of space," he said.