Hundreds of billions of isolated planets are believed to be floating alone in space, following an international survey of the sky.
A team from Japan and New Zealand scanned the center of the Milky Way galaxy during 2006 and 2007, and for the first time found evidence of up to 10 free-floating planets, or orphan planets, roughly the mass of Jupiter. They're around 10,000 to 20,000 light years from Earth.
The team was unable to detect anything much smaller than Jupiter, but believes that smaller free-floating planets may be even more common.
"Although free-floating planets have been predicted, they finally have been detected, holding major implications for planetary formation and evolution models," said Mario Perez, exoplanet program scientist at NASA Headquarters in Washington.
Finding this many in one region indicates that there's likely to be a lot more of them about - indeed, the team estimates there are about twice as many of them as stars. They're thought to be at least as common as planets that orbit stars, implying the existence of hundreds of billions of lone planets in our Milky Way galaxy alone.
"Our survey is like a population census," said David Bennett, a co-author of the study from the University of Notre Dame in Indiana. "We sampled a portion of the galaxy, and based on these data, can estimate overall numbers in the galaxy."
While a handful of free-floating planet-like objects had been found before, these are now believed to be brown dwarfs, and more like stars than planets.
The new discoveries are beleived to represent planets that have been ejected from their solar systems because of the gravitational effects of other planets or stars. Without a star to circle, they move through the galaxy in the same way as stars, in stable orbits around the galaxy's center.
"If free-floating planets formed like stars, then we would have expected to see only one or two of them in our survey instead of 10," Bennett said. "Our results suggest that planetary systems often become unstable, with planets being kicked out from their places of birth."
There's still the possibility that some of these planets may have very distant orbits around stars, but other research indicates Jupiter-mass planets in such orbits are rare.