U.S. and Japanese astronomers say they've discovered a new class of planets -- dark, isolated Jupiter-sized bodies floating alone in space, far from any star.
University of Notre Dame astronomer David Bennett, co-author of a paper describing the discovery, believes the planets were most likely ejected from developing planetary systems, a Notre Dame release reported Wednesday.
"Our results suggest that planetary systems often become unstable, with planets being kicked out from their places of birth by close encounters with other planets," Bennet said.
The discovery stems from an analysis of observations of the Milky Way galaxy taken in 2006 and 2007 by a joint Japan-New Zealand survey.
Scientists estimate the total number of such rogue planets could be as many as 400 billion, far outnumbering main-sequence stars such as the sun.
Study co-author Takahiro Sumi, an astrophysicist at Osaka University in Japan, says the likely number of homeless exoplanets surprised him.
"The existence of free-floating planets has been predicted by planetary formation theory, but nobody knew how many there are," he said.
The study has been published in Nature.