"Lonely" planets, hurled into empty space after gravitational tussles with their siblings, may be 50 per cent more common than planets orbiting stars.
Most planet search techniques turn up worlds close to stars, because they are easier to detect. But
Takahiro Sumi of Osaka University in Japan and colleagues observed 50 million stars in the Milky Way at least once an hour for about two years to hunt for such signals. They found 10 objects of about Jupiter's mass that did not seem to have host stars.
Their technique cannot rule out the existence of host stars for these objects, but it does suggest that any stars are at least as far away from the planets as Saturn is from the sun. They argue that the paucity of planets imaged at wide separations from their host stars suggests that about three-quarters of the 10 new objects are not bound to stars.
Ten objects does not sound like a lot, but microlensing events are very rare, because they require the precise alignment of a background star, the planet "lens" and Earth. So the researchers say the new observations imply that lonely planets are 50 per cent more common than planets that have host stars and nearly twice as common as stars in the galaxy.
Steinn Sigurdsson of Pennsylvania State University in University Park is not convinced by the team's argument that 75 per cent of the newly found objects are actually not bound to any star. But he says the work does suggest that a large number of planets do float free through the galaxy.
Sara Seager of the Massachusetts Institute of Technology says the new study suggests that planets are frequently thrown out of their planetary nests through gravitational interactions with other members of their brood. Sumi agrees: "The discovery of these unbound planets inform us how many planets have been created and ejected from their orbit."
Nature, DOI: 10.1038/nature10092
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