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Free-floating planets more common than stars?
Posted: 19 May 2011

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Drifting through space far from any star, a new class of free-floating Jupiter-sized planets have been discovered that could have been ejected from growing planetary systems.

A joint Japan-New Zealand survey called the Microlensing Observations in Astrophysics (MOA) scanned an impressive 50 million stars towards the centre of the Milky Way for a period of two years, in search of gravitational microlensing events. These events occur when an object such as a star or planet passes in front of another, more distant star, warping the light of the background star and causing it to magnify and brighten. In the case of a passing planet, the background light might be distorted for just a few days or less.

Ten Jupiter-like planets have been detected in our Milky Way Galaxy without any sign of a parent star, likely booted from their birth place in turbulent, developing solar systems. Image: NASA/JPL-Caltech.

The study detected 474 individual microlensing events, ten of which lasted for less than two days, many more short-period events than expected from the known population of stars and brown dwarfs – "failed" stars that didn't quite have the mass to ignite into fully-fledged fuel-burning stars. Furthermore, for those ten events, which are attributed to Jupiter-mass objects passing in front of the distant star, there is no evidence for a possible host star, leading the authors to conclude that either these objects are on extremely wide orbits from a star, or, more likely, are not associated with any star at all, drifting through space alone.

The findings, which were independently confirmed by a second microlensing survey group, the Optical Gravitational Lensing Experiment (OGLE), suggest that there could be up to twice as many free-floating planets as stars, equating to hundreds of billions of lone planets in our Milky Way Galaxy.

“Our survey is like a population census,” says David Bennett, a NASA and National Science Foundation-funded member of the team from the University of Notre Dame in South Bend. “We sampled a portion of the Galaxy, and based on these data, can estimate overall numbers in the Galaxy.”

Brown dwarf stars some three times the mass of Jupiter have already been observed within star-forming clusters, but the newly detected objects resemble planets much more than they do brown dwarfs. The astronomers think that these planets were likely ejected from the chaotic whirlpool of early solar system formation, perhaps flung out of the system after a close encounter with another planet or star, and left to wander through the Galaxy alone.

“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,” says Bennett. “Our results suggest that planetary systems often become unstable, with planets being kicked out from their places of birth.”

Furthermore, although the present survey is not sensitive to planets smaller than Jupiter, theories suggest lower-mass planets – perhaps even Earth-mass planets – would be ejected from their stars more often than large Jupiter-sized planets.