Lone Planets “More Common Than Stars”

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We happen to live in a solar system where everything seems to be tucked neatly in place. Sun, planets, moons, asteroids, comets… all turning and traveling through space in relatively neat and orderly fashions. But that may not always be the case; sometimes planets can get kicked out of their solar systems entirely, banished to roam interstellar space without a sun of their own. And these “orphan planets” may be much more numerous than once thought.

Researchers in a joint Japan-New Zealand study surveyed microlensing events near the central part of our galaxy during 2006 and 2007 and identified up to 10 Jupiter-sized orphan worlds between 10,000 and 20,000 light-years away. Based on the number of planets identified and the area studied they estimate that there could literally be hundreds of billions of these lone planets roaming our galaxy….literally twice as many planets as there are stars.

“Although free-floating planets have been predicted, they finally have been detected, holding major implications for planetary formation and evolution models.”

– Mario Perez, exoplanet program scientist at NASA Headquarters in Washington.

From the NASA release:

Previous observations spotted a handful of free-floating, planet-like objects within star-forming clusters, with masses three times that of Jupiter. But scientists suspect the gaseous bodies form more like stars than planets. These small, dim orbs, called brown dwarfs, grow from collapsing balls of gas and dust, but lack the mass to ignite their nuclear fuel and shine with starlight. It is thought the smallest brown dwarfs are approximately the size of large planets.

On the other hand, it is likely that some planets are ejected from their early, turbulent solar systems, due to close gravitational encounters with other planets or stars. Without a star to circle, these planets would move through the galaxy as our sun and other stars do, in stable orbits around the galaxy’s center. The discovery of 10 free-floating Jupiters supports the ejection scenario, though it’s possible both mechanisms are at play.

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

– David Bennett, a NASA and National Science Foundation-funded co-author of the study from the University of Notre Dame.

The study wasn’t able to resolve planets smaller than Saturn but it’s believed there are likely many more smaller, Earth-sized worlds than large Jupiter-sized ones.

Read the full NASA news release here.

The study, led by Takahiro Sumi from Osaka University in Japan, appears in the May 19 issue of the journal Nature.