Extrapolating from a survey of planets without nearby parent stars, Takahiro Sumi at Japan's Osaka University and colleagues figured out that there are at least twice as many free-flying planets as there are stars like the sun.
"The most important thing is that we found that these free-floating planets are as common as stars," Sumi told Discovery News.
The solo planets probably didn't start out that way.
Computer simulations show that if there is more than one giant planet in a solar system's family, they will perform a kind of gravitational arm-wrestling match, making their orbits unstable. Eventually, one planet loses the battle and winds up in the nether regions of its solar system or kicked out of the parent star's orbital nest entirely.
"If they formed in the proto-planetary disk (a rotating disk of dense gas surrounding a newly formed star ) as we guess, this information is very important for the planetary formation theory," Sumi wrote in an email.
That's because planets bound to a parent star would only reflect the survivors, not how many gas giant planets actually formed.
The isolated planets, however, may not have formed around a parent star at all, points out Joachim Wambsganss, with the University of Heidelberg in Germany.
"An alternative scenario has been proposed some time ago, that objects of planetary mass could also form in isolation via a collapse of matter, similar to the way a star forms," Wambsganss wrote in an email to Discovery News. "Maybe there are even two 'production lines' for these objects."
Ten extra-solar planets that served as models for the study were found with a technique known as gravitational micro-lensing, a naturally occurring "zoom" effect caused by the bending of light around a massive foreground object.
Read more at Discovery News
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