Image courtesy of Southwest Research Institute

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ScienceShot: Why So Many Homeless Planets?

Last year, astronomers reported that extrasolar planets may outnumber stars in our galaxy by almost a two-to-one margin, and that three-quarters of these worlds are likely to be free-floaters, not bound to any star. Scientists speculated that many of these homeless planets were slung out of their parent solar systems as a result of gravitationally unstable orbits. But new computer simulations blame more exotic causes. One possibility is stars literally pushing the planets into interstellar space after the suns reach the end of their normal hydrogen-burning lives and begin expanding into red giants. Other scenarios involve gravitational perturbations, either caused by passing stars, a solar system entering and exiting our galaxy's gravitationally dense spiral arms, or even via interactions with dense molecular clouds. But the most likely reason, researchers report in a paper accepted for publication in the Monthly Notices of the Royal Astronomical Society, is that these extrasolar planets would simply be ejected by the gravitational forces that result when their parent stars get jostled about inside tightly-packed star clusters—the same clusters in which most stars are thought to be formed.

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