Thieving Stars Snatch Orphan Planets

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Our galaxy is thought to be teeming with billions of "nomad" planets. These worlds are interstellar orphans, with no stellar parent to call home. Some were likely gravitationally flung from their parent star at an early age, while others may have evolved on their lonesome, clumping from small clouds of interstellar gas and dust.

If there are so many orphaned worlds drifting alone, how often might they be snatched by the gravitational tug of a star that happens to be drifting in the same direction?

Surprisingly, say astronomers from the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and Peking University in China, it may happen more often than astrophysicists ever dreamed.

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"Stars trade planets just like baseball teams trade players," said CfA's Hagai Perets in an April 17 press release.

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In a Feb. 24 Discovery News article, space correspondent Irene Klotz detailed research based on the microlensing of nomad planets and the estimate that there may be as many as 100,000 of these interstellar orphans per star in our galaxy.

Perets' research is based on the assumption that there is only one nomad planet per star in a simulated young star cluster.

From this low estimate, Perets and collaborator Thijs Kouwenhoven (Peking University) found that 3-6 percent of the stars in their simulated cluster captured a nomad planet at some point in time. Naturally, the bigger the star, the greater its gravity, and therefore the bigger the chance a nomad planet may be snatched.

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Although it may sound as if Perets and Kouwenhoven are being overly conservative with their estimate of nomad planets (especially in light of the 100,000 planets per star estimate), they deliberately focused on a young cluster of stars, which, by their nature, would be closely packed. Also, there would be fewer nomad worlds in these early years. As clusters of stars grow older, they spread out, greatly reducing the density of stars. It's for this reason that most of the "planet snatching" would happen in the very early history of star cluster evolution — despite there being fewer nomad worlds.

During the cluster's formative years, gravitational interactions between the stars and the inevitable dynamical chaos within young star systems would cause planets to be slingshotted from their stable orbits. Then, as they travel through interstellar space, having been evicted from their orbital homes, neighboring stars traveling in the same direction may pull the nomads into new orbits.

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But the newly snatched nomad worlds wouldn't have compact orbits around their new stellar parent — it is more likely that they would orbit stars hundreds or thousands of times the distance Earth orbits the sun. Also, as they wouldn't form from the star's protoplanetary disk of gas and dust, they would likely have extremely eccentric and tilted orbits. They could even orbit their stars the "wrong" way (i.e., systems of planets that evolve from a common protoplanetary disk will always naturally orbit in the same direction).

Although there are some known exoplanets with crazy orbits surrounding stars in our galaxy, there are no clear-cut examples that suggest these worlds were snatched from interstellar space. There is, however, a binary planet (i.e., two planets that orbit one another) that was discovered in 2006, suggesting the two orphaned worlds captured each other via this mechanism. "The rogue double-planet system is the closest thing we have to a 'smoking gun' right now," said Perets. "To get more proof, we'll have to build up statistics by studying a lot of planetary systems."

Image credit: Christine Pulliam (CfA)