Nomad Planets Roam Our Galaxy

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The Milky Way may be teeming with more than 100,000 free-flying planets for every star — these are worlds that unlike our orderly solar system are not orbiting parent stars.

This is the finding from a study that extrapolates from observations of a dozen so-called “nomad” planets, which were detected when their gravity briefly contorted light of passing stars — a phenomenon known as gravitational microlensing.

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To derive the estimate, astrophysicist Louis Strigari, with Stanford University’s Kavli Institute, and colleagues factored in the gravitational pull of the Milky Way, how much material it contains and how that material might be divided up among bodies ranging from Jupiter-sized objects down to tiny worlds like Pluto.

Among the study’s most interesting conclusions is that that there are not enough solar systems in the galaxy to account for all the nomad planets, which means that not all the free-flying worlds are orphan planets ejected from their birthplace.

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The study raises new questions about how planets form and whether there are different processes for making planet-sized bodies. It also adds another twist to the discussion of habitable worlds beyond Earth.

“If any of these nomad planets are big enough to have a thick atmosphere, they could have trapped enough heat for bacterial life to exist,” said Strigari.

A headcount of the objects will be among the tasks for the next-generation of large telescopes, including NASA’s Wide-Field Infrared Survey Telescope (WFIRST) and the ground-based Large Synoptic Survey Telescope, both of which are scheduled to begin operating in the early 2020s.

Strigari’s research has been submitted to the Monthly Notices of the Royal Astronomical Society.

Image: Artist’s rendering of a free-flying planet. Credit: Greg Stewart/SLAC National Accelerator Laboratory.