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Planets form from a disk of gas and dust orbiting a star and so should not exist beyond the disk’s edge. In recent years, however, astronomers have reported giant planets more than 100 sun-Earth distances from their stars—much farther out than Pluto, whose mean distance from the sun is 39.5 times greater than Earth’s.
Now astronomers Hagai Perets of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and Thijs Kouwenhoven of the Kavli Institute for Astronomy and Astrophysics at Peking University in China say the far-out planets once roamed space free of any star, but they came in from the cold when their newfound suns captured them. Such free-floating planets arise when other planets kick them out of their homes; astronomers can detect them because their gravity magnifies the light of more distant stars. These observations suggest free-floating planets are roughly as abundant as stars.