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How Many Rogue Planets Are There in the Milky Way? - Sky & Telescope
Ejected from their home stellar system, the wandering planets now free-float through the Milky Way. Earlier estimates were more like a handful to 1, though previous studies have only counted unbound planets more massive than Jupiter. Strigari and colleagues calculated the number of unbound planets by extrapolating from the small number detected so far by direct imaging and by gravitational microlensing. The statistics remain so weak that the group’s lower limit is still one loose planet for each star in the Milky Way. Better statistics for unbound planets may come from the Wide-Field Infrared Survey Telescope (WFIRST) and the Large Synoptic Survey Telescope (LSST), two gigantic sky-survey projects that may begin within the next decade or so.
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