Rogue Rocky Planet Found Adrift in the Milky Way

Not all planets orbit stars. Some are instead “free-floating” rogues adrift in interstellar space after being ejected from their home systems. For decades astronomers have sought to study such elusive outcasts, hoping to find patterns in their size and number that could reveal otherwise hidden details of how planetary systems emerge and evolve.

Of the handful known so far, most free floaters have been massive gas giants, but now researchers may have found one small enough to be rocky—smaller even than Earth. If its rogue status is confirmed, the roughly Mars-to-Earth-mass object would be the most diminutive free-floating planet ever seen. Yet finding such small worlds could soon become routine, thanks to NASA’s upcoming Nancy Grace Roman Space Telescope, set to launch in the mid-2020s.

Most planet-hunting methods rely on observing subtle changes in a star’s light to discern any orbiting companions. But free-floating worlds, of course, have no star. Instead astronomers use a quirk of Einstein’s general theory of relativity to locate these lost planets: All massive objects warp spacetime around themselves, similar to how a bowling ball stretches a rubber sheet, and can act as lenses to magnify far-distant sources. When a “lensing” foreground planet is properly aligned with a background star, it amplifies that star’s light, causing a slight brightening. This technique is known as microlensing, and astronomers first pioneered it to find black holes.

Of the approximately 100 worlds found to date by microlensing, only four have been identified as free-floating. All the rest are planets that spin around their stars on orbits that are stretched out so long that they typically elude detection through other standard planet-hunting techniques. It is possible that the newfound wee world, known as OGLE-2016-BLG-1928, could be attached to a star. But if so, its orbit would place it at least eight times as far from its stellar host as the Earth is from the sun. Confirming the planet’s likely free-floating status will require a few more years—time enough for any potential parent star, should it exist, to shift its position so that its light can be separated from that of the background star.

“It’s really a very exciting result,” says Andrew Gould, an astronomer at the Ohio State University and an author of the preprint paper describing the result. That study, which was led by Przemek Mróz of the California Institute of Technology, has been submitted to Astrophysical Journal Letters, where it is currently under review. “It’s a huge milestone to get this planet,” Gould adds.

“This is a very robust result and almost certainly a low-mass planet,” says astronomer Scott Gaudi of Ohio State, who is leading the science team working to determine the best observing strategy for NASA’s Roman telescope and was not part of the group that found the new world. “This gives us the first little peek at the likely distribution of a population of Earth-mass planets in the galaxy,” he says.

At the “Hairy Edge”

Most planets form from the gas and dust left over