An upcoming NASA mission could find that there are more rogue planets – planets that float in space without orbiting a sun – than there are stars in the Milky Way, a new study theorizes.
“This gives us a window into these worlds that we would otherwise not have,” said Samson Johnson, an astronomy graduate student at The Ohio State University and lead author of the study. “Imagine our little rocky planet just floating freely in space – that’s what this mission will help us find.”
The study was published in The Astronomical Journal (“Predictions of the Nancy Grace Roman Space Telescope Galactic Exoplanet Survey. II. Free-floating Planet Detection Rates”).
The study calculated that NASA’s upcoming Nancy Grace Roman Space Telescope could find hundreds of rogue planets in the Milky Way. Identifying those planets, Johnson said, will help scientists infer the total number of rogue planets in our galaxy. Rogue, or free-floating, planets are isolated objects that have masses similar to that of planets. The origin of such objects is unknown, but one possibility is they were previously bound to a host star.
“The universe could be teeming with rogue planets and we wouldn’t even know it,” said Scott Gaudi, a professor of astronomy and distinguished university scholar at Ohio State and a co-author of the paper. “We would never find out without undertaking a thorough, space-based microlensing survey like Roman is going to do.”
The Roman telescope, named for NASA’s first chief astronomer who was also known as the “mother” of the Hubble telescope, will attempt to build the first census of rogue planets, which could, Johnson said, help scientists understand how those planets form. Roman will also have other objectives, including searching for planets that do orbit stars in our galaxy.
That process is not well-understood, though astronomers know that it is messy. Rogue planets could form in the gaseous disks around young stars, similar to those planets still bound to their host stars. After formation, they could later be ejected through interactions with other planets in the system, or even fly-by events by other stars.
Or they could form when dust and gas swirl together, similar to the way stars form.

Image Credit:  NASA

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