'Rogue' planets could outnumber stars in the Milky Way, study suggests

A newly published study suggests there could be more “rogue” planets, those that do not orbit a star, than there are stars in the Milky Way galaxy.

The research, published in the Astronomical Journal, notes the upcoming launch of the Nancy Grace Roman Space Telescope, slated to happen in 2025, could find billions, perhaps trillions of these objects.

“As our view of the universe has expanded, we’ve realized that our solar system may be unusual,” the study’s lead author, Ohio State University graduate student Samson Johnson, said in a statement.  “Roman will help us learn more about how we fit in the cosmic scheme of things by studying rogue planets.”

The telescope will continue to search narrow patches of sky for “free-floating planetary-mass objects (FFPs) [that] will be detected as isolated microlensing events with timescales shorter than a few days,” the authors wrote in the study’s abstract.

It could find objects with masses as small as Mars to those as large as gas giants with timescales as short as a few hours to “several tens of days,” they added, thanks to the sensitivity of the lens.

“The microlensing signal from a rogue planet only lasts between a few hours and a couple of days and then is gone forever,” study co-author Matthew Penny added. “This makes them difficult to observe from Earth, even with multiple telescopes. Roman is a game-changer for rogue planet searches.”

So far, only a few rogue planets have been found, including two found a couple of years ago, according to a 2018 study.

Little is known about rogue planets, including what they are comprised of. NASA theorizes they could “form in isolation from clouds of gas and dust, similar to how stars grow,” but that’s just a theory.

It will be up to Roman to get a better idea of just how they form and how many there are. The researchers believe the mission could “detect hundreds of rogue planets,” though they estimated the rogue planet count could be 10 times more precise than current estimates, which range from “tens of billions to trillions” in the Milky Way.

“The universe could be teeming with rogue planets and we wouldn’t even know it,” study co-author Scott Gaudi explained. “We would never find out without undertaking a thorough, space-based microlensing survey like Roman is going to do.”

More than 4,000 exoplanets have been discovered by NASA in total, approximately 50 of which were believed to potentially be habitable as of September 2018. They have the right size and the right orbit of their star to support surface water and, at least theoretically, to support life.


Kevin has been a part of the journey ever since Feed Savvy started. As a strong learner and passionate writer, he contributes his editing skills for the website. He also jots down intellectual pieces from science category.

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