Our Galaxy Could Have 50 Billion Rogue Planets

Scientists have known about free-floating planets that aren't attached to any star, but a new study tells us just how many there are.

Free-floating planet, illustration
MARK GARLICK/SCIENCE PHOTO LIBRARYGetty Images

Our galaxy has planets aplenty. Our Sun possesses at least eight of them, and in recent years we’ve learned that most other stars have some, too.

But according to , a staggering number of planets aren’t orbiting any star at all. Instead, there could be 50 billion rogue planets are adrift in the Milky Way.

Rogue planets have been known to science for a while. Astronomers for centuries have suspected that rogue planets exist, and in recent years we’ve even found a few of them. But as a class, rogue planets are still somewhat of a mystery.

How do rogue planets form? How do they become untethered from their host stars? What does life as a rogue planet look like? These questions are tough to answer because rogue planets are necessarily tough to find, let alone study—they're far away from any light source and could be located anywhere. Even with our most powerful telescopes, the biggest rogue planets are nothing more than the faintest of dots.

To get past these hurdles, a group of astronomers at the University of Leiden of 1,500 stars in a real place called the Orion Trapezium star cluster. About 500 of these simulated stars contained between four and six planets, giving the sim a grand total of 2,522 planets. When the scientists ran the simulation forward, they found that gravitational effects from the closely packed stars kicked about 350 of those planets outside their respective star systems.

If that’s the case, and you extrapolate that result across the Milky Way, then there could be billions of rogue planets careening throughout the galaxy undetected. Our galaxy has about 200 billion stars, and most of them were born in a cluster similar to the Orion Trapezium. Even our own Sun was part of such a cluster once, although our stellar siblings have long since drifted apart.

If all stellar clusters produce rogue planets at the same rate as the simulated Orion Trapezium cluster, the University of Leiden astronomers estimate, then our galaxy could have as many as 50 billion rogue planets. One or two of them may have even come from our Sun, although at this point there’s not much way to tell.

With this many rogue planets peppering our galaxy, astronomers should have an easier time finding candidates to study. Perhaps we’ll learn a great deal about rogue planets in the near future, once our next-generation telescopes start coming online.

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