Illustration courtesy Caltech/NASA

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A rogue planet is seen in an artist's conception.

Illustration courtesy Caltech/NASA

"Nomad" Planets More Common Than Thought, May Orbit Black Holes

Between 3 and 6 percent of stars host rogue planets, simulations show.

Stars and even black holes likely harbor "rogue," or "nomad," planets that were kicked out of the star systems where they were born, new simulations suggest. At the same time, a separate study suggests nomad planets are much more common than thought.

Astronomers used to shrug off notions of rogue planets, also called free-floating planets. Yet in the past few years, indirect observations of these galactic wanderers—combined with detailed supercomputer simulations—have suggested they do exist.

A study released Thursday, for instance, hints rogue planets might outnumber the Milky Way's 200 to 400 billion stars by a mind-boggling 100,000 to 1. Scientists previously thought there were only about two drifter planets for every star.

But what was still poorly understood was whether stars, black holes, and even other planets can capture such free-floating planets.

To find out, two astrophysicists simulated the evolution of several sizes and densities of star clusters. Eventually such clusters would dissipate as a galaxy's gravity pulled them apart.

The simulations suggested that between 3 and 6 percent of stars host rogue planets—a much higher number than thought.

Tracking Nomad Planets

Rogue planets can get kicked out of their star systems in a variety of ways. Those include interactions with other planets, a star that drifts too close, or a larger star that goes supernova and collapses into a black hole. (See black hole pictures.)

So far there's no way to know for sure whether a given planet came from outside of its current star system.

But astrophysicists Hagai Perets of Harvard University and M.B.N. Kouwenhoven of China's Peking University suspect that any captured planets would have extremely large, distant, and elliptical orbits.

These rogue planets might gently sink into orbit around an object at distances perhaps ten thousand times greater than Earth is from our sun.

"The evidence for free-floating planets tells us these planets don't exist close to any star," Perets said.

Planets Are Black Hole Captives?

In the simulation, the more massive and dense a star, or former star, was, the better it captured wandering planets—making black holes almost twice as good as other celestial objects in capturing planets. A higher density makes a star or black hole's gravity well deeper, making it harder for drifting planets to escape its clutches.

About half of black holes weighing between 5 and 15 times the sun's mass likely harbor captured stars, and between 5 and 10 percent of the black holes have probably captured a wandering planet, the simulations showed.

Eric Ford, a University of Florida astrophysicist who was not involved in the study, said the results are compelling.

But he'd like to see the simulations account for the nuanced timing involved in the evolution of planets, stars, and star clusters.

"The art form here is what size clusters do most stars form in, and how are the clusters structured? There's a crucial element of timing here," Ford said.

He noted that planets form during specific periods of a star's life cycle, which in turn fits into the life cycle of a star cluster. If a cluster spreads out too quickly, the chance for planetary capture probably plummets, thanks to the growing distances between stars.

It can take tens or hundreds of millions of years for planets to form, so if stars in a cluster dissipate before a planet can form and be ejected, the planets may not reach high enough speeds to reach other stars systems, Ford said.

Rogue Planet Unlikely to Hit Earth

"Instead of thinking these planets are oddballs, we now think they're common," Ford added. But it's unlikely that our sun harbors a rogue planet, the scientists said.

"There are lot of other things I'd be worried about before a free-floating planet collides with Earth," Ford said.

"A good old-fashioned comet or asteroid wiping us out is something I'd be much more worried about."

Peret and Kouwenhoven's study of nomad-planet capture, which has been submitted to The Astrophysical Journalis available in preprint format at The study finding nomad plants are common has been submitted to the Monthly Notices of the Royal Astronomical Society.