How Many Unbound Planets Roam the Milky Way?

Life as we know it exists on a cozy planet in a stable orbit around a sun shining brightly in its sky. But a new study hints that the most common life in the universe might exist deep inside eternal-night worlds far from any star, adrift in the icy dark of interstellar space.

The Milky Way likely hosts billions, and possibly trillions, of unbound planets, some of which may have atmospheres thick enough to support bacterial life. Loose planets may even outnumber stars in the galaxy, but a more precise count awaits future telescopes such as WFIRST and LSST.

Caltech / NASA

Researchers at the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) at Stanford University estimate that "nomad" planets, ejected from their home stellar system and now free-floating through the Milky Way, could outnumber stars by as many as 100,000 to 1. Earlier estimates were more like a handful to 1, though previous studies have only counted unbound planets more massive than Jupiter.

To estimate the number of unbound planets as small as Pluto that could be roaming the galaxy, Louis Strigari (KIPAC), lead author of the study, began with a basic rule of nature: where a few big objects are found, there are many more small, just like a few boulders may be surrounded by thousands of pebbles. Strigari and colleagues calculated the number of unbound planets by extrapolating from the small number detected so far by direct imaging and by gravitational microlensing.

Direct imaging has severe limits because planets are so faint. Microlensing offers more promise. It looks for the characteristic brightening and fading of a background star when an object, even one as wimpy as Pluto, passes nearly in front of it and bends its light slightly by gravity. So far, 24 planet-mass objects have been detected by microlensing — 14 bound to their parent stars, 10 apparently not. Microlensing offers hope for detection of loose objects large and small even if they are completely dark, and even at great distances across the galaxy.

Media outlets have jumped on the vast number of loose planets predicted by this study. But the authors themselves acknowledge the large uncertainty in their result. The statistics remain so weak that the group’s lower limit is still one loose planet for each star in the Milky Way.

"There is no evidence for a large population of unbound planets, but then again there is nothing to rule it out," says Scott Gaudi (The Ohio State University), expert on gravitational microlensing and extrasolar planets, who was not involved with the study.

Exoplanet specialists think that some 80% of planetary systems go through an early period of gravitational chaos that flings some of their worlds into interstellar space. The survivors are left in a mess: in highly elongated orbits that are often not even in the same plane, as exoplanet hunters are discovering. Our solar system was one of the minority that avoided such a violent episode.

Still, skeptics have a hard time imagining how tens of thousands of objects larger than Pluto could originate for each star — regardless of whether they stay with it or get flung off. Our solar system has only 17 known objects that qualify — eight planets, seven large moons, and the two largest Kuiper Belt objects.

Better statistics for unbound planets may come from the Wide-Field Infrared Survey Telescope (WFIRST) and the Large Synoptic Survey Telescope (LSST), two gigantic sky-survey projects that may begin within the next decade or so.

"It's not implausible that a few nomads, most not much different than the size of Pluto, might lie within one light-year of the sun," says Strigari.

If more planets wander interstellar space than orbit stars, what might that mean for life in the universe? As it turns out, not all who wander are lost. Even with no sun in the sky, life could still eke out an existence under a thick atmosphere or ice crust at the depth where the temperature is pleasant, maintained for billions of years by the heat flowing from the planet's interior. Microbial life, at least, might thrive under these conditions.

All of this raises a question: If a planet leaves the system in which it was born, is it still a planet? The literature seems to give a qualified "yes," using terms like free-floating or rogue planets, though Strigari and colleagues prefer the term "nomad planet."

See their paper, which will be published in the Monthly Notices of the Royal Astronomical Society.

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Monica Young has joined Sky & Telescope as an editorial intern. She is a recently minted astronomy PhD (from Boston University) with an extensive background in X-ray astronomy, education, and science writing.

Posted by Monica Young, February 29, 2012