How to Find a Rogue Planet


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Approximately 80 light years from Earth, a planet wanders alone. Without a star to tether it to a single orbit, the newly discovered PSO J318.5-22 drifts aimlessly across the sky at a sluggish 20 kilometers per second. This rogue planet has six times the mass of Jupiter and is the nearest free-floating planetary-mass object ever seen. The international team of astronomers that discovered PSO J318.5-22 has been tracking it for two years; their study is published this week in the journal Astrophysical Journal Letters.

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So how does a planet end up all alone? And how is it possible to find a dark world in the middle of nowhere?

Finding a Star-less Planet

The fact that PSO J318.5-22 doesn't orbit a star means there is little light to obscure it from searching telescopes. But it also means there's little light to illuminate the planet.

The team that found this rogue planet started by looking for brown dwarfs, or failed stars. Using the Pan-STARRS wide-field telescope atop the Haleakala volcano on Maui, they surveyed huge swaths of space and mined a database of billions of objects.

"Pan-STARRS is surveying the sky using a CCD digital camera with 1.4 billion pixels, and it takes images using filters that select certain colors of light," says Katelyn Allers, an astronomy professor at Bucknell University and coauthor of the paper. "We searched the incredibly large Pan-STARRS database to find objects that seemed like they would have cool, planet-like temperatures, which is indicated by red colors."

PSO J318.5-22 showed up in those infrared readings as faint and red, but much redder than any brown dwarfs on record. Allers and the rest of the team knew they'd spotted a good candidate for a free-floating planet.

"The thing that's really interesting about our new discovery is that it looks just like the young planets that have been directly imaged around other stars," says team leader Michael Liu of the Institute for Astronomy at the University of Hawaii.

Once the astronomers caught a glimpse of PSO J318.5-22's infrared signature using Pan-STARRS, the team switched to smaller telescopes to image the planet in more detail. "Once we were able to get a spectrum, we could then estimate the temperature and age and very precisely measure the distance," Liu tells PopMech.

How It Got There

The researchers estimate that PSO J318.5-22 is 12 million years old. But they don't know how it wound up free-floating—though there are two possibilities that astronomers think are plausible.

The first idea is that PSO J318.5-22 formed the same way the Earth and the planets of our solar system did: in the circumstellar disk of a star. Then, Allers says, somehow the planet got separated from its parent star and shot off into space. According to a study in Monthly Notices of the Royal Astronomical Society, this could happen when a passing star exerts its gravitational pull and yanks the planet out of orbit. It's also possible that the parent star, upon ending its hydrogen-burning life and expanding into a red giant, could push a planet out of its system.

The second possibility is that the planet has been a loner from the beginning. It might have formed in isolation from a cloud of collapsed matter, one that did not have enough mass to ever reach the stage of fusing hydrogen, at which point it could have become a star.

"Some recent work of Viki Joergens [of the Max-Planck Institute for Astronomy in Heidelberg, Germany] has shown evidence that some planetary-mass objects form like stars, and accrete material from their own circumstellar disks," Allers says. "But we'll need to obtain additional data to try and distinguish between these two formation mechanisms for PSO J318.5-22."

How Many Rogue Planets Are There?

Similar free-floating planetary-mass objects have been discovered in the past, but while their infrared signatures are typically associated with brown dwarfs, PSO J318.5-22 most closely resembles a planet. Brown dwarves, like stars, form from collapsing clouds of gas and dust, but do not coalesce into dense enough objects to trigger nuclear fusion. They are too big to be planets, yet too small to be proper stars. Brown dwarves are thought to be as prevalent as stars, and much more abundant than free-floating planets.

Still, there are probably plenty of rogue planets. Thousands have been discovered over the past decade, but the majority of these have been too far away to capture by direct imagingio9 reports that there could be billions of free-roaming planets formed by incredibly dense, fast-moving globulettes in the Rosette Nebula, which is 4600 light years from Earth. These small clouds of matter could have collapsed upon themselves, forming planets that have broken away from the nebula.

Liu says that while the researchers are unsure of the exact number of potential free-floating planets, now that they've found PSO J318.5-22, they can use it's infrared signature as a template to find other candidates among the catalogues amassed by Pan-STARRS. And the chances are good there are more rogue planets closer to home. Judging from information gathered from star-forming regions, Allers says, it appears that there is about one planetary-mass object for every 10 sun-like stars.

"This very roughly corresponds to about a dozen free-floating planets closer than 100 light years to us," she says. "We've definitely got more work to do to find all of them."

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