A free-floating planet dubbed PSO J318.5-22, just 80 light-years from Earth, has a mass only six times that of Jupiter.
Back in 1998, the astronomy world was abuzz with the apparent discovery, thanks to the Hubble Space Telescope, of a planet racing away from its parent star and into the vastness of interplanetary space. Later observations showed that this runaway object, designated TMR-1C, was most likely just a background star. Yet, even today, astronomers really aren't sure what TMR-1C is.
What's clearer now is that free-floating planets are probably very, very common in our galaxy. In fact, just last year Stanford researchers proposed that they could outnumber stars by up to 100,000 to 1. This jaw-dropping estimate is a statistical extrapolation based on a handful of unbound planets — just 10, in fact — found using microlensing techniques.
Now an international team led by Michael Liu (Institute for Astronomy, University of Hawaii) has released images of a young planet that's drifting through space on its own. Dubbed PSO J318.5-22, it's about 80 light-years away in the direction of southeastern Capricornus.
The observers found it in 2011 using the Pan-STARRS 1 telescope atop Haleakala on the Hawaiian island of Maui. They weren't looking for rogue planets but rather the exotic stellar wannabes known as L-type dwarfs.
PSO J318.5-22 turned up in that survey, but follow-up observations showed that it's far redder and dimmer than other L-type candidates. In fact, spectroscopic scrutiny conducted this past summer suggests that this lonely object has much in common with young massive planets enveloped in dusty cocoons. The team's findings will appear in Astrophysical Journal Letters.
"We have never before seen an object free-floating in space that that looks like this," Liu notes in an Institute for Astronomy press release. "It has all the characteristics of young planets found around other stars, but it is drifting out there all alone.”
This is not the first planet-mass object to be found drifting through interstellar space. But the other candidates are thought to be sub-brown dwarfs, literally failed stars that couldn't pull together nearly enough mass to trigger fusion in their cores.
Liu and his colleagues insist that the spectrum of PSO J318.5-22 is distinct from those of other L dwarfs, and its calculated temperature, roughly 1160 Kelvins (1630°F), is significantly cooler. They estimate that it has a mass only 6½ times that of Jupiter and must have formed just 12 million years ago. Based on its motion, it probably escaped from one of the members of the Beta Pictoris Moving Group, a clutch of 17 young stellar systems averaging about 115 light-years from Earth.