An international team of astronomers has discovered a young planet that is not orbiting a star. This free-floating planet, dubbed PSO J318.5-22, is just 80 light-years away from Earth and has a mass only six times that of Jupiter. The planet formed a mere 12 million years ago—a newborn in planet lifetimes. The discovery will be published in the Astrophysical Journal Letters.
the team concluded that PSO J318.5-22 belongs to a collection of young stars called the Beta Pictoris.
It was identified from its faint and unique heat signature by the Pan-STARRS 1 (PS1) wide-field survey telescope on Haleakalā, Maui. Follow-up observations using other telescopes in Hawaiʻi show that it has properties similar to those of gas-giant planets found orbiting around young stars. And yet PSO J318.5-22 is all by itself, without a host star.
“We have never before seen an object free-floating in space that that looks like this. It has all the characteristics of young planets found around other stars, but it is drifting out there all alone,” said team leader Michael Liu, a University of Hawaiʻi at Manoa’s Institute for Astronomy associate astronomer. “I had often wondered if such solitary objects exist, and now we know they do.”
PSO J318.5-22 was discovered during a search for the failed stars known as brown dwarfs. Due to their relatively cool temperatures, brown dwarfs are very faint and have very red colors. To circumvent these difficulties, Liu and his colleagues have been mining the data from the PS1 telescope. PS1 is scanning the sky every night with a camera sensitive enough to detect the faint heat signatures of brown dwarfs. PSO J318.5-22 stood out as an oddball, redder than even the reddest known brown dwarfs.
“We often describe looking for rare celestial objects as akin to searching for a needle in a haystack. So we decided to search the biggest haystack that exists in astronomy, the dataset from PS1,” said Eugene Magnier, an associate astronomer at the Institute for Astronomy and a co-author of the study.
During the past decade, extrasolar planets have been discovered at an incredible pace, with about a thousand found by indirect methods such as wobbling or dimming of their host stars induced by the planet. However, only a handful of planets have been directly imaged, all of which are around young stars (less than 200 million years old). PSO J318.5-22 is one of the lowest-mass free-floating objects known, perhaps the very lowest. But its most unique aspect is its similar mass, color and energy output to directly imaged planets.
“Planets found by direct imaging are incredibly hard to study, since they are right next to their much brighter host stars. PSO J318.5-22 is not orbiting a star so it will be much easier for us to study,” said Niall Deacon of the Max Planck Institute for Astronomy in Germany and a co-author of the study. “It is going to provide a wonderful view into the inner workings of gas-giant planets like Jupiter shortly after their birth.”