Astronomers using the Pan-STARRS 1 wide-field survey telescope on Haleakala, Maui, have discovered a very young free-floating planet named PSO J318.5-22.

Located approximately 80 light-years away in the constellation Capricornus, PSO J318.5-22 has a mass only six times that of Jupiter. It belongs to a collection of young stars called the Beta Pictoris moving group that formed about 12 million years ago.

In fact, the eponymous star of the group, Beta Pictoris, has a young gas-giant planet in orbit around it. PSO J318.5-22 is even lower in mass than the Beta Pictoris planet and probably formed in a different fashion.

The exoplanet 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.

PSO J318.5-22 is extremely cold and faint, about 100 billion times fainter in optical light than Venus. Most of its energy is emitted at infrared wavelengths.

“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. It is going to provide a wonderful view into the inner workings of gas-giant planets like Jupiter shortly after their birth,” said Dr Niall Deacon of the Max Planck Institute for Astronomy in Germany, co-author of the paper accepted for publication in the Astrophysical Journal Letters (arXiv.org).

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, the astronomers have been mining the data from the Pan-STARRS 1 (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 co-author Dr Eugene Magnier of the Institute for Astronomy at the University of Hawaii at Manoa.

The scientists followed up the PS1 discovery with multiple telescopes on the summit of Mauna Kea on the island of Hawaii. Infrared spectra taken with the NASA Infrared Telescope Facility and the Gemini North Telescope showed that PSO J318.5-22 was not a brown dwarf, based on signatures in its infrared light that are best explained by it being young and low-mass.

“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. I had often wondered if such solitary objects exist, and now we know they do,” said lead author Dr Michael Liu, also from the Institute for Astronomy at the University of Hawaii at Manoa.

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Bibliographic information: Michael C. Liu et al. 2013. The Extremely Red, Young L Dwarf PSO J318-22: A Free-Floating Planetary-Mass Analog to Directly Imaged Young Gas-Giant Planets. ApJ Letters, accepted for publication; arXiv: 1310.0457