If you think being stuck in a strange town late at night after the last bus has gone is lonely, then give a thought for the exoplanet PSO J318.5-22. Discovered this year by astronomers at the University of Hawaii, this planet was found floating through interstellar space without a parent star and is one of the smallest free-floating objects seen outside of the Solar System.

PSO J318.5-22 isn't that far away, cosmically speaking. At a distance of 80 light years in the constellation of Pictor, it’s six times the size of the planet Jupiter and has a temperature of about 1,160° K (1.630° F, 887° C). From this, astronomers have deduced that it’s a planet rather than a protostar because it's too small for fusion to occur.

"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," says team leader Dr. Michael Liu of the Institute for Astronomy at the University of Hawaii at Manoa. "I had often wondered if such solitary objects exist, and now we know they do."

Unlike other exoplanets, PSO J318.5-22 was seen directly. That’s because the light reflected from most exoplanets is overwhelmed by their much brighter parent stars, so their presence is usually discovered by measuring dips in the brightness of the star as the planet passes in front of it. PSO J318.5-22, on the other hand, is floating along in interstellar space. Like the planet Jupiter, it generates its own heat, so its infrared signature is visible to the right telescopes on Earth.

In the case of PSO J318.5-22, the astronomers came across it while they were hunting for what are called brown dwarfs. These are a class of “sort of” stars that are too small to ignite the kind of fusion reaction that powers the Sun and other stars. Instead, the best they can hope to manage is a much lower level kind of nuclear fusion. If they’re too small, with a mass below 13 times that of Jupiter, then they can’t manage fusion at all and become giant planets rather than small stars.

PSO J318.5-22 was discovered by sifting through data from the University of Hawaii’s Pan-STARRS 1 (PS1) infrared telescope on Haleakala on the Hawaiian island of Maui, which is carrying out a sky survey for brown dwarfs and has amassed about four petabytes of data so far, so it wasn't an insignificant task.

"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," says Dr. Eugene Magnier of the Institute for Astronomy at the University of Hawaii at Manoa

PSO J318.5-22 showed up as a very red object – too red even for a brown dwarf. Other telescopes were brought in to confirm the discovery, including multiple scopes on the island of Hawaii, the NASA Infrared Telescope Facility, the Canada-France-Hawaii Telescope, and the Gemini North Telescope. The spectrographs collected not only proved the existence of the planet, but many of its characteristics. From the spectra, the astronomers could calculate its temperature, its brightness and other factors that allowed them to calculate that it’s 80 light years from Earth in the vicinity of the star Beta Pictoris, which is an area filled with young stars. This allowed them to deduce its 12 million-year age.

The discovery of PSO J318.5-22 is of interest because this is one of the few exoplanets that can be seen directly. "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.

Another reason astronomers are excited is because PSO J318.5-22 is one of the lowest-mass free-floating objects found, if not the lowest.

The results of the discovery of PSO J318.5-22 were published by Astrophysical Journal Letters.

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