"We have never before seen an object free-floating in space 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 research team leader 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."
The researchers, whose study was published in the Astrophysical Journal Letters, identified the planet from its faint and unique heat signature using the Pan-STARRS 1 wide-field survey telescope on the Haleakala volcano of Hawaii's Maui island.
They suggested the newly found planet may have the lowest mass of all known freely floating objects.Other telescopes in Hawaii showed that the planet has similar properties to those of gas giants orbiting around young stars, but PSO J318.5-22 lacks a host star.
During the past decade, researchers have found about a thousand extrasolar planets using indirect methods, including planet-induced wobbling or dimming of their host stars.
But only a handful of these planets have been observed directly since most are orbiting around young stars less than 200 million years old and thus very bright.
PSO J318.5-22 "is going to provide a wonderful view into the inner workings of gas-giant planets like Jupiter shortly after their birth," said co-author Niall Deacon of the Max Planck Institute for Astronomy in Germany.