Dubbed PSO J318.5-22, the planet without a host star is 80 light-years away from Earth and has a mass only six times that of Jupiter. It formed a mere 12 million years ago, researchers said -- a newborn in planet lifetimes.
It was identified from its faint, unique heat signature by telescopes in Hawaii, and found to have properties similar to those of gas-giant planets found orbiting around young stars.
"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," research team leader Michael Liu of the Institute for Astronomy at the University of Hawaii said. "I had often wondered if such solitary objects exist, and now we know they do."
One of the lowest-mass free-floating objects discovered to date, the planet's most distinguishing aspects is its similar mass, color and energy output to directly imaged planets orbiting young stars.
"Planets found by direct imaging are incredibly hard to study, since they are right next to their much brighter host stars," study co-author Niall Deacon of the Max Planck Institute for Astronomy in Germany said. "PSO J318.5-22 is not orbiting a star so it will be much easier for us to study."