The free-floating planet is just 80 light-years from Earth and about six times the size of Jupiter.
It is young - about 12 million years old. And unlike any other planet that has ever been discovered, it is not in orbit around a host star.
"This thing is floating in space like our sun floats in space," said Eugene Magnier of the University of Hawaii at Manoa, coauthor of a study about the lonely planet. "It is drifting around through the galaxy."
Astronomers are not yet sure how this rogue planet came to be out there in space, all by itself. One theory is that it formed from a clump of hydrogen gas that condensed. Another, less likely thought is that it started its life in the vicinity of a star and got bumped out of its orbit.
The planet was discovered with the Pan-STARRS 1 telescope in Hawaii, which is especially adept at finding dim objects in the night sky. Magnier and the team's leader, Dr. Michael Liu, who also works at the University of Hawaii at Manoa, were scanning the sky for failed stars known as brown dwarfs. These are substellar objects that never got big enough to sustain hydrogen-1 fusion reactions in their cores.
Magnier said their newly discovered planet might also be considered a brown dwarf. "We still don't have a good idea about where to draw the line between a planet and brown dwarf," he said.
He explained that the size of this object, which is extremely small in the scheme of brown dwarfs, means that it has been considered its own planet.
According to a release from the Institute for Astronomy at the University of Hawaii, the planet is one of the lowest-mass free-floating objects known.
"I had often wondered if such solitary objects exist," said Liu in a statement. "Now we know they do."
Magnier said the team would continue to study the lonesome planet to see whether it has any companion objects in its orbit.
"Most objects in the universe don't form all by themselves. They are in binary or multiple systems," he said. "It is reasonably likely that it has a companion."
Which means this lonely planet may have a friend after all.
A paper detailing the planet's discovery was published in the Astrophysical Journal Letters.
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