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takyon writes:

Astronomers have spotted a relatively small gas giant that may have formed in a protoplanetary disk around a brown dwarf:

A team of researchers from two collaborations, KMTNet and the OGLE, or Optical Gravitational Lensing Experiment, announced their new planet-brown dwarf pair, calling it the "OGLE-2017-BLG-1522 microlensing event." Brown dwarfs are either the lightest, dimmest stars or the heaviest, hottest planets. They have been spotted with companions before, but this would be largest mass difference between a brown dwarf and its companion yet, with the brown dwarf being perhaps over 60 times the size of the orbiting planet.

"OGLE-2017-BLG-1522Lb could be the first giant planet orbiting around a brown-dwarf host having a planetary mass ratio," the authors write in the paper published on the arXiv preprint server.

That means that the planet could have formed from a ring of dust around the brown dwarf, rather than the two objects forming together as a sort of binary. "That's the fundamental question," Jennifer Yee, astrophysicist at the Harvard-Smithsonian Center for Astrophysics, told Gizmodo. "That's why it's important. We don't know how you would form these things," she said, referring to planets orbiting brown dwarfs.

The presumed brown dwarf, or host object, has an estimated mass of 46 Jupiter masses with a range of 21 to 125 Jupiter masses. The authors estimate that the probability of the object being a brown dwarf is 76%. The planetary-sized companion has an estimated mass of 0.75 Jupiter masses with a range of 0.35 to 2.01 Jupiter masses.

From the abstract (arXiv:1803.05095):

Planet formation scenarios combined with the small companion-host mass ratio q ∼ 0.016 and separation suggest that the companion could be the first discovery of a giant planet that formed in a protoplanetary disk around a brown dwarf host.

Original Submission