Astronomers using the Keck and Subaru telescopes on Mauna Kea have found a distant object that occupies a mysterious realm between planets and failed stars known as brown dwarfs.
Called ROXs 42Bb, the object, 440 light-years from Earth and with nine times the mass of Jupiter, has led astrophysicists to reassess their understanding of what is and is not a planet. It orbits 30 times farther from its host star than the distance between Jupiter and the sun.
“This situation is a little bit different than deciding if Pluto is a planet,” said Thayne Currie, a post-doctoral fellow at the University of Toronto and principal author of a report on the discovery published last week in Astrophysical Journal Letters. “For Pluto, it is whether an object of such low mass amongst a group of similar objects is a planet. Here, it is whether an object so massive yet so far from its host star is a planet. If so, how did it form?”
Most astronomers believe that gas giant planets like Jupiter and Saturn formed by core accretion, when a solid core attracts a massive gaseous envelope. That process works best closer to the parent star due to the time required to form the core.
Another theory is that gas around a young star collapses under its own gravity into a planet. This mechanism would work best farther away from the parent star.
“It’s very hard to understand how this object formed like Jupiter did,” Currie said in a statement. “However, it’s also too low mass to be a typical brown dwarf … It may represent a new class of planets or it may just be a very rare, very low-mass brown dwarf.”