New Celestial Object Challenges Definitions of Planets and Stars

A celestial object may challenge traditional understandings about how planets and stars form. Astrophysicists have discovered an object that defies traditional categories of what is and is not a planet, showing that defining something isn't as easy as you might think.

The object is located near a very young star about 440 light-years away from our own sun. It's likely orbiting this young star, named ROX 42B. With about nine times the mass of Jupiter, the object possesses mass that's below the limit most astronomers use to separate planets from brown dwarfs. Yet at the same time, it's located 30 times further away from the star than Jupiter is from the sun.

"We have very detailed measurements of this object spanning seven years, even a spectrum revealing its gravity, temperature and molecular composition," said Thayne Currie, one of the researchers, in a news release. "Still, we can't yet determine whether it is a planet or a failed star--what we call a 'brown dwarf.' Depending on what measurement you consider, the answer could be either."

Most astronomers believe that gas giant planets like Jupiter and Saturn formed by core accretion. This method involves the planet forming a solid core that then accretes a massive gaseous envelope. Core accretion operates most efficiently when the planet is closer to the parent star due to the length of time it takes to first form the core.

A second theory for forming gas giants is disk instability. A fragment of a disk gas surrounding a young star directly collapses under its own gravity into a planet. This particular mechanism works best further away from the star, so it's possible that this could be what created the object-assuming it is indeed a planet.

"This situation is a little bit different than deciding if Pluto is a planet," said Currie in a news release. "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?"

This new object has blurred the distinction between planets and brown dwarfs, but this isn't necessarily a bad thing. It's begun to fill the gap between the two.

"It's very hard to understand how this object formed like Jupiter did," said Currie in a news release. "However, it's also too low mass to be a typical brown dwarf; disk instability might just work at its distance from the star. It may represent a new class of planets or it may just be a very rare, very low-mass brown dwarf formed like other stars and brown dwarfs: a 'planet mass' brown dwarf. Regardless, it should spur new research in planet and star formation theories, and serve as a crucial reference point with which to understand the properties of young planets at similar temperatures, masses and ages."

The findings are published in the Astrophysical Journal Letters.