An unusual stellar object named ROXs 42Bb may represent a new kind of planets or it may be a very rare planet-mass brown dwarf, according to a group of astronomers from Canada, Germany, Japan and the United States.

ROXs 42Bb is located near or orbiting the very young star ROXs 42B about 440 light-years away. The star is a member of the ρ Ophiuchus star-forming region located in the constellation Ophiuchus.

“We have very detailed measurements of this object spanning 7 years, even a spectrum revealing its gravity, temperature, and molecular composition,” said Dr Thayne Currie from the University of Toronto, the lead author of the paper published in the Astrophysical Journal Letters (arXiv.org).

“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.”

ROXs 42Bb is about 9 times the mass of Jupiter, below the limit most astronomers use to separate planets from brown dwarfs, which are more massive.

However, it is located 30 times further away from the star than Jupiter is from the Sun.

Dr Currie said: “this situation is a little bit different than deciding if Pluto is a planet. 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 such as Jupiter and Saturn formed by core accretion, whereby the planets form from a solid core that then develops a massive gaseous envelope. Core accretion operates most efficiently closer to the parent star due to the length of time required to first form the core.

An alternate theory proposed for forming gas giant planets is disk instability – a process by which a fragment of a disk gas surrounding a young star directly collapses under its own gravity into a planet. This mechanism works best farther away from the parent star.

Of the dozen or so other young objects with masses of planets observed by astronomers, some have planet-to-star mass ratios less than about 10 times that of Jupiter and are located within about 15 times Jupiter’s separation from the Sun.

Others have much higher mass ratios and/or are located more than 50 times Jupiter’s orbital separation, properties that are similar to much more massive objects widely accepted to not be planets.

The first group would be planets formed by core accretion, and the second group probably formed just like stars and brown dwarfs.

In between these two populations is a big gap separating true planets from other objects.

“The new object starts to blur this distinction between planets and brown dwarfs, and may lie within and begin to fill the gap,” Dr Currie said.

“It’s very hard to understand how this object formed like Jupiter did. 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.”

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Thayne Currie et al. 2014. Direct Imaging and Spectroscopy of a Candidate Companion Below/Near the Deuterium-burning Limit in the Young Binary Star System, ROXs 42B. ApJ 780, L30; doi: 10.1088/2041-8205/780/2/L30