Located nearly 500 light-years away, ROXs 42Bb is a newly discovered object that astronomers are struggling to define — a unique celestial body that's challenging conventional notions about how planets and stars form.

Using observational data from telescopes at Mauna Kea, Hawaii and the W.M. Keck Observatory, an international group of astronomers detected the object in a distant orbit around a very young star, ROXs 42B. The planet, or brown dwarf, or failed star — or whatever the heck it is — is located 30 times further away from its host star than Jupiter is from the sun. That's significant — about 156 AU or 14.5 billion miles. By comparison, Pluto is 32.4 AU or 3 billion miles from the Sun.

Image: Thayne Currie et. al.


ROXs 42Bb is also huge — but not so huge that it can be classified as a conventional brown dwarf. It's approximately nine times the mass of Jupiter, below the limit most astronomers use to distinguish planets from brown dwarfs — objects too large to be considered planets, but too small to spark the internal nuclear reactions required to become full-blown stars. This is why some scientists refer to them as "failed stars." Typically, brown dwarfs range in size from about 13 to 80 Jupiter masses. But at 9 MJ, ROXs 42Bb is an anomaly.

This news is reminiscent of last year's discovery of Kappa Andromedae b — a super-Jupiter that, at 13 MJ, also skirts the line between planet and star. Relatedly, astronomers have recently had to re-classify some super-Earths as mini-Neptunes.


But what's more perplexing to the astronomers is how it formed given its extreme distance to it's star. Gas giants typically form via core accretion in which planets form from a solid core that then develops a massive gaseous envelope. This process is most efficient when it happens closer to the host star on account of the timescales involved to first form the core.

To explain ROXs 42Bb, the astronomers have deferred to disk instability theory — a process that works best farther away from the parent star whereby a fragment of a disk gas surrounding a young star directly collapses under its own gravity into a planet.

In between the two populations of objects — those that form via core accretion and those that form via disk instability — there's 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," noted lead author Thayne Currie.

It's very likely that the astronomers will have to settle on a new classification, something like a 'planet mass' brown dwarf.

Read the entire study at The Astrophysical Journal: "Direct Imaging and Spectroscopy of a Candidate Companion Below/Near the Deuterium-burning Limit in the Young Binary Star System, ROXs 42B."

Top image: Artistic impression of a brown dwarf by Jon Lomberg