We Just Found the First Normal Planet to Orbit a Brown Dwarf

We Just Found the First Normal Planet to Orbit a Brown Dwarf

Scientists have announced the first planet found to be orbiting a brown dwarf. It’s a significant discovery and a fascinating window into the evolution of a type of substellar object we don’t know much about.

A brown dwarf — sometimes referred to as a failed star — is an object of near-Jupiter size, but substantially greater mass. Unlike a normal star, it is not massive enough to fuse hydrogen and does not emit much visible light. Brown dwarfs can fuse deuterium and lithium, but never reached the size required to sustain hydrogen fusion in their cores.

As you might imagine, there are questions regarding when a super-planet (a planet of greater than Jupiter mass) gathers enough material to qualify as a brown dwarf. Because they aren’t particularly luminous, they aren’t all that easy to spot. The researchers in question note: “Our galaxy could also be teeming with brown dwarfs, which cannot sustain hydrogen fusion and thus are very faint.”

Comparison: most brown dwarfs are only slightly larger than Jupiter (10–15%) but up to 80 times more massive due to greater density. The Sun is not to scale and would be larger. Image and caption by Wikipedia.
Comparison: most brown dwarfs are only slightly larger than Jupiter (10–15%) but up to 80 times more massive due to greater density. The Sun is not to scale and would be larger. Image and caption by Wikipedia.

Now, here’s the catch: We’ve actually found very large planets orbiting brown dwarfs before. As the researchers note, however:

[The] large mass ratios (q & 0.1) between the host and the companion would suggest that they are formed as binary systems either by the dynamical interaction in unstable molecular clouds (Bate 2009, 2012) or by the turbulent fragmentation of molecular cloud cores (Padoan & Nordlund 2004). This implies that the companions could be considered as substellar objects rather than planets (Chabrier et al. 2014). By contrast, the low mass ratio of this event (q ∼ 0.016), combined with recent reports of the massive disks (& MJ) around young brown dwarfs (Hervey et al. 2012; Andr´e et al. 2012; Palau et al. 2014), suggest that the planetary companion of the event may be formed by planet formation mechanisms.

In other words: In the past, we’ve found planets that appear to have formed around brown dwarfs very differently than how we believe our sun and solar system were created. This planet appears to have been formed in the more normal fashion, suggesting that brown dwarf systems may be able to host planets (or at least a planet) as well.

The chances of such planets hosting life, however, are pretty slim. Brown dwarfs are not thought to be capable of emitting significant amounts of heat, since they contain relatively low amounts of fusable material in the first place. Old brown dwarfs are cool enough to have an atmosphere, though it isn’t one you want to breathe (it can rain iron in the atmosphere of a brown dwarf). Still, a brown dwarf with a planet around it is a surprising find and could tell us more about how planets form, even around exceptionally low-mass, not-quite-stars.

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