IAU day 2: What makes a brown dwarf?

Posted by Korey Haynes
on Saturday, August 08, 2015

A giant planet and a brown dwarf look very similar to astronomers, but probably form very differently. How and where should astronomers draw the line between them? // J. Pinfield/RoPACS network/University of Hertfordshire
[Updated August 10]

My second day at the International Astronomical Union's (IAU) general assembly meeting featured one of my favorite parts about attending science conferences: watching scientists fight! OK, astronomers are a pretty friendly bunch, so by "fight," I mean "spirited but polite debate." Nonetheless, this kind of dialog is why conferences are important, so I'm always excited to see it in action. In this particular case, I was enjoying a talk about the boundary between planets and brown dwarfs, failed stars that we've covered recently at Astronomy.

The speaker, Paul Wilson, from the Paris Institute of Astrophysics, was specifically looking to understand a quirk of the universe astronomers call the "brown dwarf desert": Brown dwarfs found around stars don't seem to orbit with periods shorter than about five years, stranding them out at more than three times the average Earth-Sun distance — maybe much farther, but almost never closer. We do see planets larger than Jupiter on tight orbits all the time, though astronomers are confident they don't form there, but rather migrate in later in their lives. So what's the difference in how these two kinds of objects form, evolve, and interact with their parent stars? Wilson went hunting for stars with low-mass companions — perhaps brown dwarfs, perhaps giant planets, in order to study a large sample and figure out what might be going on. And he's still working on the larger problem.

But aside from the question of the desert, the audience seemed almost more interested in his introduction, where he laid out the cutoffs between stars, brown dwarfs, and planets. A star burns hydrogen. This is an easy and uncontroversial limit, with an equivalent mass cutoff around 73 Jupiter masses*. But the planet-brown dwarf boundary is trickier. Many astronomers have used the limit for deuterium burning (deuterium is a hydrogen atom plus a neutron), but Wilson pointed out this actually is fairly arbitrary. Unlike hydrogen burning, deuterium burning doesn't affect the object too much, and this makes it a somewhat pointless distinction just for the sake of drawing a line somewhere. The benefit is that it is a distinct line (and sets the mass cutoff on the small end around 13 times the mass of Jupiter).

Wilson favors making the divide instead based on formation — did the object in question form like a star would have, or like a planet? It gets more to the root of the question, but of course the big holdup here is that astronomers are not omniscient and are left making only (educated) guesses about how a now mature object formed a few billion years ago when they weren't around to watch it.

The Q&A period was spent almost exclusively talking about, in true IAU form, what makes a planet. One questioner was quick to point out that the IAU currently favors the deuterium burning cutoff for the definition of a brown dwarf. Another pointed out that the rules for planethood were developed for our solar system, which has nothing like a super-Jupiter or a brown dwarf, so the rules are both biased and incomplete. Building on this, Wilson pointed out that if you moved the Moon to Mercury's orbit, it would be considered a planet. So should we look forward to different rules if a brown dwarf orbits a star or is found by its lonesome in the cosmos?

We probably shouldn't, and yet definitions require hard cutoffs that nature simply doesn't always cooperate with. We don't know enough yet to answer definitively, but that's OK for now. Conferences aren't always about getting all the answers. Often, they're about asking the interesting questions.

*Originally incorrectly stated brown dwarf upper mass limit of 73 solar masses.

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