Worlds Around Brown Dwarfs: When is a Planet a Planet?

I’ve always found brown dwarfs to be fascinating objects, and as such I’ve recently been re-reading papers about their formation and unusual chemistry. Brown dwarfs are strange, transitionary objects; they’re sort of in between being a planet and a star. They start out like a normal star would, forming from a collapsing cloud of interstellar dust and Hydrogen. The only difference is that the object in the center of this collapsing cloud is not big enough to achieve the crazy-high temperatures and pressures in its core to start fusing Hydrogen into Helium as stars do; instead, it merely glows from the heat of its birth and the energy released through gravitational contraction. As a result, brown dwarfs are very dim. Most of the light that they emit is in the invisible infrared part of the spectrum, and many would appear to glow magenta to the human eye.

 

brown_dwarf

My interpretation of a world transiting its brown dwarf host.

 

Early in their history, some of the largest brown dwarfs may be capable of fusing Hydrogen’s bigger brother isotope Deuterium, or even Lithium, but this process is short-lived. Most, if not all of a brown dwarf’s energy comes from the release of heat from its highly compressed interior. They are, in essence, failed stars.

Incidentally, you may have heard it said that Jupiter is a failed star; that had it been just a little more massive, it would have ignited into a second Sun. That’s not quite true, as you’d actually need to toss about 80 more Jupiters into Jupiter before it would become massive enough to approach the smallest known true stars. 80 Jupiter masses is about how small most astronomers think a star can get and still be a star. Any lighter and you can’t get high enough pressures and temperatures to fuse Hydrogen. Brown dwarfs occupy the mass range between about 13 and 80 Jupiter masses (although the lower bound of 13 Jupiter-masses is rather fuzzy and is more of a rule-of-thumb).

There’s a lot of really cool things (pun intended) about brown dwarfs, many of which deserve their own posts (and probably will get them eventually). But something that’s been on my mind recently is just what do you call an object that orbits one? Brown dwarfs aren’t technically stars, but they’re not planets either. So would an object in orbit around a brown dwarf be a planet, or a moon? This post will be a weird mix of both astronomy, linguistics, and categorization, so I apologize in advance.

There are examples of brown dwarf companions that have been discovered over the last few years. Most of these are very large objects with several times the mass of Jupiter. In terms of the system’s mass ratio, these are more similar to a binary star system than to a planetary system. But there is no reason why a brown dwarf couldn’t form smaller, terrestrial planets. In fact, given their low mass, brown dwarfs are expected to preferentially form terrestrial worlds over larger gas giants (and because of the nature of expected chemistry in a brown dwarf’s protoplanetary disk, those planets are likely to be very different than anything we know… but that’s a story for another post). However, there is right now at least one example of a low-mass companion orbiting a brown dwarf: OGLE-2013-BLG-0723LB/Bb. This world is about 0.7 Earth-masses, a little smaller than Venus, which would make it a terrestrial planet assuming it was small and rocky. But is it a planet, or is it a moon? The situation is further complicated by the fact that the brown dwarf itself orbits a low-mass star. Is this then a binary system of star and brown dwarf with a planet, or a single system of one star containing a brown dwarf with a large moon?

This might seem like a simple matter of semantics, but classification and taxonomical schema are important in trying to understand things in nature. Unfortunately, nature is very rarely, if ever, neatly organized into discrete categories that can fit nicely into little boxes. There’s a lot of fuzziness, nuance, and overlapping categories. Everyone remembers when the International Astronomical Union (IAU) reclassified Pluto to a “dwarf Planet”. While I agree with creating a new bin for objects like Pluto (which are more similar to each other than they are to any other object in the Solar System), doing so opened up a huge can of worms, and underscored just how vague our ideas of “planet-ness” are.

The IAU decided that in order to be an official planet, an object had to:

  1. Orbit the Sun (or another star; in which case it was an exoplanet)
  2. Be sufficiently massive enough to be rounded out into a spheroid by its own gravity
  3. Be gravitationally dominant over all other objects in its orbit (to be able to “clear the neighborhood”)

There are plenty of reasons why these criteria alone are insufficient to define a planet, but it presents a thorny issue when it comes to objects orbiting brown dwarfs. Again, they’re not stars, so anything orbiting them fails to meet the IAU’s definition of planet by virtue of not orbiting a star. But the brown dwarf is not a planet either, so their companions can’t quite be called moons. Yet the objects we find orbiting brown dwarfs are very obviously planet-like in terms of size and mass, especially when they’re several Jupiter-masses and therefore gas giant worlds. And certainly no one would ever mistake a gas giant for a moon. Clearly, we need a better system for classifying these objects. Or at the very least a better definition of planet. Right now, it’s all quite a mess, and frankly a little hand-wavy.

I’m not sure exactly how we should draw the line between planet and not-planet, or if such a line can really be meaningfully drawn (I truthfully don’t think it can). There are far too many outlier cases, like pulsar planets that orbit stellar remnants, planets orbiting black holes, binary planets, dwarf planets, and even rogue planets that don’t even orbit anything but instead roam the galaxy unbound to any object. But my thought is that if an object orbiting a brown dwarf meets the other two requirements in the IAU’s definition, then it’s a planet. After all, the IAU’s definition hasn’t stopped astronomers from calling any of the above “planets”. This might spawn a separate post about the need to overhaul our definition of planet entirely, as I feel that’s a discussion the astronomical community is going to have to have at some point.

Until next time, keep looking up.

References and Additional Reading:

Pascucci, I., Herczeg, G., Carr, J. S., & Bruderer, S. (2013). The Atomic and Molecular Content of Disks Around Very Low-mass Stars and Brown Dwarfs. The Astrophysical Journal, 779(2). Retrieved from http://iopscience.iop.org/article/10.1088/0004-637X/779/2/178/pdf

Bolmont, E., Raymond, S. N., & Leconte, J. (2011). Tidal Evolution of Planets Around Brown Dwarfs. Retrieved from https://arxiv.org/abs/1109.2906.

Udalski, A., & Jung, Y. K. (2015). A Venus-Mass Planet Orbiting a Brown Dwarf: A Missing Link Between Planets and Moons. The Astrophysical Journal. Retrieved from http://iopscience.iop.org/article/10.1088/0004-637X/812/1/47/pdf

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One Response to Worlds Around Brown Dwarfs: When is a Planet a Planet?

  1. astronist says:

    You may like to compare your thoughts with my own post on planets here:
    http://www.astronist.co.uk/astro_ev/2015/ae109.shtml
    and here:
    http://www.astronist.co.uk/astro_ev/2015/ae110.shtml
    I agree with you that a planet-sized body orbiting a brown dwarf should be described as a planet. Plus, the term “brown dwarf” is a starlike one (analogous to “red dwarf” etc).

    Like

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