The Other End of the Planetary Scale

Article Updated: 24 Dec , 2015
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The definition of a “planet” is one that has seen a great deal of contention. The ad-hoc redefinition has caused much grief for lovers of the demoted Pluto. Yet little attention is paid to the other end of the planetary scale, namely, where the cutoff between a star and a planet lies. The general consensus is that an object capable of supporting deuterium (a form of hydrogen that has a neutron in the nucleus and can undergo fusion at lower temperatures) fusion, is a brown dwarf while, anything below that is a planet. This limit has been estimated to be around 13 Jupiter masses, but while this line in the sand may seem clear initially, a new paper explores the difficulty in pinning down this discriminating factor. For many years, brown dwarfs were mythical creatures. Their low temperatures, even while undergoing deuterium fusion, made them difficult to detect. While many candidates were proposed as brown dwarfs, all failed the discriminating test of having lithium present in their spectrum (which is destroyed by the temperatures of traditional hydrogen fusion). This changed in 1995 when the first object of suitable mass was discovered when the 670.8 nm lithium line was discovered in a star of suitable mass.

Since then, the number of identified brown dwarfs has increased significantly and astronomers have discovered that the lower mass range of purported brown dwarfs seems to overlap with that of massive planets. This includes objects such as CoRoT-3b, a brown dwarf with approximately 22 Jovian masses, which exists in the terminological limbo.

The paper, led by David Speigel of Princeton, investigated a wide range of initial conditions for objects near the deuterium burning limit. Among the variables included, the team considered the initial fraction of helium, deuterium, and “metals” (everything higher than helium on the periodic table). Their simulations revealed that just how much of the deuterium burned, and how fast, was highly dependent on the starting conditions. Objects starting with higher helium concentration required less mass to burn a given amount of deuterium. Similarly, the higher the initial deuterium fraction, the more readily it fused. The differences in required mass were not subtle either. They varied by as much as two Jovian masses, extending as low as a mere 11 times the mass of Jupiter, well below the generally accepted limit.

The authors suggest that because of the inherent confusion in the mass limits, that such a definition may not be the “most useful delineation between planets and brown dwarfs.” As such, they recommend astronomers take extra care in their classifications and realize that a new definition may be necessary. One possible definition could involve considerations of the formation history of objects in the questionable mass range; Objects that formed in disks, around other stars would be considered planets, where objects that formed from gravitational collapse independently of the object they orbit, would be considered  brown dwarfs. In the mean time, objects such as CoRoT-3b, will continue to have their taxonomic categorization debated.

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17 Responses

  1. Jorge says:

    Hang on a minute.

    The terminological limbo implied here would be a range between 11 and 13 Mj, which is an area of overlap between those objects that undergo deuterium fusion and those that don’t, right? Anything more massive than that would undoubtly be a planetar (I much prefer this term to brown dwarf), anything less massive would be a planet. CoRoT-3b would then be a planetar, no question about it.

    The only thing limbic about it is that some folks think we shouldn’t be calling planet to objects formed through gravitational collapse and we shouldn’t call star to objects formed through core accretion. And this planetar’s orbit suggest core accretion. Still, this seems to be a rather minoritary position these days.

  2. James says:

    So in that hypothetical nomenclature scheme, what would an object that formed in a disk with enough mass to fuse hydrogen be called? A planet or a close binary star?

    IANAA, but to me it would make sense to restrict the term planet to rocky worlds and not use it to describe failed stars at all.

  3. Catsceo says:

    What would Jupiter and Neptune be then?

  4. gopher65 says:

    This is why a branching spectrum definition is needed instead of an arbitrary “line in the sand” definition, which simply doesn’t work.

    Rather than saying “if an object is round, clears it’s orbit, and can’t burn deuterium then it is a planet”, the definition should simply run the entire spectrum of possibilities. The spectrum of objects should include everything from a single grain of dust all the way up to a red/blue supergiant star. When objects of the same mass have different properties, a branch is created.

    It’s simple. We already do this for stars. Why not planets too?

  5. Trippy says:

    @Jorge:

    The terminological limbo implied in the article is complex.

    Corot 3b has an orbit of 4.25 days.
    Brown dwarves that close to the star are rare (have a look for ‘brown dwarf desert’) but most planets that close to their primary have highly eliptical orbits where Corot 3b’s orbit is nearly circular.

    To add to the confusion, some models suggest that planets as big as 25-30 Jupiter masses can form by core accretion.

    So the problem with Corot-3b is that is has some of the properties that are more characteristic of planets, and some of the properties that are more characteristic of brown dwarves.

  6. SteveZodiac says:

    Regardless of teh categorisation debate this has lead to a better understanding of the conditions required to create a star. I was taught that Brown Dwarfs were objects which were unable to create SUSTAINED fusion I guess the term “burn” means that in this context.

  7. agmartin says:

    Since objects that formed like brown dwarf but are not massive enough to fuse deuterium are too small to be actual brown dwarfs I propose they be called dwarf brown dwarfs.

  8. Dark Gnat says:

    Perhaps the OBAFGKMLT should be eexpanded to incluse no fusing objects, such as planemos, planets, etc maybe all the way down to dwarf planets.

  9. Jorge says:

    @trippy

    I think it’s yet to be proven that the method of formation of any given object really has such an impact in its properties to make it all that different from another object with the same mass formed through some other method, especially when we are talking about two methods, like gravitational collapse and core accretion, that, after a point, do act exactly the same way. We may find markers of how they formed, but in general I find it very likely that a core accreted 20 Mj planetar is pretty much the same as a gravitationally collapsed 20 Mj planetar of the same metallicity. So I don’t really see the need to distinguish one from the other.

    In my view, if two objects are similar, they should be called the same, regardless of how they formed, where they are, what they revolve around (if anything), how many other objects reside in the same area and so on and so forth. Everything else is part of the object’s history, which is always unique to each object in the universe and therefore shouldn’t be considered relevant for classification (only for subclassification, maybe). You don’t have to know how many birds nested on a tree to call it a tree, even if you may have a hard time deciding if something in between is a small tree or a tall bush.

  10. Torbjorn Larsson OM says:

    … highly dependent on the starting conditions. … One possible definition could involve considerations of the formation history of objects in the questionable mass range; Objects that formed in disks, around other stars would be considered planets, where objects that formed from gravitational collapse independently of the object they orbit, would be considered brown dwarfs.

    I’ve noted this before here, but this type of pathway definition is what enables biology to make sense. If they used characteristics, they would group whales with fish, which makes no hereditary sense.

    And here is an excellent example of why the same thinking will be fruitful in astronomy, because in some instances the pathway (starting conditions) do make a difference.

    So I’m all for this.

  11. Torbjorn Larsson OM says:

    @ Gopher65

    “if an object is round, clears it’s orbit, and can’t burn deuterium then it is a planet”

    I believe I see what you want to say. (And I agree in principle, except that I note that in practice the subsequent naming system for individual grains will be a bitch. :-D) But this is a strawman argument.

    The current IAU definition for planets applies for planets only, not exoplanets. (Which is presumably why they don’t bother with brown dwarfs, no one has been found associated with our system yet.)

  12. Jorge says:

    I’ve noted this before here, but this type of pathway definition is what enables biology to make sense. If they used characteristics, they would group whales with fish, which makes no hereditary sense.

    Actually, no, not at all. It’s because biology uses characteristics that it groups whales with mammals and not with fish. Basic characteristics, dealing almost exclusively with genetic constitution these days, but before that with anatomy, and the inner workings of organs and cells.

    It’s a grave error to equate astronomical taxonomy with biological taxonomy. Because biology is inherently a historical phenomenon in the sense that there are no strict and simple laws determining what kinds of organisms will be produced. There are some biophysical constraints, but the variability that can be acheived within these constraints is huge, and everything a biological lineage was in the past as a profound impact on what it is in the present and will be in the future. This is not true with astronomy. An object above a given mass will be rounded by its own gravity, inevitably. An object above a certain mass and below a certain temperature will trap volatiles and grow an atmosphere, inevitably. An object above a certain mass will grab huge quantities of the lighter (and most abundant) elements, and will become enormous balls of hidrogen and helium, inevitably. One such object above a certain mass will be compressed to the point of nuclear ignition, inevitably. On a very basic level, what an astronomical object is depends very much on its mass and very little on how that mass came to be. History plays a part, but it’s a pretty minor one. That’s biology turned pretty much inside out; so no, it’s not smart to use the same kind of approach to classification.

  13. Jorge says:

    Errata: Please read “everything a biological lineage was in the past has a profound impact” instead of “everything a biological lineage was in the past as a profound impact “.

    And please read “An object above a certain mass will grab huge quantities of the lighter (and most abundant) elements, and will become an enormous ball of hidrogen and helium” instead of “An object above a certain mass will grab huge quantities of the lighter (and most abundant) elements, and will become enormous balls of hidrogen and helium”

    Still no edit fuction… sigh… 🙁

  14. vagueofgodalming says:

    I can’t say how the usage will evolve, but I will make one confident prediction: there will not be an official definition of ‘brown dwarf’, or of the upper limit to ‘planet’, for a very long time.

    The IAU definition of 2006 has, so far as I can see, had zero practical impact on science: the only thing it affects is how the IAU’s own naming committees go about their work. As far as I know, there’s no official definition of ‘star’, for example, yet astrophysics, amazingly enough, is not crippled by this grave oversight. I’m sure the IAU will learn the lesson of Prague and let sleeping dogs lie.

  15. Aqua says:

    Fusion as the demarcation between planets and stars sounds right, but what if we eventually detect deuterium fusion processes ongoing within Jupiter or Saturn? Perhaps emission rates would then become the ‘line in the sand’?

  16. Jon Voisey says:

    Aqua: That’s essentially the point I was working at here. I didn’t dwell too much on it, but I alluded to it in the article saying, “just how much of the deuterium burned, and how fast”.

    The authors of the paper pointed out that Deuterium is so easy to fuse, there will pretty much ALWAYS be some, but how much of it gets burned, and how quickly is the issue. Even Jupiter is likely burning some, but so slowly as to be inperceptible.

  17. Aqua says:

    Addendum: http://antwrp.gsfc.nasa.gov/apod/astropix.html

    While not fusion, instead fission, there are nuclear processes close at hand.

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