Cooler-Than-Steam Brown Dwarf Blurs The Line Between Star & Planet

By Patrick Morgan | March 10, 2011 6:20 pm

Planetar. Substar. Failed star. Sub-stellar object. Astronomers have pinned each of these monikers on brown dwarfs, a category that has always perplexed scientists because it raises questions about what it means to be a star or a planet. And if that wasn’t enough, now they’ve discovered the coldest brown dwarf yet, blurring the line between planet and star even further.

It’s name is CFBDSIR J1458+1013B, and may be cooler than the boiling point of water (at the pressure of Earth’s atmosphere). This strange body is about 75 light-years from us, where it orbits its binary partner, another brown dwarf. Using the infrared capabilities of the 10-meter Keck II Telescope on Mauna Kea, University of Hawaii researcher Michael Liu and his team estimated the brown dwarf’s temperature, and have a ballpark range for its mass: between 6 and 15 times the mass of Jupiter.

It’s special because it may be a class Y dwarf (temperature less than 225 degrees Celsius (440 F)), a type of object whose existence astronomers had predicted but never actually found. Before this candidate arose, the coolest known brown dwarf was in the T spectral class; while there have been a few Y-class candidates in the past, scientists have a better grasp on the temperature of this one: 97 degrees Celsius, plus or minus 40C.

Another cool (ahem) thing about this particular brown dwarf is its mass. An object less than 13 Jupiter masses is too light to fuse atoms of deuterium, a heavy isotope of hydrogen; objects above 13 Jupiter masses can fuse deuterium. The uncertainty over CFBDSIR’s mass—estimated as between 6 and 15 Jupiter masses—could put it on either side of the line. And to top it off, it may be so cool that its gases could form clouds, a very planet-like thing to do.

So much still remains to be known about this particular brown dwarf and brown dwarfs in general, but one thing is set, at least for now: It’s the coolest one we’ve ever seen, and it may help us sort out this vague and messy mystery about the smudgy line between stars and planets.

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Image: NASA

MORE ABOUT: brown dwarf, planets, stars
  • Brian Too


  • Zombie

    What exactly is the ambiguity between star and planet? (sincere question, not being sarcastic)

    I’ve always assumed “star” meant stable fusion and anything not big/dense enough to fuse wasn’t a star. I’m assuming nobody wants a definition that begs the question of whether Jupiter-like objects are planets or brown dwarf companions/n-aries.

    Are there conditions where an object could start fusing briefly but blow itself out, as it were?

  • Iain

    Zombie – Perhaps the answer to your question lies in the definitions of the two objects. I assumed that stars emit visible light and planets reflect it.

  • Amos Zeeberg (Discover Web Editor)

    Zombie and Iain, you’re both right about the overall differences between stars and planets, but there’s ambiguity in the middle, where brown dwarfs are. Some heavy brown dwarfs have quite a bit of fusion, but not as much as proper stars; e.g., stars will fuse all of their lithium quite quickly, while dwarfs will not. And the cooler, lighter dwarfs don’t emit any visible light but do emit measurable infrared—much more than a planet.

    Discoveries over the past 15 years show that while there are of course big differences between stars and planets, it’s not easy to pick a bright line that divides and defines the two. Quick explanation here on Wikipedia.

  • Aaron

    How can the line be blurred? If it can fuse deuterium it’s a star; if not, it’s a planet! At some point, we have to come up with an objective classification.

    In any case, this thing is really cool–pun most certainly intended.

  • nick

    Planaster! Asteret!

    I predict we will eventually find enough stars/planets/other objects to fill all the vacancies in currently know stellar/planet/object scale gaps.

  • Amos Zeeberg (Discover Web Editor)

    @Aaron: That’s fine, a rational way to draw the distinction, but that doesn’t cover all of the differences that come to mind (and that have been measured) when people think about stars and planets. For instance, chemical separation into layers and formation of clouds both seem to be things that planets do, but the cut-offs for those won’t be at exactly the same place as for fusion of deuterium (i.e., 13 Jupiter masses). It’s fine if you want to use just the fusion cut-off, but you might end up with “planets” with star-style convection and/or stars with clouds—which would certainly be controversial.

  • Flonkbob

    That’s the problem with trying to produce discrete definitions for objects that exist on a continuum. I’m sure that over time we will find every possible state between ultra-hot blue stars and barely warm dwarfs, and a definition to cover it all is not going to be easy to develop.

    “It is the mark of an educated mind to rest satisfied with the degree of precision that the nature of the subject admits, and not to seek exactness where only an approximation is possible.” <– Badly Quoted Brilliant Greek Dude (name slips my mind)

  • Neon Sequitur

    It looks as if the only real ambiguity here is that of the object’s mass: between 6 and 15 Jupiter masses? No wonder its status is uncertain.

  • Dan

    @Flonkbob. I’m old enough to remember how to use a slide rule. If more people had the skill they would be less inclined to seek the absolute precision that digital technology has led them to expect in everything. Fuzziness IS!

  • Joseph G

    @#8 Flonkbob: That’s the problem with trying to produce discrete definitions for objects that exist on a continuum. I’m sure that over time we will find every possible state between ultra-hot blue stars and barely warm dwarfs, and a definition to cover it all is not going to be easy to develop.

    Very well put! Ultimately, even though we like to have clear dividing lines for everything, large planets and small stars are both just blobs of matter held together by their own gravity. They’re all heated by gravitational collapse for an extended period of time. If they’re massive enough, they begin stable fusion at some point. But even fusion is dependent on temperature, which may drop over time as the energy of gravitational collapse is used up and equilibrium is reached.
    And what if you drop an asteroid on a very very very large planet that’s on the cusp of deuterium fusion – does it instantly become a brown dwarf? What if it MIGHT have fused deuterium for a bit if the asteroid had hit it earlier? At some point the universe just plain defies our attempts to pigeonhole its inhabitants.

  • Demian W

    What I find fascinating about this article is the tangible examples that are becoming avaliable for us to even measure in the first place. I for one commend the search for precision, however there is a concept that we all must remember, a new discovery can change everything. Even if it just adjusts our definition of what is a star and a planet is or it forces us to create an entirely new classification, we will never be able to move forward in our understanding without tangible examples with which we can measure. I for one would like to congradulate the astronomers working at the Keck II telescope on Mauna Kea who found this unprecedented and previously theoretical brown dwarf for us to argue about.

  • amphiox

    Another point of ambiguity in the planet/star definition divide has to do with the formation process of the object. Stars form from the gravitational collapse of gas clouds. Planets form from the debris ring (protoplanetary disc) that forms around a protostar.

    Brown dwarfs form the same way stars do. But it is not known if there is a theoretical limit to how small an object can form in such a fashion. So it may be possible for cloud collapse to form even smaller objects, maybe even as small as Jupiter. This particular dwarf is in a binary with another brown dwarf, which means you could argue to call it a planet, (it orbits a star, and even on the high end of the mass estimate, there are known planets around that mass. However, the dynamics of its orbit suggest that it and its partner formed like binary stars, each from a cloud collapse, rather than as star-planet pairs do). But what if you found a lone dwarf of this mass range? Then what should we call it?

    This is basically the other end (big rather than small) of the planet definition controversy that reclassified Pluto. Stars had previously been pretty definitely defined by hydrogen fusion (and it’s easy to determine if a star is fusing hydrogen, because that’s what makes it shine). Brown dwarfs sort of shred this definition, since they don’t fuse hydrogen. And they don’t necessarily fuse deuterium either, they only potentially fuse deuterium, and only in the early phases of their existence. And the mass cut-off is also theoretical, as there are other factors that affect the ability to fuse deuterium. The 13J limit is theoretical only, as in something bigger may be able to fuse deuterium, but might not, depending on circumstances. You can have two 13J brown dwarfs of identical mass, but one will be fusing deuterium, and one won’t be. And a brown dwarf that is sufficiently old (and this age, compared to the age of other stars, can be actually quite young) won’t be fusing deuterium because it’s used up all it’s fusable deuterium. (And with increasing age, less and less infrared radiation.)

    Finally, large planets also can radiate infrared (that is not reflected starlight). Even Jupiter, cold as it is, radiates more infrared than it should from solar reflection alone.

  • Amos Zeeberg (Discover Web Editor)

    Thanks for the great comments. I think this isn’t the last we’ve heard of this version of the what’s-a-planet/star discussion.

  • Messier Tidy Upper

    Cool news – & write-up here. Thanks! :-)

  • teh winrar

    STAR FAIL!!!!!!!!


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