Dense Exoplanet Creates Classification Calamity

Article written: 6 Oct , 2008
Updated: 24 Dec , 2015


Given all the fervor over the definition of Pluto (planet? dwarf planet? snowball?), let’s hope the debate over the discovery of a planet that lies in an equally hazy area of classification is a little calmer. The COROT satellite recently discovered an extrasolar planet named Corot-exo-3b. It’s quite a curiosity as far as exoplanets are concerned, and its characteristics – such as a density twice that of lead – may force astronomers to rethink the distinction between massive planets and low-mass brown dwarfs.

Corot-exo-3b is orbiting close to its star, and takes 4 days and 6 hours to complete one orbit. For comparison, Mercury orbits the Sun every 88 days. It’s also roughly the same size as Jupiter, but far more dense, totaling a whopping 21.6 times Jupiter’s mass. This makes classification of the object a bit tricky.

“COROT-exo-3b might turn out to be a rare object found by sheer luck. But it might just be a member of a new-found family of very massive planets that encircle stars more massive than our Sun. We’re now beginning to think that the more massive the star, the more massive the planet,” said Dr Francois Bouchy, from the Institut d’Astrophysique de Paris (IAP), a member of the team that discovered the object.

Because of its extreme density, Corot-exo-3b lies in the shady area of classification between planet and brown dwarf. Brown dwarfs are massive bodies (between about 13 and 80 times the mass of Jupiter) that don’t make the cut for fusing hydrogen in their cores – and thus don’t shine in optical wavelengths – yet are much more massive that what is normally classified as a planet. Brown dwarfs can fuse deuterium even at lower masses (above 13 Jupiter masses), and lithium in masses above 65 that of Jupiter.

Planets generally form out of a disk of dust and gas that surrounds the early star they orbit, and then are pulled in closer due to friction with the debris that lies in their orbit. The close orbit and very short orbital period of Corot-exo-3b was likely caused by this effect.

The COROT satellite initially discovered the planet by measuring the change in the brightness of the host star as the planet passes in front of it. As the planet moves in front of the star, it slightly darkens the visible light, and then the star brightens once again as the planet moves behind it. The bigger the planet, the more it will darken the light coming from the star. The pull of a planet as it moves around its star can also redshift or blueshift the light coming from the star, and this shift can give information as to the mass of the planet.

Follow-up observations of the planet were done by a collaboration of scientists from around the world, led by Dr. Magali Deleuil from the Laboratoire d’Astrophysique de Marseille (LAM). Their results will be published in the journal Astronomy and Astrophysics.

Author’s note: Due to technical errors in the original posting of this article, the original was removed from UT, but the link may still show up in your feed reader. Be assured that this corrected version is the real, much more accurate one.

Source: ESA

31 Responses

  1. mandydax says

    Didn’t I read on here somewhere that planets aren’t able to get much larger volume-wise than Jupiter? The reason is that as more material collects to add to the planet’s mass, it increases the gravity and further compresses the core material. Not until mass sufficient for fusion is achieved is there any force opposing gravity other than the electromagnetic force keeping the individual atoms from fusing, at which point the release of energy from the fusion into the material tends to expand the planet into a star. I think that was how the hypothesis went.

  2. mandydax says

    Ah, yes! andy commented exactly what I was thinking of over at Bad Astronomy.

  3. pantzov says

    “…We’re now beginning to think that the more massive the star, the more massive the planet…”

    ummm… excuse me? i’m going to go out on a limb and predict that this will be disproven 🙂

    this object is fascinating. there are several things that it could turn out to be. we need to find more of them to conduct further research.

  4. Kevin F. says

    Failed star? Captured neutron star/white dwarf/brown dwarf?

  5. j.h.wegener says

    Why should planets have much in common at all? Perhaps to discuss “planets” as if it is one sort of object can be compared to label all water animals “fish” or all flying animals “birds”. Perhaps “planets” only have central stars in common?

  6. Jorge says

    Well, if we define the upper boundary of “planet” as the mass at which deuterium fusion becomes possible, then there’s little doubt that this object is indeed a brown dwarf.

    The only difficulty this poses is to those that think there is a fundamental difference in the way planets and stars form. Since I’ve never bought that one, I’m cool. That object It’s a fascinating brown dwarf, as far as I’m concerned. And I can’t help wondering what kinds of extreme processes would take place deep inside exo-3b. Twice as dense as lead is an average density of the whole BD: deep inside it, density would probably be a LOT larger. The mind boggles.

  7. Peter says

    The article doesn’t mention heat output. Is there an indication of some fusion? What does it appear as in infrared? I think we’ll find a lot of these as we look more closely at known stars. They probably will end up as the “other” type of solar system.

  8. Andrew says

    I doubt that the people observing the planet have found much more information than that presented here.

    I’m guessing that the planet in question just happens to have a whole lot more metal in it’s core than Jupiter has, but is otherwise still a gas giant.

  9. Chuck says

    RE: “What does it appear in the infrared?”

    With a four day orbital period It’s probably much to close to its star to image with anything we have now.

  10. Steve says

    Hmm methinks we could have a rather large solid planet here, likely a huge iron core.

    Its possible to be a brown dwarf, but a neutron star is not an option, since an object the size of the earth likely contains more mass than our sun.

    Im sticking with huge terrestrial planet, or small brown dwarf

  11. Louis says

    4 days and 6 hours!!! wow!! even faster than Mercury. Is it because it’s in a closer distance or because of the gravitational pull of the star? This is so intriguing!! 😀

  12. DestroyAllHumans says

    Why do we assume this thing is a natural object? Other than the fear of being ridiculed by mainstream astronomers who want to keep their jobs, that is.

  13. John Mendenhall says

    Occam’s razor. The chances of it being a natural object are much greater than it being a ‘made’ object, especially since we know of no non-human manufactured objects.. The simplest explanation is probably the correct one.

  14. Bill says

    At twice the density of lead, this is made of Unobtainium.

  15. Donald D says

    Gas giant planets formed when H and He were still abundant in our solar system. If they grow large enough, they might become a brown dwarf, possibly initiate nuclear fusion. Terrestrial planets like the Earth grew after H and He largely dissipated, which limited their overall mass to what heavy elements (C, O and heavier) were available. If such a terrestrial planet attracted much more of these elements after H and He were gone, its size and gravity would account for the higher density (21 times lead). To me this type of planet is very different from a brown dwarf and could never initiate fusion energy because not sufficient H would be present.

  16. DestroyAllHumans says

    I know it probably is “just” some kind of new planet we haven’t seen before because we have so far to go in our exploration of the Universe.

    But I cannot help but retain the feeling that we (or at least astronomers with big scopes0 are staring at artifical objects out there all the time and don’t know it – or are too afraid of ridicule and job loss to even suggest it out loud.

    Some day we will grow up, then we’ll be allowed into the Galactic Empire.

  17. Donald D says

    @DestroyAll. This planet is the size of Jupiter, but 21 times as massive. That’s 6600 Earth masses. Quite an accomplishment for an artificial body.

  18. What is a planet?

    A planet, as defined by the International Astronomical Union (IAU), is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals (wikipedia, fine definition).

    What is a brown dwarf?

    Brown dwarfs are sub-stellar objects with a mass below that necessary to maintain hydrogen-burning nuclear fusion reactions in their cores, as do stars on the main sequence, but which have fully convective surfaces and interiors, with no chemical differentiation by depth and, in discussion, that have experienced fusion at some point in their history. (again, a good definition in Wikipedia).

    I think that that is the first criterion to consider about the nature of the objects: is it a star or a planet?

    Now, two stars circling a common gravity center that is outside both bodies make a double star system… but what if that center of gravity is inside the more massive one?

    Well, that doesn’t mean that de less masive one is not an stellar object, for sure.

    And so? Well, let’s say that it is a close double star system.

    Anyway, we’ll have to wait until we resolve the first question.

  19. …and, of course, excuse me for some little mistakes in my writing; English is not my mother language, and it shows, more often than I would like…

  20. Bill says

    i find it really hard to believe that men of science can fall into such speculation and conjecture when it comes to the totally unbelievable prospect that they can prove that these so called planets are just that, planets. Give me proof, I want to see some solid evidence that these are more than just a guess on their part. I want pictures of just one of these planets. if not, then they should not make such outlandish claims and stick to the plain of reality.

  21. jerry says

    Unobtainium or disproven 250 year old theorioum?

    Just one more data point trying to tell us Newtonian estimates of mass, based upon orbital velocities without correctly factoring in the mass of the system are wrong.

    Likewise, Mercury is less dense, Jupiter and all outer planets more dense, and Titan is just what it looks like: A rocky, sandy desert.

  22. DestroyAllHumans says

    Donald D – If we can envision the possibility that advanced ETI could turn the entire galaxy into a Dyson Shell, we can imagine a dense artifact the size of Jupiter.

    Bill – There have been a few images of objects similar to this new “planet”, but if you look at how hard it is to image these things, you won’t be wondering why for long. Don’t you think astronomers would rather see the actual planets than just their effects?

  23. Excalibur says

    Cold gaseous objects (like gas planets) will not become very much larger in size than Jupiter is, maybe as much as twice as large, before the compression in the center becomes enough that adding more mass would only make the object shrink in size. The effect is due to degenerasy in the core. Clearly hot gaseous objects can become significantly larger.

    Ofc if the object also have a large rocky/metal based core, this scenario will only be stronger.

    So it shouldnt really come as any surprise at all that a planet with 21.6 times the mass of Jupiter turns out to be just about the same size.

  24. Imeanreally says

    What’s the issue with classifying this object?
    A Brown Dwarf is pretty much an extra large gaseous giant that cannot ignite into a star because it is not massive enough.
    The high density of this object clearly indicates it is not gaseous and is therefore a large planet made of rock and metals.

  25. RetardedFishFrog says

    21.6 times the mass and the same size as Jupiter doesn’t bother me. It’s the “twice as dense as lead” part that doesn’t pass the small test. Clearly, we need more, and more accurate, data on this object.

  26. Donald D says

    I also question the given density of 21 times Jupiter for this planet. It’s difficult for me to see how an average specific gravity (S.G.) of ~28 (density 21 times Jupiter) is possible in a planet this size. If it is gas (H, He) like Jupiter, why would it have much greater S.G. compared to Jupiter, yet be the size of Jupiter? It cannot be degenerate matter like a white dwarf, for the density is too low. If it is a super Earth, made of heavier elements, the question is whether high internal pressure could increase the density of any abundant element that high. I doubt it. (Making it of e.g., pure Osmium, with a S.G. of ~22, is essentially impossible.) The S.G. of the Earth’s core is about 13, or about 65% higher than iron under normal pressure. Even if this planet is pure iron (very unlikely), its S.G. in the planet core would have to be much greater than the average planetary S.G. of 28. It seems unlikely that iron or any mixture of abundant metals could be compressed that dense. (Such high pressures cannot be duplicated in the lab, and their effects on solid matter is estimated from equations of state.) There is a limit to increased S.G. of a solid with higher compression. The electrons and crystal structure can only be squeezed so much, and it requires an enormous pressure to degenerate matter and separate electrons from atomic nuclei (as in white dwarfs).

  27. Excalibur says

    Even the core of Jupiter is beginning to become degenerate – even the core of earth is somewhat degenerate. This is weak degeneracy.

    Heavily degenerate objects like White Dwarfs, and Neutron Stars, are the endproduct of extreme pressures.

    The reason densities starts to rise when under these tremendous pressures are that the electrons are beginning to cave in – they begin to occupy states that are closer in to the nucleon even when its a higher energy state, and eventually they simply fall loose – that is what degeneracy is. And the more the electrons cave in, the deeper the degeneracy – White Dwarfs and Neturon Stars are the extremes. Even Brown Dwarfs, and smaller Red Dwarfs are expected to have highly degenerate cores.

    But i dont believe this ‘planet’ is a pure gas-giant, sounds more reasonable that it does have a large rocky core, surrouded by huge amounts of gas. Im quite convinced its not a rock through-and-through though.

  28. RetardedFishFrog says

    I meant to say “doesn’t pass the SMELL test.”

    Excalibur – Thanks for the explanation. This new super-dense planet is a lot less smelly now.

  29. ruf says

    It is not a planet, for it does not orbit the sun. (See IAU rule #1 on the definition of a planet.)

  30. troy says

    didn’t we just go through this with Pluto.

    If it orbits a star, as opposed to orbiting a common object, binary stars it must be a planet. My interpretation is that dwarfs are failed stars and not planets. I quote “if orbits star, clears path, round and other requirements not remembered at the time of this typing it is a PLANET.

    Whats good for the goose is good for the gander.

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