Posted by: mattburleigh | August 26, 2011

It’s not a planet. It’s a white dwarf.

Yesterday Science announced the amazing discovery of an incredibly dense object that appears to be made of a crystalline form of carbon: possibly, ultra-dense diamond (Bailes et al. 2011, Science, DOI: 10.1126/science.1208890).

The object orbits a recently-discovered pulsar, PSR J1719-1438, every two hours and ten minutes. It has a slightly higher mass than Jupiter (technically, its minimum mass), but the lack of evidence for direct interaction with the pulsar places a maximum limit on its diameter of 55,000km, or about 4x the diameter of Earth (Jupiter is about 10x the diameter of Earth). This means its density is on average at least 23 grams per cubic centimeter (or about twice that of lead). If it is made largely of carbon (see below), then compressed to this high density it should literally be squeezed into a crystalline form: probably, diamond.

Obviously the discovery of a unique, exotic object like this is incredibly exciting and has the wow factor that attracts press attention: see Sky and Telescope , Astronomy Now and New Scientist (including a video made by lead institute, Swinburne University, Australia) for starters. Also check out Steinn Sigurdsson’s blog.

What you’ll immediately notice is how the object is labelled a planet, or even better, a Diamond Planet. The densest planet ever discovered. This is understandable: it has the mass of Jupiter (in truth, most likely between 1 and a few Jupiters), squeezed into a body with the diameter of no more than 4 Earths. It orbits a star (a neutron star). Therefore it is a planet.

Or is it?

You’ll also notice how some of the media articles explain that the “Diamond planet was once a star”. This is indeed true. Here’s how this object formed. Once upon a time there was a binary system of two relatively normal stars, albeit one of them was pretty massive. The massive one collapsed and exploded as a supernova, and a pulsating neutron star (pulsar) was left behind. The pulsar, having a very high gravity, began to strip material off its neighbour. Basically it ate it. When the pulsar accreted this material, it also gained angular momentum, and spun up to rotate once every 5.7 milli-seconds. Hence it’s name, a milli-second pulsar. Eventually, all that was left of the star it accreted from was the core, which collapsed to become a White Dwarf, the kind of star I’ve spent my career studying.

There are many milli-second pulsars known with white dwarf companions. These white dwarfs are often much less massive than the majority of white dwarfs which evolve from ordinary, isolated Sun-like stars. Say about 0.1 solar masses (or 100 x Jupiter) compared to 0.5 solar masses (500 x Jupiter) in a common-or-garden white dwarf. That’s because the pulsar has stripped away so much mass. What we have here in PSR J1719-1438 is a white dwarf that has been stripped to an absolute extreme. There’s so little of it left that the atmosphere, which once consisted of H and He, has gone, and just the core remains.

But as Sky and Telescope ask, “Can a stripped-down star be called a “planet”?”. Indeed, beyond the title of their paper,
“Transformation of a Star into a Planet in a Millisecond Pulsar Binary”, the discovery team are a bit reluctant to make that claim. In their abstract they call it an “ultra-low-mass carbon white dwarf”, which is exactly what I am arguing it is. Lead author Matthew Bailes of Swinburne University in Australia told Sky and Telescope: “…. technically it is [also] a stellar remnant.”

But then Bailes also says: “… But all the rocky planets are composed of elements that were once in stars.” For me this is a bit disingenuous. Yes, all the planets in our solar system are composed of atoms once made in previous generations of stars, but that’s not the same as a stripped-down star. Our planets were born in a disk of dust and gas that surrounded the early Sun. For me, that distinction is important and Alex Wolszczan, the discoverer of the first planets around a pulsar, gets the description right: “This spectacular discovery provides yet another demonstration of nature’s cleverness in creating planet-mass bodies,” (my italics).

But there are other views. Sara Seager of MIT told Sky and Telescope: “If the mass is less than 13 Jupiter masses, the exoplanet community would certainly agree this is a planet.” I don’t agree with Sara. 13 Jupiter masses is an entirely arbitrary limit based on the minimum mass needed for any kind of nuclear fusion (to fuse deuterium). It used to be the point at which we called an object a brown dwarf. But then we started finding isolated brown dwarfs with masses less than 13 Jupiters. And transiting planets with masses greater than 13 Jupiters. What’s probably more important in defining a planet (and I’m not getting into the Pluto debate here, which is about minimum mass) is the formation mechanism. Did it form in a disk of gas and dust around a star? Interestingly, the pulsar planets found 20 years ago by Wolszczan and Frail quite possibly did form from a disk of material left around the pulsar after the supernova explosion that created it.

I had an exchange of tweets yesterday with Marc Kuchner (@marckuchner) who, together with Sara Seager, predicted the existence of carbon planets (arXiv:astro-ph/0504214). Marc’s view was that “it’s how the pizza tastes, not how it’s made”. My reply was, “If it looks like a white dwarf, smells like a white dwarf and tasted like a white dwarf, it’s a white dwarf”. Clearly, we beg to differ, although Marc also suggested that “Maybe it’s both a planet AND a white dwarf!”.

Does any of this matter? Well, not in the sense that astronomy in general doesn’t really matter. If the economy really collapsed tomorrow, we astronomers would soon be out of a job. On the other hand, wealthy, advanced, cultured societies like ours are happy to spend a small amount of money indulging the pursuit of knowledge, to benefit from any technological and scientific spin-offs, and to inspire and educate. For this reason, I think it is important to debate these things. There is another point. All of us scientists are under increasing pressure to produce papers, to gain more funding, to show our work has impact and relevance. In short, to justify ourselves to our political masters. Hence, we are all tempted or indeed need to “sex up” our results.

I’ll leave it there. Maybe you agree with Marc Kuchner and Sara Seager, or maybe with me? Hell, why not have a completely unscientific vote

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Responses

  1. If there are objects with essentially identical properties but that could form via two different processes then I might agree that the formation mechanism isn’t that important when considering the type of object. There could be objects with masses a few times that of Jupiter that formed via gravitational collapse in a molecular cloud core and in which sedimentation leads to the formation of a solid core. Similar objects could form in a disc around a star. I wouldn’t be too bothered about whether we called the first a brown dwarf and the second a planet or called them both planets. What is important (at least to me) is that we try to understand the various processes that can lead to the formation of these object.

    My understanding is that this isn’t the case for the “diamond planet”. A quick scan of Marc and Sara’s paper (which I gather was never published) is discussing the possibility of carbon planets with masses up to a few tens of Earth masses forming in moderately high metallicity discs around young stars. The properties of these objects would be completely different to that of the white dwarf around the pulsar. As far as I’m aware there is essentially only one process that can form the object around the pulsar. The only similarity it has with a planet is that it has a planetary-mass and a planetary radius. If there was a planet-like formation mechanism that could have formed this object, then I might feel that calling it a planet is fine. As there is not, there seems to be no obvious scientific reason why we want to regard this as a planet.

  2. Interesting article! I think the definition of a planet (nevermind an extrasolar planet) is sufficiently vague that this remnant could happily be both planet + WD. And in interest of full disclosure, I’m one of the authors on this paper 🙂

  3. Your heading of this post will most surely bias your results.

  4. How about:

    “chimera” = Compact HIgh density Matter Exotic Remnant “-a”
    (this would also be fitting as a chimera is “any mythical animal with parts taken from various animals and, more generally, an impossible or foolish fantasy” (thank you Wikipedia). And this object has “parts” that are both white dwarf-life and planet-like.

    “corpsar” = Carbon Oxygen Remnant Pulsar Sibling (uhhh), “-ar”.

  5. I should have also said, for the record, I think you’re probably right to say a planet *should* have some kind of definition which includes formation mechanism. But for now, it doesn’t (as far as I know?)

  6. […] Perhaps when there’s not a shred of evidence that it’s actually made of diamond? Yesterday Science announced the amazing discovery of an incredibly dense object that appears to be made of a crystalline form of carbon: possibly, ultra-dense diamond (Bailes et al. 2011, Science, DOI: 10.1126/science.1208890). The object orbits a recently-discovered pulsar, PSR J1719-1438, every two hours and ten minutes. It has a slightly higher mass than Jupiter (technically, its minimum mass), but the lack of evidence for direct interaction w … Read More […]

  7. I agree: by definition, this is a planet. The definition of a planet may of course be in need of revision. But any stellar remnant is also called a star, so by definition this is also a planet. Lucky thing.

    I note that brown-dwarf-sized companions in close orbits around white dwarfs are commonly called ‘brown dwarfs’, although there are a variety of ways in which they could have gained or lost mass during their previous evolution – we do not know whether these are stellar remnants, overgrown planets, or even that they did start out as brown dwarfs. So there is a precedence for calling this object a planet based on its current characteristics.

    • Hi Albert. I totally agree that the definition of a planet is in need of reform in light of the discoveries of massive transiting objects, very low mass isolated brown dwarfs (eg in Upper Sco) and in wide binaries with other brown dwarfs (2M1207), and now this “chimera” as Eric Mamajek has suggested it might be called. Although you’re quite right, we call objects supported by electron degeneracy “stars”, as well as having neutron “stars”, neither of which are undergoing nuclear fusion.

      As far as I know and understand, the few known brown dwarf companions to white dwarfs in non-interacting close binaries most likely did start life as bona fide brown dwarfs, and probably have changed little despite going through common envelope evolution. The stellar envelope they orbited within was extremely tenuous and little mass was likely accreted (or stripped). On the other hand, the known “brown dwarfs” in interacting cataclysmic variable binaries may have had at least two evolutionary channels, one in which they started out as a genuine brown dwarf, and another in which they were stripped down from a stellar mass to the current mass in the brown dwarf regime. There’s a couple of observational papers by Stu Littlefair discussing likely examples of these different evolutionary channels.

  8. This is a diamond core remnant of a low mass low temp white dwarf star, common in binary systems. All the gas was stripped away by EM forces by the pulsar’s magnetic field. Carbon has been discovered with new techniques, in the forms of buckyballs and graphene. Carbon nanotubes are excellent conductors of electric currents, and diamond is an insulator. This discovery should be the first real opportunity to actually know what the core of a white dwarf contains. Magnetic fields are proven in stars, and carbon is the 4th most abundant atom in the universe. Our galaxy is 0.46% carbon by mass, which is a lot when considering how electric and magnetic fields are strongly organized by carbon atoms. Graphene near absolute zero temp conducts electrons in ways that mimic photons. See my story at:
    http://holographicgalaxy.blogspot.com

  9. Dear all, thanks for the comments, this is a most interesting debate on a fascinating object. There’s a couple of other excellent blog posts about it that you might like to read:
    Centauri Dreams “In the Sky with Diamonds” – http://www.centauri-dreams.org/?p=19600
    Professor Astronomy “A Diamond Planet…I dunno…” – http://blog.professorastronomy.com/

  10. […] doesn’t make the discovery any less exciting – after all, this planet is probably the core of a white dwarf with its atmosphere stripped away, something we’ve never seen in the wild before.  Any details we can glean from this gives us […]


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