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

.