It’s a Diamond Planet! That Isn’t Diamond! Or a Planet!

The white dwarf Sirius B is the small dot at the lower left of Sirius (the Dog Star). The "diamond planet" is probably also a white dwarf, albeit one with highly unusual properties.

Everyone not anxious about the approach of Hurricane Irene has been making jokes about the newly-announced “diamond planet“. My friend who prompted me to write this post offered to pay me if I can find a way to make the planet hers; others have announced that Donald Trump or deBeers will be making an offer for it soon.

I didn’t follow up immediately for several reasons, including Irene (I live in Richmond, Virginia) and a promised new post for the Scientific American guest blog, but a few things didn’t seem right to me about the announcement:

  • We don’t know the chemical composition of most planets orbiting other stars (though we can make guesses based on our Solar System). The data we do have is from looking at absorption of the host star’s light through the outer layers of the planet’s atmosphere, so the surface or interior of the planet isn’t something we have direct information about.
  • Rocky planets — Mercury, Venus, Earth, and Mars — are made of iron and other minerals, so it’s odd to infer composition radically different from that for a planet orbiting another star.
  • Diamond is a carbon compound, but with atoms arranged in a specific way. Graphite and buckyballs are also carbon compounds — the atoms are exactly the same, but the crystal structure is very different. If determining chemical composition at such a great distance is hard, determining the crystal structure would be even more difficult.

As with so many things in the internet era, all I had to do was wait, and let someone else figure it all out. Today’s hero is science blogger Professor Astronomy, whose post goes into a lot more detail than I will do here, so read what he says if you don’t trust me. (Actually, read what he writes anyway.)

To summarize: the “diamond planet” is neither diamond nor a planet. It appears to be a white dwarf: the highly-compressed core of a star similar to our Sun that has run out of fuel for nuclear and shed its outer layers. A typical white dwarf is the size of Earth but with a mass closer to that of the Sun — in other words, they are very dense objects. White dwarfs are normally composed of carbon, with a smaller amount of oxygen, but because of the highly compressed nature, everything is squeezed together to resemble a crystal, which is where the “diamond” idea comes from. However, strictly speaking it’s not diamond, because the crystal structure isn’t like a diamond. White dwarfs probably aren’t sparkly, in other words — just like most forms of carbon (graphite, charcoal, etc.) aren’t very aesthetically pleasing. (They are rather bright, though, since they have very high surface temperatures.)

This particular white dwarf is weird, even by white dwarf standards: its mass is closer to Jupiter than to the Sun. Instead of orbiting around a normal star, it orbits a pulsar, itself the dense core of a much more massive star that went supernova sometime in the past. In fact, that’s how astronomers were able to determine so much about the white dwarf: pulsars are roughly the size of a city on Earth, but with masses greater than our Sun, so they exert very intense gravitational attractions. By measuring variations in the pulsar’s motion, astronomers determined the mass of the white dwarf companion, but also how close it orbits — very close, as a matter of fact, and that’s why they think it’s a white dwarf rather than a planet.

It does kind of fit the IAU’s criteria for a planet, though I would say anyone classifying a white dwarf as a planet should rethink their definition. The reason it’s so strange is probably because of interacting with the pulsar: when two objects are so close, the gravitational pull on the near side is stronger than on the far side, which in mild cases produces tides (as with Earth’s oceans) and in extreme cases can destroy one or both objects (which is probably what produced Saturn’s rings). The “diamond planet” is an intermediate case: some of the atoms evidently got stripped off the white dwarf by the pulsar’s tidal pull, and what is left is much smaller than the original stellar core. (As an aside, I worked on a project a few years ago with fellow blogger Louis Rubbo in which we modeled white dwarf binary systems, including transfer of mass from one to the other. The project never was satisfactorily completed, so it remains unpublished.)

So we can understand why the press releases call this object a “diamond planet”, why that name is misleading, and why the full story is more intriguing — as is often the case. Until now, nobody has ever seen an object quite like this, and to me (all apologies to my friend) that is worth more than diamonds.


11 responses to “It’s a Diamond Planet! That Isn’t Diamond! Or a Planet!”

  1. The original story on the discovery that I read suggested that the object technically wasn’t actually a white dwarf proper, but the dead leftover core of one that had lost most of its mass to the pulsar–hence the low mass.

    1. We don’t really have a good name for it yet, so I stuck with “white dwarf” for simplicity. Perhaps I am as guilty as the “diamond planet” crew then!

  2. How small can a white dwarf be, and still be supported by electron degeneracy? I did not know that such a low-mass white dwarf could exist. As the mass is reduced by tidal stripping, the surface gravity of the remaining white dwarf will go down. White dwarfs are generally at high temperature; why won’t regular gas pressure begin to become interesting relative to surface gravitational force? Has the theory actually been worked out for such low mass white dwarfs?

    1. That’s a good question; as far as I know, nobody has developed anything beyond a heuristic equation of state for white dwarfs. I’m not even sure who to ask, but that’s what the internet is for.

  3. Apparently the theory that it rains diamonds on Neptune has also been called into question. But whatever is happening there, I’m sure it’s pretty interesting, what with all those hydrocarbons, ultra-high pressures, and low temperatures!

    1. We need to send another mission to Neptune — there’s far too much about it we don’t know. I’m also interested in knowing how common planets of that type are throughout the universe.

  4. […] get science right, comparing his discovery with the treatment of climate scientists. (Here’s my post on the “diamond planet” and why it’s cool, despite the misleading […]

  5. If this object was once a star, it cannot really be considered a planet. Alan Stern draws the line at the question of whether the object ever conducted fusion, even if it is not doing so now. Dead stars are still stars, as are brown dwarfs which may once have fused hydrogen or deuterium but no longer do so.

  6. […] White dwarfs can only grow so large before gravity overwhelms the pressure holding them from collapse. This limit is known as the Chandrasekhar limit (for the great astrophysicist Subrahmanyan Chandrasekhar), and it’s the most massive any white dwarf can become. If enough mass is added to any white dwarf (from a companion star), it will explode in a similar fashion: a supernova of type Ia. The name comes from its spectrum, which makes it possible for scientists to distinguish white dwarf supernovas from the explosions of massive stars, which are not at all uniform in the way they explode. Astronomers know how bright these white dwarf explosions are, so if one happens in a distant galaxy, they can accurately determine how far that galaxy is from us. […]

  7. […] at equal distances, you get a hexagonal lattice, with different directions of symmetry. Graphite, a form of carbon, is made of sheets of hexagonal lattices stacked on top of each other; the bonds within the sheets […]

  8. […] times the density of Earth. That’s a lot more dense than the Sun, and even more dense than a white dwarf, which is the compact core of a star similar to the Sun that has exhausted its usable supply of […]

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