Like a diamond in the sky

A sparkling new discovery for an international team of scientists – including Professor Matthew Bailes, Dr Ramesh Bhat and Dr Willem van Straten from Swinburne University in Victoria – was the finding of a planet that’s probably made out of diamond.

Now there are many facets to this discovery, but one that immediately comes to mind is: how do they know it’s made out of diamond?

Well, they’re actually rather clever. The object is orbiting a pulsar – a magnetic, rotating neutron star which emits radio pulses – about 4,000 light years towards the centre of our galaxy. From modulations (or twinkles) in the pulses, it’s possible to tell that there’s something orbiting it every 2 hours and 10 minutes.

From this orbital frequency and an estimate for the mass of the pulsar (all pulsars are between 1.4 and 2 times the mass of our Sun), it’s possible to calculate not only how far away its planet is, but how much its mass is too. We can also calculate its maximum possible radius: anything bigger than the so-called Roche lobe radius will be pulled off by the pulsar.

So we have its mass and its radius, which allows us to calculate its density, from which we can deduce that it must be made of mostly crystalline carbon and oxygen. And of course, crystalline carbon is diamond.

That’d be a diamond with a mass greater than that of Jupiter, around 2.29×1027 kg, or, if you like, 1.14×1031 carats.

For the rock-hard technical details, see Bailes M, Bates SD, Bhalerao V, Bhat NDR, Burgay M, Burke-Spolaor S, D’Amico N, Johnston S, Keith MJ, Kramer M, Kulkarni SR, Levin L, Lyne AG, Milia S, Possenti A, Spitler L, Stappers B & van Straten W, “Transformation of a star into a planet in a millisecond pulsar binary”, Science, published online 25 August 2011, doi:10.1126/science.1208890 [PDF 1.7 MB]

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