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Diamond planet found (if you like it then you should have put a ring on it)

Stars shine, for sure, but PSR J1719-1438 is sporting some serious bling. Robyn Beck/AFP

A planet has been found in our Milky Way galaxy that may be made entirely of diamond.

As reported in Science today, an international astronomy team led by Swinburne University’s Matthew Bailes, has discovered a low-mass but dense object in orbit around a rapidly-rotating neutron star.

This binary system (the name given to any two objects in space orbiting a common center of mass) exists some 4,000 light‐years away in the constellation of Serpens (the Snake).

The unusual circumstances that gave rise to the system could have resulted in the universe’s largest diamond – a whopping 1031 carats worth.

To put this in context: one carat of diamond can cost up to $5,000, and 1031, written in full, is 10,000,000,000,000,000,000,000,000,000,000.

Do the math and you’re looking at one expensive rock.

So what exactly is this diamond object and how was it formed? Well, first we need to understand a couple of key concepts.

First, a neutron star is made from the superdense remnants of a supernova – the catastrophic explosion that ends the lives of medium-size stars.

With masses of approximately 1.5-times that of our sun, but only 20 kilometres across, neutron stars are the densest objects we know of.

They are only detectable thanks to the chance alignment of their lighthouse-like beams of radio emissions, which sweep across the Earth once every rotation period.

One of the objects in the newly-discovered binary is a pulsar – a rotating neutron star – with the rather unwieldy name of PSR J1719-1438. For the sake of brevity, let’s call it Beyoncé.

Staggeringly, Beyoncé rotates 173 times a second, so by the time the Earth has rotated once (24 hours), Beyoncé has rotated approximately 15 million times.

The pulsar system was discovered in observations made at New South Wales’s Parkes Radio Telescope, a dish made famous by the 2000 Aussie film The Dish.

The research team – made up of astronomers from Swinburne University, CSIRO and several other institutions worldwide – has been using the Parkes telescope to search the entire southern sky for the cosmic lighthouses indicative of neutron stars.

So, just how do neutron stars reach such phenomenal speeds? The theory is that they “spin up” over many years spent stealing mass from their companion. So far, almost 200 examples of this phenomenon have been documented by astronomers worldwide.

Neutron stars are such exquisite clocks that the orbital period (time to complete one orbit) of any companions can be determined extremely precisely.

This is done by measuring the arrival time of each radio pulse from the star. In the case of Beyoncé, the researchers found that the companion completes one orbit every two hours and 10 minutes.

Typical binary companions to neutron stars are somewhat less massive than our sun, but the mass of Beyoncé’s companion is even smaller, approximately that of Jupiter.

With a diameter approximately four times that of Earth and a mass greater than Jupiter, Beyoncé’s diamond companion has a density at least twice that of lead, and more than ten times that of Jupiter.

For a planetary-mass object to reach such high densities requires quite an unusual evolutionary history.

A possible precursor is a so-called “ultracompact binary” – a neutron star system which orbits around its centre of mass in less than an hour.

Normal (main-sequence) stars, such as our own sun, are too big to fit in such tiny (and therefore quick) orbits.

Instead, the companions in such ultracompact systems are probably some flavour of white dwarf, comprised of helium, or carbon and oxygen.

In a binary system of this kind, mass is often transferred from one star to the other and once the transfer ceases, the orbital period of the system increases.

The binary system discovered by Bailes and colleagues might well have been a white dwarf binary whose orbital period slowed from less than an hour, to the present value of two hours and ten minutes.

So what about Beyoncé’s sparkly friend?

Although the precise composition of the planet hasn’t yet been confirmed, at such high densities the material must be crystallised, just as in salt, sand (quartz) or diamond here on Earth.

But if the companion object is made of high-density carbon, as the researchers suspect, Beyoncé might just own the biggest diamond in the known universe.

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