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25 August 2011

Astrophile: The diamond as big as a planet

Video: Diamond planet found orbiting a pulsar



Object type: Exoplanet
Mass: Equal to Jupiter


Cruising through the Milky Way in your reconnaissance craft, your sensors pick up a powerful radio beacon. Altering your course to take a closer look, you find not a ship in distress, but an ultradense sphere of neutrons, packing a sun’s worth of mass into something the size of a city.


A closer look reveals it to be an orb with the mass of Jupiter and about half as wide. Sensors indicate it’s made of – wait, this can’t be right – diamond! Your instruments don’t lie. You’ve just stumbled upon a 1031-carat diamond.

Glitter ball

Fanciful as it may sound, a team led by Matthew Bailes of Swinburne University of Technology in Melbourne, Australia may have made a similar discovery – via telescope, not a starship.

Their radio survey of the sky detected the pulsar in December 2009, using the CSIRO Parkes radio telescope in New South Wales, Australia. A month later, follow-up observations with the Lovell radio telescope in Cheshire, UK, revealed periodic variations in the pulsar’s signals, indicating the existence of an orbiting companion with the mass of a planet.

That in itself was a rare find: of the 1800 or so pulsars known, only two had previously been found to harbour planets. Further analysis pointed to an even more astonishing possibility – a diamond planet.

The variations in the pulsar’s signals, which stem from the planet’s gravity tugging on the pulsar, revealed that the planet’s mass is roughly equal to Jupiter’s and that it orbits the pulsar at a distance of 600,000 kilometres, 1.5 times the distance of the moon from Earth.

Danger zone

The latter point is crucial. The planet orbits so close to the pulsar that it skirts the danger zone within which the star’s gravity would rip it apart.

Wait a minute, though. If it were a gas giant the size of Jupiter, part of its atmosphere would actually be inside the gravitational destruction zone, and the planet would not have survived long enough for Bailes’s team to detect it. So it must be less than about 60,000 kilometres in diameter, roughly 40 per cent of Jupiter’s width.

That in turn means it is much more compact than Jupiter, which has an average density only slightly greater than water.

The extremely fast rotation of the pulsar supports this conclusion. Pulsars that rotate many times each second are thought to spin up to such tremendous speeds as a result of stealing matter from a companion star. But there is no sign of such a massive companion today, so the planet is likely all that’s left of a star that was whittled down by the pulsar.

Hard-core bling

The core of a stripped down star would be mostly carbon, with a dash of oxygen. With the mass of Jupiter, such an object would be under high pressure because of its own gravity. And this would cause it to crystallise – most likely into diamond, just as carbon does deep inside the Earth.

If it is a diamond, does the planet glitter like an Earthly gem? “It’s highly speculative, but if you shine a light on it, I can’t see any reason why it wouldn’t sparkle like a diamond,” says Travis Metcalfe of the National Center for Atmospheric Research in Boulder, Colorado. He previously found a white dwarf – the remnant of an old star – with a carbon-crystal core that was under higher pressure than the new planet, producing a crystalline structure distinct from diamond.

Moshe Mosbacher, president of the Diamond Dealers Club in New York says he has “no clue” how much a diamond of this size would fetch, without first knowing its quality. But he is intrigued. “If there’s some way to transport it to New York and cut it, it doesn’t make a difference if it’s from inner space or outer space.”

Journal reference: Science, DOI: 10.1126/science.1208890