A planet made of diamond?

Astronomers watching a millisecond pulsar – a small dead star spinning at an extremely fast rate – have found a dense companion orbiting it, which they believe to be a planet made of diamond. This dense gem is likely all that remains of a once-massive star, most of whose matter might have been siphoned off towards the pulsar. Though rare, the “diamond planet” is in accord with current theory of how certain binary star systems form.

The pulsar and its planet are part of the flat plane of our Milky Way galaxy and lie 4,000 light-years away in the direction of the constellation of Serpens (the Snake).

Artist’s illustration of the pulsar and its orbiting planet. The blue line represents radio waves, and the gold circle represents the circumference of our sun. Image Credit: Swinburne Astronomy Productions

An international team of researchers, led by Matthew Bailes of Swinburne University of Technology in Melbourne, Australia first detected the unusual pulsar – PSR J1719-1438 – using the Parkes radio telescope in Australia. They followed up their discovery with the Lovell radio telescope in the UK and one of the Keck telescopes in Hawaii.

As pulsars spin, they emit a beam of radio waves. As the radio beam sweeps repeatedly over Earth, radio telescopes can detect a regular pattern of pulses, akin to the pulsing light from a lighthouse.

As they watched PSR J1719-1438, the astronomers noticed that the arrival times of the pulses were systematically modulated. They attributed the modulations to the gravitational pull of a small companion planet, orbiting the pulsar in a binary system.

Parkes radio telescope. Image Credit: David McClenaghan, CSIRO

The modulations in the radio pulses tell astronomers several things about PSR J1719-1438’s hypothetical diamond planet.

First, it orbits the pulsar in just two hours and ten minutes, and the distance between the two objects is 372,823 miles (600,000 km) – a little less than the radius of our sun.

Second, the companion must be fewer than 34,175 miles (55,000 km) in diameter – that’s about five times the Earth’s diameter. The planet is so close to the pulsar that, if it were any bigger, it would be ripped apart by the pulsar’s gravity.

But despite its small size, the planet has slightly more mass than Jupiter. According to Bailes, the high density of the planet provides a clue to its origin.

A Star is Torn

Astronomers think it is the companion that, in its star form, transforms an old, dead pulsar into a millisecond pulsar by transferring matter and spinning it up to a very high speed. Pulsar J1719-1438 spins more than 10,000 times per minute and has a mass of about 1.4 times that of our sun, but is only 12.4 miles (20 km) in diameter. About 70 percent of millisecond pulsars have companions of some kind.

Pulsar J1719-1438 and its companion are so close together that the companion can only be a very stripped-down white dwarf, one that has lost its outer layers and over 99.9 percent of its original mass.

Researcher Michael Keith said:

This remnant is likely to be largely carbon and oxygen, because a star made of lighter elements like hydrogen and helium would be too big to fit the measured orbit.

This kind of density means the material is certain to be crystalline – that is, a large part of the star may be similar to a diamond.

Team member Benjamin Stappers from the University of Manchester said:

The ultimate fate of the binary is determined by the mass and orbital period of the donor star at the time of mass transfer. The rarity of millisecond pulsars with planet-mass companions means that producing such exotic planets is the exception rather than the rule, and requires special circumstances.

Results of the diamond planet study appear in the August 25, 2011 online issue of the journal Science.

Bottom line: An international team of researchers, led by Matthew Bailes of Swinburne University of Technology in Melbourne, Australia, detected an unusual pulsar – PSR J1719-1438 – using the Parkes radio telescope in Australia, later discovering a companion planet, likely composed of diamond. The results of their research appear in the August 25, 2011 issue of Science.

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August 26, 2011

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