A planet made of diamond ?

A once-massive star that's been transformed into a small planet made of diamond: that's what astronomers think they've found in our Milky Way.

The discovery, reported in Science, was made by an international research team led by Professor Matthew Bailes, Pro Vice-Chancellor (Research) at Swinburne University of Technology in Melbourne and the 'Dynamic Universe' theme leader in a new wide-field astronomy initiative, the ARC Centre of Excellence for All-sky Astrophysics (CAASTRO).

The researchers, from Australia, Germany, Italy, the UK and the USA, first detected an unusual star called a pulsar which are small spinning stars about 20 km in diameter -- the size of a small city -- that emit a beam of radio waves.

As the star spins and the radio beam sweeps repeatedly over Earth, radio telescopes detect a regular pattern of radio pulses.

For the newly discovered pulsar, known as PSR J1719-1438, the astronomers noticed that the arrival times of the pulses were systematically modulated. They concluded that this was due to the gravitational pull of a small companion planet, orbiting the pulsar in a binary system.

The pulsar and its planet are part of the Milky Way's plane of stars and lie 4,000 light-years away in the constellation of Serpens (the Snake). The system is about an eighth of the way towards the Galactic Centre from Earth.

The modulations in the radio pulses tell astronomers several things about the planet.

First, it orbits the pulsar in just two hours and ten minutes, and the distance between the two objects is 600,000 km -- a little less than the radius of our Sun.

Second, the companion must be small, less than 60,000 km (that's about five times 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."This high density of the planet provides a clue to its origin," said Professor Bailes.

The team thinks that the 'diamond planet' is all that remains of a once-massive star, most of whose matter was siphoned off towards the pulsar.

Pulsar J1719-1438 is a very fast-spinning pulsar -- what's called a millisecond pulsar. Amazingly, it rotates more than 10,000 times per minute, has a mass of about 1.4 times that of our Sun but is only 20 km in diametre. About 70 per cent of millisecond pulsars have companions of some kind. 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. The result is a fast-spinning millisecond pulsar with a shrunken companion -- most often a so-called white dwarf.

"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 orbiting times," said Dr Michael Keith (CSIRO), one of the research team members.The density means that this material is certain to be crystalline: that is, a large part of the star may be similar to a diamond.

The team found pulsar J1719-1438 among almost 200,000 Gigabytes of data using special codes on supercomputers.

The discovery was made during a systematic search for pulsars over the whole sky.

- ScienceDaily