A small planet-sized object made of solid diamond has been found orbiting a pulsar 4,000 light years away in the constellation of Serpens.

The detection of pulsar PSR J1719-1438 was made using the 64-metre radio telescope in Parkes, Australia, and confirmed with the Lovell radio telescope in the UK and one of the Keck telescopes on Hawaii. Pulsars are extremely dense city-sized stars, cramming 1.4 solar masses into a diameter of just 20 kilometres and spinning rapidly; PSR J1719-1438 rotates at a dizzying 10,000 times per minute.

The astronomers, from Australia, Germany, Italy, the UK and USA, noticed that the arrival times of the pulses were systematically modulated, suggesting the gravitational pull of a small companion object, such as a planet. By analysing the modulations of the pulses the astronomers learned that the companion completes one circuit around the pulsar in just two hours and ten minutes, and that the distance between the two objects is 600,000 kilometres. The object must also be no more than 60,000 kilometres wide – or else it would be ripped apart by the pulsar's gravity – but has a Jupiter-like mass.

The density of the body is consistent with one composed largely of carbon and oxygen and is thought to be a stripped down white dwarf star which has had the majority of its mass ripped away by the pulsar.

"There is no doubt it was once a star," Benjamin Stappers of the University of Manchester, UK, tells Astronomy Now. "So far this is the only system that we know about that formed this way, and so we believe that it is quite rare."

About 70 percent of millisecond pulsars have companions of some kind, but in this new example astronomers think that it is the companion in its original star form that transformed an old, dead pulsar into a millisecond pulsar by transferring matter and spinning it up.

“We know of a few other systems, called ultra-compact low-mass X-ray binaries, that are likely to be evolving according to the scenario above and may likely represent the progenitors of a pulsar like J1719-1438,” says team member Andrea Possenti, Director at INAF-Osservatorio Astronomico di Cagliari.

The ultimate fate of the binary is determined by the mass and orbital period of the donor star at the time of mass transfer. "We believe that the system is now in a stable phase of its life cycle and so could stay like this from now on," says Stappers. "We certainly will be studying the system further. We would like to do deeper infrared observations and also try and measure the sped with which the object moves across the sky which may tell us more about the origin and the future of the system."

The discovery was made during a systematic search for pulsars over the whole sky and found among 200,000 gigabytes of data. The results are published in this week's issue of the journal Science.