A girl's best friend may have just gotten a whole lot bigger with the news that an international research team has discovered a small planet they think may be made of diamond. Although the planet is calculated to have a diameter of less than 60,000 km - which is about five times the diameter of Earth - it has slightly more mass than Jupiter. With the planet likely to be made largely of oxygen and carbon, its high density means it is almost certainly crystalline, meaning that a large part of the planet may be similar to a diamond.

The discovery was made by a team of researchers from Australia, Germany, Italy, the U.K. and the U.S., led by the Swinburne University of Technology's Professor Matthew Bailes. Using the CSIRO Parkes radio telescope in Australia, Lovell radio telescope in the U.K. and one of the Keck telescopes in Hawaii, they identified an unusual star called a pulsar known as PSR J1719-1438 located 4,000 light-years away in the constellation of Serpens in our Milky Way galaxy.

Pulsars are small spinning stars only around 20 km (12 miles) in diameter that emit a beam of radio waves. As the star spins, the emitted radio waves sweep repeatedly over Earth where radio telescopes are able to detect a regular pattern of radio pulses.

The astronomers noticed that the arrival times of the pulses from PSR J1719-1438 were systematically modulated and concluded that the gravitational pull of a small companion planet orbiting the pulsar in a binary system was to blame. The modulations tell the astronomers that the planet orbits the pulsar in just two hours and ten minutes, and that the distance between the two objects is 600,000 km - which is a little less than the radius of our Sun.

Because if it were any bigger it would be ripped apart by the pulsar's gravity, they also know that the companion planet must be small at less than 60,000 km in diameter. With slightly more mass than Jupiter, which has a diameter of almost 143,000 km, it is the planet's high density that Professor Baines says provides a clue of its origin.

The researchers believe that the "diamond planet" is the remnant of a once-massive star, most of whose matter was siphoned off towards the pulsar. PSR J1719-1438 is what is known as a millisecond pulsar because it spins very fast - rotating more than 10,000 times a minute. It also has a mass roughly 1.4 times that of our Sun, yet is only 20 km in diameter.

With around 70 percent of millisecond pulsars having companions of some kind, astronomers think that it is the companion in its star form that 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, which is most often a white dwarf. However, because PSR J1719-1438 and its companion are so close together, the researchers say the companion must be one that has lost its outer layers and over 99.9 percent of its original mass.

"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 the CSIRO's Dr Michael Keith (CSIRO), one of the research team members.

Although there are a lot of stars "twinkling like a diamond in the sky," don't expect "diamond planets" to be all that common.

"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," said Dr Benjamin Stappers from the University of Manchester.

The "diamond planet" discovery is reported in the journal Science.