The discovery of a small, fast-spinning star known as a pulsar has led astronomers to detect a companion planet, likely made of diamond.

This is only the second time a planetary mass has been found orbiting a pulsar. The first planetary system outside of our solar system was discovered around a pulsar in 1992.

The international research team led by Matthew Bailes of the Swinburne University of Technology in Melbourne published its findings in the journal Science.

Co-investigator Willem van Straten, also at Swinburne, said that this millisecond pulsar (MSP), so named because it rotates thousands of times per second, was detected in the Milky Way galaxy by the Parkes radio telescope.

"To date, we've discovered 20 millisecond pulsars in the High Time Resolution Universe survey. All of them are in binary systems [where one object orbits around another], but only this one has a companion with a planet-like mass," said van Straten.

He added that pulsars have a strong gravitational field and are about the size of a small city, 20 kilometres in diameter.

"If you were to squash a 13-metre cube of steel down into a teaspoon — this is the density of a pulsar."

As pulsars spin, they emit a regular, clock-work radio beam — similar to the light from a lighthouse. Tiny fluctuations in the radio signals from the pulsar, known as PSR J1719-1438, suggested something was pulling on it.

Further information about the pulsar was collected by two other radio telescopes, one in England and the other in Hawaii. This revealed that a small companion planet is orbiting the pulsar every 2.2 hours.

Former star

It is likely the planetary body is all that remains of a once massive star that has lost 99.9 per cent of its original mass to leave it a shadow of its former self — just five times bigger than Earth.

But, despite its small size, the planet has slightly more mass than Jupiter.

"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 CSIRO scientist and study co-author Michael Keith.

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 also calculated that the distance between the pulsar and its planetary companion is approximately 600,000 kilometres, 1.5 times the distance between the Earth and the Moon.

Both are located 4,000 light years away in the constellation of Serpens (the snake), one eighth of the distance to the centre of our galaxy.

"We are still interested in seeing if we can see the companion planet — if one side is [always] facing the pulsar [tidally locked], we should be able to see it being heated up by the radiation. An orbital variation in the brightness would tell us that one side of the planet is basically being roasted," says van Straten.

According to van Straten, the Parkes telescope was used for the survey because it can scan the sky 13 times faster than "your average telescope."

"We point the telescope at a patch of sky for nine to 10 minutes and look for radio signals that occur periodically. You end up with a lot of candidates and then have to rule out interference," he says.

For every 10 minutes of data collected it takes several hours of computing power to sift through it. The astronomers estimate enough data to fill 40,000 high-capacity DVDs was generated in the search for PSR J1719-1438.

But he added that the "holy grail" of the survey would be to find a pulsar that is orbiting a black hole.

"This would be the biggest prize because you could study the structure of space time in the vicinity of the black hole ... and it would allow us to test Einstein's theory of gravity, known as general relativity."