Pulsars represent the very last stages of star formation. They are rapidly rotating neutron stars the size of a small city which emit a highly focused beam of radio waves. As the star rotates, the beam sweeps over Earth -- like the beam of light from a lighthouse -- and radio telescopes detect a regular signal which seems to pulsate: thus the name pulsar.

With the newly discovered pulsar, which is known as PSR J1719-1438, the astronomers noticed a regular modulation in the arrival times of the signals. This "disturbance" is caused by the gravitation of a low-mass companion. The type of modulation tells the researchers quite a lot about the small celestial body: it has a diameter of only 60,000 kilometres and is therefore half as big as Jupiter. It orbits the pulsar in just two hours and ten minutes, at a distance of 600,000 kilometres -- that's only slightly less than the radius of our sun. Thus the planet orbits so close to the pulsar that its gravity should really rip it apart.

"The density of the planet is at least as great as that of platinum; this tells us a lot about its origin," says the team leader, Matthew Bailes from Swinburne University of Technology in Australia. The scientists believe that the companion planet is the tiny core of a once-massive star. It only just managed to escape destruction as the rest of its material was being siphoned off by the pulsar.

J1719-1438 is an extremely rapidly rotating type of neutron star, which is known as a millisecond pulsar. It spins around its own axis more than 10,000 times per minute, has 1.4 times the mass of the sun, but a radius of only 20 kilometres or so. Around 70 percent of pulsars have partners of various types. The astronomers suspect that it is these companions which transfer mass when they are still stars and thus accelerate an old, slowly rotating pulsar to a very high orbital speed. The result is a rapidly rotating millisecond pulsar with a companion whose mass is greatly diminished -- often a white dwarf.

In PSR J1719-1438 the pair is so close together that the companion can only be a white dwarf with greatly reduced mass which has lost all its outer layers and more than 99.9 percent of its original mass. The remnant must consist predominantly of carbon and oxygen because lighter elements, such as hydrogen and helium, cannot explain the data obtained from the observations. The density derived leads to the conclusion that the material is certain to be in a crystalline state; a large part of the star could therefore have a diamond-like structure.

"The ultimate fate of this binary star depends on the mass and orbital period of the donor star at the time the mass was transferred. The rare occurrence of millisecond pulsars with planet-mass companions means that the formation of such exotic planets is the exception rather than the rule, and requires a combination of special circumstances," says Benjamin Stappers from the University of Manchester.

The data on the pulsar-planet pair were confirmed with follow-up observations with the Lovell radio telescope in Great Britain and one of the two Keck telescopes on Hawaii. The system is around 4,000 light years away towards the Serpens (Serpent) constellation in the plane of our Milky Way. The pulsar itself was identified among a total of 200,000 gigabytes of data -- with the help of special analytical programs on supercomputers at Swinburne University of Technology, the University of Manchester and the INAF-Osservatorio Astronomico di Cagliari on Sardinia.

The project is part of a systematic search for pulsars in the entire firmament in which the 100-metre Effelsberg radio telescope, belonging to the Max Planck Institute for Radio Astronomy, contributes measurements in the northern hemisphere. "What we have here is the largest and most sensitive mapping of pulsars in the whole sky ever conducted," says Michael Kramer, Director at the Max Planck Institute in Bonn. "We anticipate a series of exciting new findings with this programme. It is good to see that a start has already been made and more will follow. After all, we want to find out a great deal more about pulsars and fundamental physics in the years to come."

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