Artist's impression of the newly-discovered pulsar.

Credit: Swinburne Astronomy

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SYDNEY: A massive star that has been transformed into a small planet made of diamond has been discovered in the Milky Way.

The planet is orbiting a pulsar called PSR J1719-1438, which is 4,000 light-years away in the constellation of Serpens. It was discovered by an international team of researchers using the CSIRO Parkes Observatory telescope, 380 km west of Sydney, to scrutinise 90,000 different patches of sky for nine minutes each.

Pulsars occur when a star in a binary star system has collapsed to a small, degenerate neutron star. They are generally 20 km in diameter - the size of a small city - and emit a beam of radio waves that are picked up by telescopes on Earth in the form of radio pulses. If a pulsar has a companion planet, the planet's gravitational pull will regulate these pulses, which is how astronomers are able to detect them. About 70% of pulsars have stellar companions, but only about 1% of them have planetary
companions.

"We looked at an enormous amount of data/sky very quickly - competing surveys are not able to cover the sky so quickly," said lead author Matthew Bailes from Swinburne University in Melbourne, of the study published in Science today.

Greater density than Jupiter

By analysing the modulation of the radio pulses, astronomers can pick up the time it takes for a planet to orbit a pulsar, the distance between the two objects and the size of the planet.

The researchers determined that the 'diamond' planet must be less than 60,000 km in diameter and 600,000 km away from its pulsar. If the planet was any bigger, the pulsar would have torn it apart. The planet takes just two hours and ten minutes to complete its orbit.

Despite its relatively small size, the planet has a greater density than Jupiter, which has a diameter of 142, 984 km. "This high density of the planet provides a clue to its origin," said Bailes.

Super-fast pulsar

The planet is the remnant of a massive star, which used to form a binary system with Pulsar J1719-1438. The researchers think that as the star drew closer and closer to the pulsar, it caused the pulsar to start spinning at a very high speed while siphoning matter from the surface of the star. In this process, the star lost both its outer layers and over 99.9% of its original mass.

The star ended up a small, cold and fusion-less planet, known as a white dwarf, and Pulsar J1719-1438 became what's known as a fast-spinning millisecond pulsar. "It spins at about 10,000 revolutions per minute," said Bailes. Millisecond pulsars are more massive than the Sun, but with all their mass compressed into an object a few kilometres across.

"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," said team member Andrea Possenti, director at the Italian National Institute for Astrophysics' Observatory in Cagliari.

The 'diamond planet'

The planet has been nicknamed the 'diamond planet', which refers to its carbon-based core. "We think it has a very evolved core that contains carbon in a crystalline structure, like diamond," said Bailes. Judging by the orbiting times of the planet, the researchers were able to determine that it is largely made up of carbon and oxygen, and judging by its density, they concluded that the material is certain to be crystalline.

"The ultimate fate of the binary is determined by the mass and orbital period of the donor star at the time of mass transfer. 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 co-author Benjamin Stappers from Britain's University of Manchester.

"Normally if we found an object of this mass we would assume that it was indeed a planet similar to Jupiter, a gas-giant planet. However, in this case the scientists are able to show that it is much denser than any normally planet could be," commented Jeremy Bailey from the University of New South Wales in Sydney, who was not involved in the study.

"They conclude that it is instead the remaining core of a star which has had all its outer layers stripped away leaving an object quite unlike anything else we know, one with the mass of a planet but a composition like that of a white dwarf star. They describe the process as being the 'transformation of a star into a planet'. However, I suspect it will add to the controversy about exactly what is a planet and what is a star."