DIAMOND AS BIG AS A PLANET found, say Aussies

Colossal Jupiter-weight gemstone orbits far-flung pulsar

Dr Who fans will remember the episode “Midnight”, set on a diamond planet. That’s what a team of astronomers from Australia, Germany, Italy, the UK and the USA believe they’ve found circling a star 4,000 light-years distant.

Don’t expect the object to become a tourist destination anytime soon, however: not only is it too far away to reach, its orbit around its star is believed to be smaller than the radius of the Sun.

The researchers first detected the unusual star using the 64-meter Parkes radiotelescope in Australia, and confirmed their findings with follow-up observations from the Lovell radiotelescope in the UK and one of the Keck telescopes in Hawaii.

The “diamond planet” (actually a former star) was detected circling a newly-discovered pulsar, PSR J1719-1438. The modulation of its pulses suggested to the astronomers that the pulsar had an orbiting companion.

The characteristics of that companion turned out to be surprising. From their observations, the scientists say the companion’s orbit is 600,000 km away from the pulsar; its own radius has to be less than 60,000 km or tides would destroy it; but although small, the object has more mass than Jupiter.

Source: CSIRO

“The high density of the planet provides a clue to its origin,” says professor Matthew Bailes of Swinburne University of Technology in Melbourne.

The team believes the orbiting companion of the millisecond pulsar (that is, it rotates once per millisecond) is a formerly-massive star that’s had most of its mass ripped away by the pulsar. PSR J1719-1438 has quite a pull, packing a mass 1.4 times that of the Sun into a package that’s just 20 kilometers in diameter.

The companion will have contributed to that high speed, causing the host pulsar’s rotation to accelerate as it grabs mass from its companion, turning it into a white dwarf.

According to the researchers, the companion and PSR J1719-1438 are so close together that the white dwarf must have lost 99.9 percent of its mass. And that’s what leads to the “diamond planet” conclusion.

“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 into the measured orbit,” said team member Dr Michael Keith of CSIRO. Its density is such that much of the carbon will be crystalline, making much of the star similar to diamond.

“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 Dr. Benjamin Stappers from the University of Manchester. The pulsar was found in an analysis of more than 200,000 GB of data, conducted on supercomputers at Swinburne, the University of Manchester, and the INAF-Osservatorio Astronomico di Cagliari, Italy.

Also involved in the search is the 100 meter Effelsberg radiotelescope at the Max Planck Institute for Radioastronomy in Germany.

The discovery is reported today in Science. Swinburne University has posted an animated impression of the pulsar and its companion here. ®

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