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Over The Horizon

Pulsar Planets

A UK group believes they may have solved the difficult question of how planets may occur around pulsars.

Pulsars are small, massively dense neutron stars formed after certain types of stars collapse. Much of the star's original mass is blown off, destroying any planets.

Millisecond pulsars spin hundreds of times a second. They may have been made to spin so fast by streams of gas falling onto them from a companion star.

Last year, a US team spotted two planets orbiting the millisecond pulsar PSR 1257+12. This pulsar has no companion star, and the UK group suggests that the pulsar has destroyed its stellar partner, producing planets in the process.

As gas from the companion fell onto the pulsar, the pulsar sped up and become a powerful radiation source, boiling away its companion's outer layers. A "black widow" pulsar discovered in 1988 is currently doing this to its companion.

As the companion star is "eaten", its interior swells out and gets stripped away, until the star is completely disrupted. The remains form a narrow gas ring around the pulsar. The inner portions fall on the pulsar, while the outer regions cool, thinning into a disc from which planets may be formed.

PSR 1257+12, in the constellation of Virgo, is thought to have two planets of about three times the Earth's mass orbiting the pulsar in 67 and 92 days respectively. Astronomers have treated the observation with caution, having been stung by the revelation that a previous planet discovered around another pulsar was simply caused by a mistake in analysis.

The formation theory now provides an understanding of how planets can form around fast pulsars. The theory predicts that many rapidly rotating pulsars will have planets, and the UK team are waiting to see if more planets will be found around millisecond pulsars.

Graham Blow, Frank Andrews, Tim Banks, Carter Observatory

Tim Banks is a PhD student at Victoria University, doing work at Carter Observatory.