Earth: no longer the lonely planet


image The question of whether we're alone in the universe just got a lot bigger.

Two astronomers from the University of New South Wales, Australia - Dr Charles Lineweaver and Daniel Grether - have found that at least 25 per cent of Sun-like stars have planets.

"This means there are at least 100 billion stars with planets in our Galaxy," says Dr Lineweaver, a Senior Research Fellow at the University's School of Physics.

Until now, astronomers believed that only five to 15 per cent of Sun-like stars had orbiting planets, but Lineweaver and Grether's work shows that previous estimates under-reported the proportion of so-called extrasolar planets.

The Astrophysical Journal, the world's leading journal of astrophysics, has accepted their research for publication.

Astronomers have been carefully monitoring 2,000 nearby stars for the presence of orbiting extrasolar planets.

"To date, they've detected a hundred or so, meaning the fraction of stars with extrasolar planets was around five per cent," says Dr Lineweaver.

"But most planets are too small or take too long to orbit their host stars to be detected. For example, if the Sun were one of the stars being monitored, we still wouldn't have detected any planets around it.

"Using a new method to correct for this incompleteness, we found that at least 25 per cent of Sun-like stars have planets."

Dr Lineweaver believes that the figure of at least 100 billion stars with orbiting planets could be on the low side when it comes to cosmic counting. It could be that close to 100 per cent of stars have planets.

"Given that there are about 400 billion stars in our Galaxy alone, it means there could be up to 400 billion stars with planets," he says.

"With about 100 billion galaxies in the observable universe, our result suggests that there are at least 10 trillion planetary systems in the Universe."

'What Fraction of Sun-like Stars have Planets?' by Charles H Lineweaver and Daniel Grether will be published later this year. It is available online.

Dr Lineweaver is an ARC Senior Research Fellow and Senior Lecturer, School of Physics, UNSW. Daniel Grether is working on a PhD.

Media contact: Dan Gaffney mob 0411 156 015 or UNSW Media Office ph (02) 9385 2864

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