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Monday, 15 October, 2001, 15:40 GMT 16:40 UK
Eight planets found around nearby stars
Pparc David A Hardy
Artist's impression of one of the newly discovered worlds
Image by David A Hardy

By BBC News Online science editor Dr David Whitehouse

An international team of astronomers has discovered eight new extrasolar planets. Three of them are Jupiter-sized planets in circular orbits around their parent stars.

These planets constitute a new class of extrasolar giant planets with orbits similar to those of the Earth and Mars in our own Solar System.

The eight new worlds bring the total number of planets known to be circling other stars to 74. With them, astronomers are starting to understand the statistics and formation of planetary systems.

"The previous planets discovered were very different from our own," said Dr Hugh Jones, of Liverpool John Moores University, UK. "We're now starting to see, if not twins, then second cousins."

Star 'wobble'

The new worlds were found by the Keck telescope in Hawaii, the Anglo-Australian Telescope (AAT) in New South Wales, and the Lick telescope in California.

To find evidence for the existence of these planets, the astronomers used a high-precision technique, developed by Dr Paul Butler, of the Carnegie Institute of Washington, and Dr Geoff Marcy, of the University of California at Berkeley, that measures how much a star "wobbles" in space as it is affected by a planet's gravity.

As an unseen planet orbits a distant star, the gravitational pull causes the star to move back and forth in space. This wobble can be detected in the star's light.

The Keck Planet Search found five of the new objects and the Anglo-Australian Planet Search found three. They range in mass from 0.8 to 10 times the mass of Jupiter, the largest planet in our Solar System.

How common

They have orbital periods between six days and six years. They orbit their stars at distances ranging from about 0.7 to 3 times the Earth-Sun distance - 149 million kilometres (93 million miles).

Astronomers say that the discoveries strengthen the statistics of planets, showing that 7% of all stars have Jupiter-size planets within three Earth-Sun distances of their star.

The long-term goal of this project is the detection of Solar System analogs - that is Jupiter-like planets in Jupiter-like orbits.

The discovery of such planets within the next decade will help astronomers assess the Solar System's place in our galaxy and whether planetary systems like our own are common or rare.

The first extrasolar planet ever discovered was found by the Swiss team of Michel Mayor and Didier Queloz in 1995.

See also:

16 Nov 99 | Sci/Tech
Extrasolar planet detected
25 Nov 99 | Sci/Tech
We saw it too, say astronomers
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