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Professor Geoffrey Marcy
"We know it is there"
 real 28k

Tuesday, 16 November, 1999, 12:08 GMT
Extrasolar planet detected
Planet The planet orbits its star once every 3.523 days (Artist's impression)

By BBC News Online Science Editor Dr David Whitehouse

Astronomers have watched a planet passing in front of a distant star. It is the first visual proof that planets exist outside our Solar System.

"For the first time in human history, we have conformation of a planet orbiting another star," Geoffrey Marcy, a professor of astronomy at the University of California, Berkeley, told the BBC.

We will be studying this thing like crazy for the next year
Geoffrey Marcy
"Indeed, we now have inferences of some 29 planets orbiting the nearest stars, and this tells us that our Milky Way galaxy contains billions - probably 100bn - planets, and it means that some of those planets must have the right conditions for life on them."

Astronomers look for a 'wobble' in stars as evidence for the possible presence of an orbiting planet. The wobble is the product of the gravitational interaction between the star and a nearby, large body.

Since 1995, Marcy and other astronomers have used this technique with great success. But last week they obtained the first direct evidence that a planet exists by observing the shadow cast as it passed in front of its host star.

'Shadow' planet

"This is the first planet we have been able to verify is real," Marcy said.

"This lends credibility to all the other planets we have found, and which may harbour life. It is very exciting."

Astronomers have yet to name the planet - this will be done later by an International committee - but Marcy wants it to bear the name "Silhouette" or "Shadow" in tribute to the image captured by telescopes on 7 November.

The planet is 18 times the size of the Earth and has a surface temperature of about 2,000 C (3,500 F). It circles a star similar to ours every 3.5 Earth days in an orbit that is 20 times closer than the Earth is to the Sun. The star is on our cosmic doorstep, only 150 light years away.

"We will be studying this thing like crazy for the next year," Marcy said.

Arizona observatory

The discovery took place after Marcy and his colleagues, Paul Butler of the Department of Terrestrial Magnetism at the Carnegie Institution, Washington, and Steve Vogt of University of California, Santa Cruz, and Lick Observatory, first detected a wobble in the star called HD 209458 on 5 November.

Believing that the wobble was due to a nearby planet, the team immediately contacted Greg Henry, an astronomer at the Tennessee State University.

Henry turned one of his automated telescopes in Arizona on HD 209458 at precisely the time Marcy and colleagues predicted from their orbital calculations the planet might cross the face of the star.

The calculations were spot on and Henry observed a 1.7% dip in the star's brightness on 7 November.

Bloated planet

"This planetary transit occurred at exactly the time predicted from Marcy's observations, confirming absolutely the presence of a companion," Professor Henry said.

"We knew that sooner or later we would find one," Marcy said. "The amount of dimming of the star's light during the transit also gives us the first-ever measure of the size and density of an extrasolar planet."

The calculations suggest the planet has only 63% of Jupiter's mass while its radius is 60% larger. This fits with theories that predict a bloated planet when, as here, the planet is very close to the star.

"This supports the theory that extrasolar planets very near their star did not form where they are, but formed farther out and migrated inward," Henry said.

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See also:
13 Oct 99 |  Sci/Tech
A planet beyond Pluto
28 Sep 99 |  Sci/Tech
Star wobbles under tug of planet
21 Apr 99 |  Sci/Tech
Out of this world discovery
25 Jan 99 |  Sci/Tech
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