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Thursday, April 15, 1999 Published at 13:04 GMT 14:04 UK


Out of this world discovery

The two known solar systems compared

By BBC News Online Science Editor Dr David Whitehouse

Astronomers have discovered the first solar system other than our own. It has three planets orbiting a star that is 44 light years away.

The BBC's Christine McGourty: The planets are huge compared to ours
The discovery of a group of planets orbiting a sun like our own will cause great excitement and raises the chances that there may be other forms of life in the Universe.

"It is one of the most important astronomical discoveries for decades," Professor Geoff Marcy, one of the discoverers of the system, told BBC News Online.

The star concerned is called Upsilon Andromedae. Although it is four hundred thousand billion kilometres from Earth, it is easily visible to the unaided eye in the night sky.

Jupiter-sized planets

Geoff Marcy: It's a very important stepping stone
In 1996, astronomers Geoff Marcy and Paul Butler of San Francisco State University (Sfsu) detected evidence that a planet, about the same mass as Jupiter, was circling the star.

Now they have discovered that two other planets are also orbiting Upsilon Andromedae.

Astronomers from the Harvard-Smithsonian Center for Astrophysics in Massachusetts have also made observations of the star and confirmed the findings.

The discovery is a momentous one. It offers the first evidence that solar systems like our own could be commonplace in our galaxy.

Planet formation

Until now, astronomers had detected many Jupiter-sized planets orbiting nearby solar-type stars but not a solar system.

Geoff Marcy: When I realised there were three planets
"It implies that planets can form more easily than we ever imagined, and that our Milky Way is teeming with planetary systems," said Dr Debra Fischer of Sfsu.

The innermost of the three planets circles Upsilon - and only about 8 million kilometres from its surface. Its 'year' would be only 4.6 days. Although the planet is estimated to be about the mass of Jupiter, it must be a completely different world.

Being so close to its parent star, it must be very hot on its starward side. Its other side may always look away from its sun and may be very cold. Huge storms would rack the planet as heat is passed around the world.

Massive planets

The other two planets are thought to be somewhat larger. One would have a mass of about 2 Jupiters and take 242 days to circle Upsilon, in an oval orbit about 129 million kilometres from the star.

The third planet is even more massive, about 4 times the mass of Jupiter. It is even further away at 400 million kilometres. It takes about 4 years to circle the star.

No current theory of planetary formation predicted that so many giant worlds would form around a star.

"I am mystified at how such a system of Jupiter-like planets might have been created," said Professor Marcy.

"This will shake up the theory of planetary formation," adds Robert Noyes, Professor of Astronomy at the Harvard-Smithsonian Center for Astrophysics.

The research data will be published in the Astrophysical Journal in July. News of the discovery has also been posted on Marcy and Butler's Website and that of the Extrasolar Planets Encyclopedia.

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