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Tuesday, 30 November, 1999, 05:30 GMT
...and then six come along at once
Keck Observatory: High up in Hawaii Keck Observatory: High up in Hawaii

By BBC News Online science editor Dr David Whitehouse

The world's most prolific team of planet hunters have found six new planets orbiting nearby stars. This brings to 28 the total number of planets astronomers have detected outside our Solar System (so-called "extrasolar planets" or "exoplanets").

The researchers also found evidence suggesting that two previously-discovered planets have additional companions.

The team consisted of Geoffrey Marcy of University of California Berkeley, Paul Butler of the Department of Terrestrial Magnetism at the Carnegie Institution of Washington DC, Steven Vogt, of UC Santa Cruz, US, and Kevin Apps of the University of Sussex, UK. They made the discoveries using the high-resolution spectrograph on the Keck I Telescope in Hawaii.

The findings are announced in an International Astronomical Union telegram.

Similar to Jupiter

The new planets orbit stars that are similar in size, age, and brightness to our Sun and are some 65 to 192 light-years from Earth.

The planets themselves range in mass from slightly smaller to several times larger than the planet Jupiter (0.8 to 6.5 times the mass of Jupiter). They are probably also similar to Jupiter in being giant balls of hydrogen and helium gas.

"These planets are at just the right distance, with temperatures in one case around 108 degrees Fahrenheit - like a hot day in Sacramento."
Professor Steven Vogt
The presence of a planet around a star is indicated by a wobble in the motion of the star as a result of the gravity exerted by the orbiting body.

The orbits of the new planets, like those of most of the extrasolar planets discovered so far, tend to be quite elliptical. One of the planets, around a star called HD 222582, has the most elliptical orbit yet known. It goes as close as 0.39 astronomical units from its parent star - an AU is the distance from Earth to the Sun - to as far as 2.31 AU during its 576-day orbit.

Some astronomers speculate that neat, circular orbits such as we see in our own Solar System are relatively rare. Significantly, for those who believe that finding exoplanets means the chances of finding life in space are also increased, five of the six planets are located within the so-called "habitable zones" of their stars.

Too hot or too cold

This is the region where temperatures would allow water to exist in liquid form. Most of the extrasolar planets the researchers have studied have turned out to be outside the habitable zone, either too close to their star or too far away, and therefore too hot or too cold.

"These planets are at just the right distance, with temperatures in one case around 108 degrees Fahrenheit- like a hot day in Sacramento," Professor Vogt said.

In addition to the discovery of six new planets, the researchers gathered new data on four previously known planets. Two of them, around the stars HD 217107 and HD 187123, showed long-term trends in their orbits indicating the presence of an additional companion.

This is significant because previously only one other system of multiple planets has been detected - that around the star Upsilon Andromedae.

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See also:
15 Nov 99 |  Sci/Tech
Extrasolar planet detected
25 Nov 99 |  Sci/Tech
We saw it too, say astronomers
22 Nov 99 |  Sci/Tech
Light detected from distant planet
25 Sep 98 |  Sci/Tech
British student shows Nasa new planet

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