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October 29, 2001

Astronomers discover new planets with solar system-like orbits

By Tim Stephens

An international team of astronomers has discovered eight new extrasolar planets, including at least two that travel in circular orbits similar to those of Earth and Mars. Planet hunters have now detected nearly 80 planets orbiting nearby stars, but most of them have elongated, or "eccentric," orbits.

This is a possible scene from a moon orbiting the extrasolar planet in orbit around the star HD23079. The planet is about three times the mass of Jupiter and orbits the star in 628 days, with a nearly circular orbit of one and a half times the Earth-Sun distance (almost the same as that of Mars). Artist's impression by David A. Hardy, courtesy of the Particle Physics and Astronomy Research Council (United Kingdom).
The latest discoveries strengthen the likelihood of finding solar systems analogous to our own, said UCSC professor of astronomy and astrophysics Steven Vogt. Vogt is a member of the team that made the latest discoveries and lead author of a paper submitted to the Astrophysical Journal describing some of the new planets.

"Most of the planetary systems we've found have looked like very distant relatives of the solar system--no family likeness at all," Vogt said. "Now we're starting to see something like second cousins. In a few years' time we could be finding brothers and sisters."

The newly detected planets range in mass from 0.8 to 10 times the mass of Jupiter, the largest planet in our solar system. They orbit their stars at distances ranging from about 0.07 to 3 astronomical units (one astronomical unit is the distance from the Sun to Earth).

"As our search continues, we're finding planets in larger and larger orbits," Vogt said.

The further a planet lies from its star, the harder it is to detect because it takes longer to complete an orbit. Only recently have astronomers begun to make the precise measurements over long observing times needed to detect such planets.

Three planets previously detected by the same team also have approximately circular orbits. Two of them orbit a star in the Big Dipper constellation called 47 Ursae Majoris and another orbits the star Epsilon Reticulum. The host stars for the two newly discovered planets with circular orbits are known as HD23079 and HD4208.

The astronomers--from the United States, Australia, Belgium and the United Kingdom--are searching the nearest 1,200 stars for planets similar to those in our solar system, particularly Jupiter-like gas giants. Their findings will help astronomers assess the solar system's place in the galaxy and whether planetary systems like our own are common or rare.

"This result is very exciting," said Anne Kinney, director of NASA's Astronomy and Physics Division. "To understand the formation and evolution of planets and planetary systems we need a large sample of planets to study. This result, added to others in the recent past, marks the beginning of an avalanche of data which will help to provide the answers."

To find evidence of planets, the astronomers use a high-precision technique developed by Paul Butler of the Carnegie Institution of Washington and Geoff Marcy of UC Berkeley to measure how much a star "wobbles" in space as it is affected by a planet's gravity.

Two groups of planet hunters, with some shared members, were involved in the latest findings. The Keck Planet Search Team is funded by the National Science Foundation and NASA, and the Anglo-Australian Planet Search Team is supported by the U.K. and Australian governments.

The Keck team includes Vogt, Marcy, Butler, Kevin Apps (University of Sussex), Debra Fischer (UC Berkeley), and Gregory Laughlin (UC Berkeley). The Anglo-Australian team includes Marcy, Butler, Hugh R. A. Jones (Liverpool John Moores University), Alan J. Penny (Rutherford Appleton Laboratory), Chris G. Tinney (Anglo-Australian Observatory), Brad Carter (University of Southern Queensland), and Chris McCarthy (Carnegie Institution).

For most of their discoveries, the astronomers have used the Keck 10-meter telescope on Mauna Kea, Hawaii; the 3-meter Shane Telescope at Lick Observatory on Mt. Hamilton; and the 3.9-meter Anglo-Australian Telescope in New South Wales, Australia.

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