Friday April 16 1:55 AM ET

Astronomers Discover Three-Planet Solar System

By Deborah Zabarenko

WASHINGTON (Reuters) - Astronomers said Thursday they had discovered the first solar system outside our own, with three massive planets orbiting a Sun-like star.

Scientists from California, Arizona, Massachusetts and Australia have all reached the same conclusion, that three giant gas planets are circling a bright star called Upsilon Andromedae in the constellation Andromeda.

None of Upsilon Andromedae's planetary companions are like Earth, and would not be hospitable to earthly life. And even if an Earth-type planet existed in this system it might risk being obliterated by its huge sister planets, astronomers said.

But planet-hunting astronomer Geoffrey Marcy said this discovery suggested that millions of Earth-like planets may reside in our Milky Way galaxy, and some of them may lie the right distance from their host stars to allow for liquid water and the possibility of life.

``Today, with the discovery of the first planetary system beyond our own, we are witnessing, I think, the emergence of a new era in human exploration,'' Marcy, a professor at San Francisco State University, said at a San Francisco news conference monitored by telephone in Washington.

The discovery also prompts profound questions about Earth's place in the galaxy, which has 200 billion stars that might possibly harbor planets, Marcy said.

``Is our own solar system unusual in some way? Is the architecture of our solar system some sort of cosmic quirk of nature?'' he said. ``We may live in a somewhat bizarre planetary system and we just don't know it yet.''

Marcy and his colleagues have been looking for extrasolar planets since 1987, and finding them since 1995. There are now at least 20 such planets, including those around Upsilon Andromedae.

But never before have several planets been detected orbiting a Sun-like star, comparable to the nine-planet grouping that contains Earth.

The Andromedae trio features one planet quite close to the star, a body about three-fourths the mass of Jupiter, which whips around its host star once every 4.6 days. The next planet is about twice Jupiter's mass and orbits the star every 242 days at about the distance of Venus from the Sun.

The third planet, about four times Jupiter's mass, orbits the star every 3.5 years at a distance about two and a half times the distance Earth is from the Sun.

The two outermost planets are similar to other extrasolar planets identified by astronomers: they have elliptical orbits that could cross the circular orbit of an Earth-like planet, if one existed, which could fling this new world into space.

The new solar system is about 44 light years away from Earth, relatively nearby in cosmic terms. A light year is the distance light travels in a year, or about six trillion miles (10 trillion km).

Upsilon Andromedae can be seen with the naked eye in the northern sky from June to February. The planets cannot be seen, even with telescopes, due to the brightness of the star and the planets' closeness to it.

Astronomers detected these extrasolar planets by monitoring the host star's characteristic wobble in space, resulting from the gravitational tugs of the planets. A planet the size of Earth would not exert enough gravitational force to cause a discernible wobble, the astronomers said.

However, Upsilon Andromedae is considered a good candidate for the upcoming Space Interferometry Mission (SIM), a project of the National Aeronautics and Space Administration scheduled for launch in 2005, and aimed at probing nearby stars for Earth-like planets.