By Shankar Vedantam
Washington Post Staff Writer
Thursday, August 16, 2001; Page A01

Astronomers have found a planetary system remarkably similar to the sun's -- two planets traveling in circular orbits around a star in the Big Dipper.

The star is similar to the sun in chemical composition, and astronomers say the circular paths and sizes of the two planets hint at the presence of smaller, Earth-like bodies in tighter orbits.

If such inner planets exist, and if any are blessed with water, life might have flourished there. Even if that is not the case, scientists said the discovery boosts hopes that planetary systems with Earth-like planets will eventually be found.

"Of all the solar systems that have been found, this is the one that looks the most like our own," said Debra Fischer, an astronomer at the University of California at Berkeley. "Hands down. Nothing else is even close."

More than 70 planetary systems have been found around stars other than the sun, including three with multiple planets, but most have orbits that are sharply elliptical. Such orbits, which tend to freeze and heat the planet to extremes of temperature as it dives close to a star and then pulls far away, are poor candidates for life to gain a foothold.

The planets around this star -- named 47 Ursae Majoris -- are large, gaseous and Jupiter-like. If they had been circling the sun, their path would have been roughly between Mars and Jupiter, scientists said.

"For the first time, we have a star with two gas giant planets that are far away from the star, and we know there isn't a gas giant planet in the inner regions of the star," Fischer said. "From our perspective, this space is empty. But when you ask, 'What can you hide in this space?' -- you could hide Earth. This is the only star that has a big empty zone in the habitable region around a star, the place where water could exist."

In the last few years, astronomers have begun a systematic search for planetary systems that resemble the solar system. They have been hindered by vast distances of space, which have made it impossible to see planets in other star systems directly.

What astronomers can see are stars. As large planets revolve around their stars, they exert a gravitational pull that causes stars to wobble. The speed of the wobble indicates how long a planet takes to circle the star.

By focusing extremely precise measuring techniques at 47 Ursae Majoris, which is about 45 light years (more than 200 trillion miles) away, astronomers measured wobbles of 36 feet per second -- about the speed of a human sprinter -- and inferred the presence of one of the large planets. The other, which is larger and closer to the star, had been discovered previously.

By comparison, Jupiter exerts a wobble on the sun of 40 feet per second. Earth, being much lighter, exerts a wobble of about 4 inches per second, Fischer said.

The tiny effect of small planets makes it impossible to use the technique to spot Earth-like planets around Ursae Majoris. The wobble is impossible to pick up. Researchers can spot wobbles as small as 10 feet per second and are working on getting the number down to about 3 feet per second, which will allow them to spot planets that are about 10 to 20 times the mass of Earth.

Astronomers said it was possible the large planets around Ursae Majoris could crowd out smaller ones but that large planets often imply the presence of smaller, Earth-like ones. The solar system has two large gaseous planets -- Jupiter and Saturn -- and a host of smaller planets, such as Earth, Mercury, Venus and Mars, that have orbits closer to the sun.

"Finding the large ones is like finding boulders and rocks strewn along the beach from afar," said Geoff Marcy, a professor of astronomy at the University of California at Berkeley who also worked on the discovery. "If you see them a mile offshore, that means there's probably sand there, too."

Large, gaseous planets may also help create the proper conditions for life on smaller ones. Paul Butler, a staff scientist in the department of terrestrial magnetism at the Carnegie Institution of Washington, said that in the development of the solar system, Jupiter played the essential roles of traffic cop and garbage disposal.

Jupiter's enormous mass -- larger than all the other planets combined -- sucked in meteors, comets and debris that would otherwise have constantly pelted Earth. And it enforced lane discipline -- all the planets in the solar system have roughly circular orbits and revolve in the same direction around the sun in a single plane.

"Jupiter acts as a bouncer in the solar system; it enforces circular orbits," said Butler, who worked with the Berkeley team on finding new planets. "If any planet's orbit should become non-circular, Jupiter will eject it."

It will probably be several years before researchers can tell whether Ursae Majoris has smaller planets. Two techniques are being discussed.

One involves positioning a telescope in space that would constantly take photos of thousands of stars. If a star has an Earth-like planet, about 1 in 200 stars, by sheer chance, will have planets that pass the star along Earth's line of vision. Such planets will block some light from the star. Scientists hope to use that tiny signal to detect the presence of smaller planets.

Butler said another proposal that could take even longer to implement involves assembling multiple telescopes in space that can block out light from a star and make its planets dimly visible, much in the way that blocking out the headlights of an oncoming car can make it possible to spot the car itself.

Scientists would then directly study the light spectrum of the planets, and search for the telltale signs of water and oxygen, the harbingers of life.