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Cosmic discovery may swell harvest for planet hunters

Astronomers say they've found a planet around a double-star system, byusing other galaxies as telescopes.

Scientists hunting alien planets have a new string for their bows. For the first time, the ability of an object's gravity to act like a lens has revealed another world, some three times more massive than Jupiter.

Hunters have identified nearly two-dozen planets by studying the wobbles they impose on the motion of their parent stars toward and away from Earth. Now gravitational lensing - an effect Einstein predicted - gives them another way to locate their quarry.

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Moreover, the new planet is the first ever found orbiting two stars. That's a bonus for members of the Microlensing Planet Search (MPS) team, who had expected their technique to reveal Jupiter-class planets around single stars. Instead, their report in today's issue of Nature may lead to the discovery of a whole new cache of planets never before seen.

The "result suggests that Jovian planets may be more common in short-period binary stars than in single-star systems," notes the report.

Astronomers have used gravitational lensing for decades in galactic research. Generally, the gravity of a foreground galaxy makes multiple images of a more remote galaxy that lies along the same line of sight from Earth.

Recently, though, astronomers realized that lensing can also be used to look at closer star systems within our own Milky Way galaxy. They call it microlensing.

Several research teams now patrol the skies looking for microlensing events. They hope these will help identify some of the so-called dark matter that surrounds our galaxy. Dark matter makes up most of the galaxy's mass, yet it is invisible to infrared, radio, optical, and X-ray telescopes.

Astronomers know that it's there because of the strength with which the Milky Way's gravity pulls on other nearby galaxies. If a dark matter object moves in front of a background star, the star appears to brighten as microlensing focuses its light.

Planet research is a potential secondary payoff. If the object being looked at is a star system, any planets inside would influence the lensing with their gravity. That's why dark matter researchers alert planet hunters to suspicious microlensing events. The MPS collaboration between American and Australian astronomers is one of several teams that follow up the clues.

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It reported tentative identification of an alien planet during an American Astronomical Society meeting in January. What it is announcing today is the much more definitive identification of a lensing system that, according to computer analysis, includes two stars and a planet.

"We're pretty confident the model is correct," says David Bennett, a physicist at the University of Notre Dame in Indiana, and a co-leader of the MPS team. Still, the group is planning further testing to make sure they're assessment is accurate.

That model shows the planet orbiting about seven astronomical units from the system's center of gravity. An astronomical unit (AU) is Earth's average distance from the sun - about 93 million miles.

Dr. Bennett says, as a detection technique, microlensing complements search by wobble, called the radial-velocity technique. "We are able to detect substantially lower-mass planets than the radial-velocity technique," he says.

Also, microlensing is more sensitive in picking up planets orbiting at Jupiter-like distances from their star. Radial-velocity analysis is better with close-in planets.

The planet reported today is not the first to be found in a binary-star system. However, the MPS team explains that these other planets orbit individual stars in their systems. It believes that "these star systems probably became binaries by gravitational capture after the planets formed." It adds that the new planet "is likely to be the first example of a planet that formed in a binary system."

Planet hunters using the radial-velocity technique have avoided binary stars. If binary star solar systems are as common as the MPS team suspects, microlensing searches should swell the harvest of alien worlds.

The microlensing planet-hunting teams are just finishing their pilot programs. Bennett says the MPS collaboration expects to expand its capability by refurbishing the old Harvard Southern Observatory site near Bloemfontein in South Africa. It has an excellent view of our galaxy's central bulge. That's the direction in which the microlensing events the team wants to observe occur.

(c) Copyright 1999. The Christian Science Publishing Society