WASHINGTON — It's boom time for planet hunters. Astronomers are bagging new worlds at an average rate of more than two a month.
As of July 20, the latest available date, 246 extrasolar planets had been detected circling other stars in the Milky Way galaxy. Among them are 25 alien "solar systems" consisting of two, three or four bodies orbiting single suns.
Four new exoplanets, as they're also called, were reported just this month; three were reported in May and 28 during the last 12 months. The smallest known exoplanet, only twice as wide and five times heavier than Earth, was revealed in April.
"Ten years ago, we knew of no extrasolar planets," said John Bally, an astronomer at the University of Colorado in Boulder. "Now we're discovering planets almost weekly."
"Extrasolar planets are everywhere in the sky," said Sylvain Korzennik, a researcher at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.
In addition to simply boosting the count of planets, new technologies are letting scientists begin to analyze the chemical makeup of their finds. Water molecules have been spotted in the atmosphere of at least one new planet. The fingerprints of elements such as carbon, oxygen, sodium, silicon and iron have shown up.
So far, none of the known exoplanets seems likely to harbor life. That's because almost all the discoveries are gas giants, as big or bigger than our own Jupiter. Most of them huddle close to their stars - inside Mercury's orbit if they were in our solar system - and are far too hot for liquid water, an essential for life as we know it. Many orbit so rapidly that their years last only a few days.
"The known exoplanets are very different from our own," said Sara Seager, a planetary scientist at the Massachusetts Institute of Technology in Cambridge.
The reason astronomers are finding mostly big, close-in planets is that they're easier to detect than Earth-size objects. But planet hunters are confident that new telescopes soon will be able to identify smaller, solid bodies in Earthlike orbits in the so-called "habitable zone": close enough - but not too close - to their stars to permit liquid water and perhaps life.
"No question there are habitable planets out there," Seager said. "But whether they are inhabited is uncertain."
The pace of discovery is bound to increase. A European planet-hunting satellite, named Corot, was launched last December and reported its first discovery May 3.
In early 2009, NASA will launch a more powerful telescope, named Kepler, that will monitor 100,000 stars in the northern sky for four years. Kepler will have "enough precision to find Earth-size planets," said William Borucki, a space scientist at NASA's Ames Research Laboratory in Mountain View, Calif.
Borucki said he hoped to find 50 Earthlike planets and 500 objects twice as massive by a relatively new technique known as "transit photometry." This method measures the tiny dip in a star's light as a planet passes in front of it as seen from Earth. Borucki compared it to detecting "a gnat flying across a headlight." So far 21 planets have been found this way.
The overwhelming majority of the new planets were identified by an older technique known as "radial velocity." This refers to the minute wobble in a star's motion caused by a planet's gravity as it orbits around it. The effect is like the motion of your body when you swing a bucket full of water around your head.
No distant planets have been imaged directly as yet. A NASA satellite called the Terrestrial Planet Finder might have been able to do the job, but it was scrapped last year because of technical problems and its high cost.
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A list of the 10 most intriguing exoplanets.