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 Friday, the latest available date, 248 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.
At least four new exoplanets, as they're also called, were reported just this month; three were found in May and 28 over the last 12 months. The smallest known exoplanet, only twice as wide and five times heavier than Earth, was discovered in April.
"Ten years ago, we knew of no extrasolar planets," says John Bally, an astronomer at the University of Colorado in Boulder. "Now we're discovering planets almost weekly."
Or as Sylvain Korzennik, a researcher at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., put it: "Extrasolar planets are everywhere in the sky."
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 what are known as gas giants, as big as 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 ingredient 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," says Sara Seager, a planetary scientist at the Massachusetts Institute of Technology.
The reason astronomers are finding mostly big, close-in planets is that they're easier to detect than Earth-size objects. Planet hunters are confident, however, 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 says. "Whether they are inhabited is uncertain."
The pace of discovery is bound to increase. A European planet-hunting satellite named Corot was launched in December and reported its first discovery May 3.
In early 2009, NASA will launch a more powerful telescope, named Kepler, that is designed to monitor 100,000 stars in the northern sky for four years. Kepler will have "enough precision to find Earth-size planets," says William Borucki, a space scientist at NASA's Ames Research Laboratory in Mountain View, Calif.
Borucki says he hopes to find 50 Earthlike planets and 500 objects twice as massive using a relatively new technique known as "transit photometry."
The method measures the tiny dip in a star's light as a planet passes in front of it as seen from Earth - like "a gnat flying across a headlight," Borucki says. At least 20 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 cost.
Intriguing Extrasolar Planets
The first planets outside our solar system were spotted in 1990. The current tally - nearly 250 - ranges from large gassy giants to small rocky worlds. Some are two-faced planets of fire and ice. Others float eerily in space, bound to no star. Here are some oddballs and record holders, as compiled by Space.com.
The youngest exoplanet yet discovered is less than a million years old. It orbits Coku Tau 4, a star 420 light-years away. Astronomers inferred the planet's presence from an enormous hole in the dusty disk that girdles the star. The hole is 10 times the size of Earth's orbit around the Sun and was probably caused by the planet's clearing a space in the dust as it orbits the star.
The oldest is a primeval world 12.7 billion years old that formed more than 8 billion years before Earth - just 2 billion years after the Big Bang. Its discovery, by Penn State researcher Steinn Sigurdsson's team, suggests planets are common in the universe and raises the prospect that life began far sooner than most scientists thought.
A year on HD209458b equals 31/2 Earth days. The planet orbits so close to its star that its atmosphere is being blown away by gales of stellar wind. Scientists estimate the planet is losing at least 10,000 tons of material every second. Eventually, only a dead core of the shrinking planet will remain.
Gliese 581 C marked a milestone in the search for worlds beyond our solar system. It is the smallest exoplanet ever detected, and the first to lie within the habitable zone of its parent star, thus raising the possibility that its surface could sustain liquid water, or even life. It is 50 percent bigger and 5 times more massive than Earth.
Video, mapping and full reports on these and other exoplanets: http://go.philly.com/science