Planethunters say it's just a matter of time before they lasso Earth's twin, whichalmost surely is hiding somewhere in our star-studded galaxy.
Momentum isbuilding: Just last week, astronomers announced they had discovered threesuper-Earths ? worlds more massive than ours but small enough to most likely berocky ? orbiting a single star. And dozens of other worlds suspected of havingmasses in that same range were found around other stars.
"Beingable to find three Earth-mass planets around a single star really makes thepoint that not only may many stars have one Earth, but they may very well havea couple of Earths," said Alan Boss, a planet formation theorist at theCarnegie Institution of Washington in Washington, D.C.?
Since theearly 1990s, when the first planetsoutside of our solar system were detected orbiting the pulsar PSR 1257,astronomers have identified nearly 300 such worlds. However, most of them aregas giants called hot Jupiters that orbit close to their stars because, simply,they are easier to find.
"Sofar we've found Jupiters and Saturns, and now our technology is becoming goodenough to detect planets smaller, more like the size of Uranus and Neptune, andeven smaller," said one of the top planet hunters on this world, GeoffMarcy of the University of California, Berkeley.
Marcy, Bossand other scientists are optimistic that within the next five or so yearsheadlines will be splashed with news of a near twin of Earth in another starsystem.
"Whatis amazing to me is that for thousands of years humans have gazed at the stars,wondering if there might be another Earth out there somewhere," Boss told SPACE.com."Now we know enough to say that Earth-like planets are indeed orbitingmany of those stars, unseen perhaps, but there nevertheless."
Twotechniques are now standard for spotting other worlds. Most of the planetsnoted to date have been discovered using the radial velocity method, in whichastronomers look for slight wobbles in a star's motion due to the gravitationaltug of an orbiting planet. This favors detection of verymassive planets that are very close to their host stars.
With thetransit method, astronomers watch for a dimming of light when a planet passesin front of its host star. Though more haphazard, this approach works whentelescopes scan the light from hundreds or thousands of stars at once.
Both methodsare limited by their ability to block out the overshadowing light of the hoststar. For instance, the sun is 100 times larger, 300,000 times more massive andup to 10 billion times brighter than Earth. "Detecting Earth in reflectedlight is like searching for a firefly six feet from a searchlight that is 2,400miles distant," writes a panel of astronomers recently in their finalreport of the Exoplanet Task Force.
Withupgrades in spectrometers and digital cameras attached to telescopes,astronomers' eyes have become more sensitive to relatively tiny stellar wobbles(measured by changes in certain wavelengths of light) and dips in starlightfrom ever smaller planets.
The discoveryof super-Earths announced last week reflects this technological leap.
"Ithink why astronomers are really excited [about the super-Earth discovery] isit just shows that technology has really matured and so they're able to seethese very subtle wobbles due to these low-mass planets," said DavidCharbonneau of the Harvard-Smithsonian Center for Astrophysics in Massachusetts."Those were fairly massive stars. If they were able to get the sameprecision on a lower-mass star, they would be able to look at even lower-massplanets and so those really would be analogs of the Earth."
To eke outeven more sensitivity from current technologies, Charbonneau suggestsastronomers look for worlds around small stars.
He andother astronomers are in fact probing the universe for transiting planetsorbiting M dwarfs, or red dwarfs, which are about 50 percent dimmer than thesun and much less massive. Red dwarfs are also considered the most common startype in the universe.
"Ithink the real opportunity there is to study low-mass stars, and that's becausewe're looking for very small planets," Charbonneau said. "Thedifficulty is the ratio between the planet's mass and the star's mass or theplanet's size and the star's size depending on how you want to find it."
The lowmass and luminosity means any changes to the star due to an Earth-mass planetare much more likely to be detected.
"Alate M star is about 10 times smaller than the sun," said Penn State'sJames Kasting, who studies planetary atmospheres and the habitable zones ofexoplanets. "So Earth going in front of an M star would give a 1 percentsignal. That's like Jupiter going in front of the sun." Kasting added,"We could conceivably find an Earth analog planet by this method withinthe next five or ten years."
Other teamsare gearing up to look for Earth-like worlds orbiting massive stars like thesun. NASA's Kepler observatory is scheduled for launch in February 2009, afterwhich the high-powered telescope will monitor about 100,000 stars in the MilkyWay looking for periodic dimming of starlight due to a planet's transit infront of the star.
The French COROTmission is already up in space working in a similar fashion.
Theultimate goal of planet-hunting projects is to find Earth twins.
"We arelooking for twins of the Earth, analogs that walk and talk and smell like ourown Earth," Marcy said during a telephone interview. He is currentlylooking for super-Earths using the W.M. Keck Observatory in Hawaii.
Such a twinwould be rocky, with a similar chemical composition to Earth, and would orbitwithin the habitablezone of its star.
Thehabitable zone defines the distance at which a planet must orbit from its starfor liquid water to exist on its surface ? not too hot like Venus, not too coldlike Neptune or Pluto.
Astronomershave found planets orbiting pretty close to the habitable zone, but none so farwithin it.
"Isuspect there are Earth-like planets with lakes and rivers and waterfalls anddeep glacial gorges and that are spectacularly beautiful," Marcy said.
Finding aplanet in the habitable zone is the first step toward finding alien life.
"Whenwe say it's a habitable world, all we're doing is saying it potentially couldhold life," Boss said. "To go beyond that to say, 'Here's a habitableworld; is it inhabited,' then you need to start studying the atmosphere of theplanet."
The JamesWebb Space Telescope (JWST), scheduled for launch in 2013, could do just that.
"Theremight be a signal in the atmosphere that could be a smoking gun and wouldsuggest that plate tectonics is there," said earth and planetary scientistDiana Valencia of Harvard University.
Hercomputer models have shown that plate tectonics, the forces that movecontinents and lift gigantic mountain ranges, are key tolife on Earth as we know it, and possibly to life on other worlds. That'sbecause as the rocky plates that form the planet's outer shell move about, theyalso recycle carbon dioxide. This greenhouse gas keeps our planet's temperaturebalmy, but not too hot. And the telltale signal would be certain levels ofcarbon dioxide, suggesting that just as on Earth, this other world relies onplate tectonics to cycle carbon.
Butfirst things first. "There's no doubt that other Earths exist, simply dueto the sheer vast numbers of other stars and galaxies in our universe,"Marcy said. "There's a deeper question ? how common are Earth-likeplanets? Are Earth-like planets a dime a dozen, or are they quite rare, quirkyprecious planets that are one in a thousand or one in a million?"
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