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AnEarth-size planet has been spotted orbiting a nearby star at a distance thatwould makes it not too hot and not too cold ? comfortable enough for life toexist, researchers announced today (Sept. 29).
Ifconfirmed, the exoplanet, named Gliese 581g, would be the firstEarth-like world found residing in a star's habitable zone ? a region where aplanet's temperature could sustain liquid water on its surface. [Illustrationof planet Gliese 581g.]
Andthe planet's discoverers are optimistic about the prospects for finding lifethere.
"Personally,given the ubiquity and propensity of life to flourish wherever it can, I wouldsay, my own personal feeling is that the chances of life on this planet are 100percent," said Steven Vogt, a professor of astronomy and astrophysics atthe University of California, Santa Cruz, during a press briefing today."I have almost no doubt about it."
Hiscolleague, Paul Butler of the Carnegie Institution of Washington, inWashington, D.C., wasn't willing to put a number on the odds of life, though headmitted he's optimistic.
"It'sboth an incremental and monumental discovery," Sara Seager, anastrophysicist at the Massachusetts Institute of Technology, told SPACE.com. Incrementalbecause the method used to find Gliese 581g already has found several planetsmost of the known planets, both super-Earths,more massive than our own world outside their stars' habitable zone, along withnon-Earth-like planets within the habitable zone.
"Itreally is monumental if you accept this as the first Earth-like planet everfound in the star's habitable zone," said Seager, who was not directlyinvolved in the discovery.
Vogt,Butler and their colleagues will detail the planet finding in the AstrophysicalJournal.
Thenewfound planet joins more than 400 other alienworlds known to date. Most are huge gas giants, though several are just afew times the mass of Earth.
Gliese581g is one of two new worlds the team discovered orbiting the red dwarf starGliese 581, bumping that nearby star's familyof planets to six. The other newfound planet, Gliese 581f, is outside thehabitable zone, researchers said.
Thestar is located 20 light-years from Earth in the constellation Libra. Onelight-year is about 6 trillion miles (10 trillion km).
Reddwarf stars are about 50 times dimmer than our sun. Sincethese stars are so much cooler, their planets can orbit much closer to them andstill remain in the habitable zone.
Estimatessuggest Gliese 581g is 0.15 astronomical units from its star, close enough toits star to be able to complete an orbit in just under 37 days. Oneastronomical unit is the average distance between the Earth and sun, which isapproximately 93 million miles (150 million km).
TheGliese581 planet system now vaguely resembles our own, with six worlds orbitingtheir star in nearly circular paths.
Withsupport from the National Science Foundation and NASA, the scientists ? membersof the Lick-Carnegie Exoplanet Survey ? collected 11 years of radial velocitydata on the star. This method looks at a star's tiny movements due to thegravitational tug from orbiting bodies.
Thesubtle tugs let researchers estimate the planet's mass and orbital period, howlong it takes to circle its star.
Gliese581g has a mass three to four times Earth's, the researchers estimated. Fromthe mass and estimated size, they said the world is probably a rocky planetwith enough gravity to hold onto an atmosphere.
The planet is tidally locked to its star,so that one side basks in perpetual daylight, while the other side remains indarkness. This locked configuration helps to stabilize the planet's surfaceclimate, Vogt said.
"Anyemerging life forms would have a wide range of stable climates to choose fromand to evolve around, depending on their longitude," Vogt said, suggestingthat life forms that like it hot would just scoot toward the light side of thatline while forms with polar-bear-like preferences would move toward the darkside.
Betweenblazing heat on the star-facing side and freezing cold on the dark side, theaverage surface temperature may range from 24 degrees below zero to 10degrees Fahrenheit (minus 31 to minus 12 degrees Celsius), the researcherssaid.
Supposedlyhabitable worlds have been found and later discredited, so what makes this onesuch a breakthrough?
There'sstill a chance that further observations will dismiss this planet, also. Butover the years, the radial velocity method has become more precise, theresearchers point out in their journal article.
Inaddition, the researchers didn't make some of the unrealistic assumptions madein the past, Seager said.
Forinstance, another planet orbiting Gliese 581 (the planetGliese 581c) also had been considered to have temperatures suitable forlife, but in making those calculations, the researchers had come up with an"unrealistic" estimate for the amount of energy the planet reflected,Seager pointed out. That type of estimate wasn't made for this discovery.
"We'relooking at this one as basically the tip of the iceberg, and we're expectingmore to be found," Seager said.
Oneway to make this a reality, according to study researchers, would be "tobuild dedicated 6- to 8-meter-class Automated Planet Finder telescopes, one ineach hemisphere," they wrote.
Thetelescopes ? or "light buckets" as Seager referred to them ? would be dedicated to spying on the nearby stars thought to potentiallyhost Earth-like planets in their habitable zones. The result would beinexpensive and probably would reveal many other nearby potentially habitableplanets, the researchers wrote.
Beyondthe roughly 100 nearest stars to Earth, there are billions upon billions ofstars in the Milky Way, and with that in mind, the researchers suggest tens ofbillions of potentially habitable planets may exist, waiting to be found.
Planetslike Gliese 581g that are tidally locked and orbit the habitable zone of reddwarfs have a high probability of harboring life, the researchers suggest.
Earthonce supported harsh conditions, the researchers point out. And since reddwarfs are relatively "immortal" living hundreds of billions of years(many times the current age of the universe), combined with the fact thatconditions stay so stable on a tidally locked planet, there's a good chancethat if life were to get a toe-hold it would be able to adapt to thoseconditions and possibly take off, Butler said.
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JeannaBryner is the Managing Editor for LiveScience,a SPACE.com sister publication.