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Distant planet judged possibly habitable  

April 24, 2007

In a find­ing that if con­firmed could stand as a land­mark in history, as­tro­no­mers have re­ported dis­co­v­er­ing the most Earth-like plan­et out­side our So­lar Sys­tem to date: a world that may have liq­uid oceans and thus life.

Swiss, French and Por­tu­guese sci­en­tists found the body, es­ti­mated as 50 per­cent wid­er than our Earth, or­bit­ing a so-called red dwarf star rel­a­tively close to Earth. The star is thought to har­bor two oth­er plan­ets al­so.

Artist's im­pres­sion of a sys­tem of three plan­ets sur­round­ing the red dwarf Gliese 581. (Cour­te­sy ESO)

The new­found exo­pla­n­et—as as­tro­no­mers call plan­ets around stars oth­er than the Sun—would be the small­est such body ev­er re­ported. 

None­the­less, the object is es­ti­mat­ed to weigh as much as five Earths, part­ly thanks to its great­er width. For the same rea­son, it would have more than twice Earth’s sur­face ar­ea. His­tor­i­cally, only large exo­pla­n­ets lend them­selves to hu­man de­tect­ion, though that is chang­ing.

Oth­er cu­ri­ous fea­tures of the new­found plan­et are that grav­i­ty at its sur­face would be around twice as strong as on Earth; and its year is just 13 Earth days long, as it comp­letes one or­bit about its sun in that time.

It’s 14 times clos­er to its star than we are from our Sun, re­search­ers said. But since its host star, the red dwarf Gliese 581, is smaller and cool­er than the Sun, the plan­et nev­ertheless would lie in its hab­it­a­ble zone—the re­gion around a star with suit­a­ble tem­pe­r­a­tures for liq­uid wa­ter. 

Av­er­age tem­pe­r­a­tures on this “supe­r-Earth” lie be­tween 0 and 40 de­grees Cel­si­us (32 to 104 de­grees Fahren­heit), “and wa­ter would thus be liq­uid,” said Sté­phane Udry of Switz­er­land’s Ge­ne­va Ob­serv­a­to­ry, lead au­thor of a pa­pe­r re­port­ing the re­sult. “Mod­els pre­dict that the plan­et should be ei­ther rock­y—like our Earth—or cov­ered with oceans,” he added.

“Liq­uid wa­ter is crit­i­cal to life as we know it,” not­ed Xa­vi­er Delfosse, a mem­ber of the team from Gre­no­ble Uni­ver­si­ty, France. “Be­cause of its tem­pe­r­a­ture and rel­a­tive prox­im­i­ty, this plan­et will most prob­a­bly be a very im­por­tant tar­get of the fu­ture space mis­sions ded­i­cat­ed to the search for extra-terrestrial life. On the treas­ure map of the Uni­verse, one would be tempted to mark this plan­et with an X.” 

The ar­row marks the ap­prox­i­mate lo­ca­tion of the red dwarf star Gliese 581 with re­spect to the con­stel­la­tion Li­bra vi­si­ble in the south­ern sky. 

The host star, Gliese 581, is among the 100 clos­est stars to us, ly­ing 20.5 light-years away in the con­stel­la­tion Li­bra (“the Scales.”) A light-year is the dis­tance light trav­els in a year. 

Gliese 581 has one third the mass of our Sun. Such small stars, called red dwarfs, are at least 50 times faint­er than the Sun and are be­lieved to be the most com­mon stars in our gal­axy. Among the 100 clos­est stars to the Sun, 80 be­long to this class.

“Red dwarfs are ide­al tar­gets for the search for such plan­ets be­cause they emit less light, and the hab­it­a­ble zone is thus much clos­er to them than it is around the Sun,” said Xa­vi­er Bon­fils, a co-re­searcher from Lis­bon Uni­ver­si­ty. Plan­ets near a star are eas­i­er to de­tect be­cause their grav­i­ta­tion­al pull af­fects the par­ent star no­tice­ably, in­duc­ing some­thing of a wig­gling mo­tion.

Red dwarfs are al­so ex­pected to live ex­traor­di­nar­ily long be­cause they burn fu­el slow­ly. A red dwarf one-third the Sun’s mass, like Gliese 581, would typ­i­cal­ly shine for some 130 bil­lion years, out­liv­ing the Sun by thir­teen times. That might re­lieve at least one source of stress for any in­hab­i­tants of a red dwarf sys­tem. We on Earth are al­ready half­way through the Sun’s life­time, though much time re­mains.

Artist's con­cept of a red dwarf, a dim star that burns slow­ly and very long. (Cour­te­sy NASA)

Two years ago, Udry and his team found anoth­er plan­et around Gliese 581, es­ti­mat­ed to weigh as much as 15 Earths—about as much as Nep­tune—and or­bit­ing the star in 5.4 days.

At the time, the as­tro­no­mers had al­ready not­ed hints of anoth­er plan­et, Udry and col­leagues said. They thus took new mea­sure­ments and found the new “supe­r-Earth,” as well as a like­ly third plan­et weigh­ing eight Earths and or­bit­ing in 84 days. The find­ings have been sub­mit­ted to the re­search jour­nal As­tron­o­my and As­t­ro­phys­ics, the sci­en­tists said.

The find was pos­si­ble thanks to an in­stru­ment known as a spec­tro­graph on the Eu­ro­pe­an South­ern Ob­serv­a­to­ry’s 3.6-meter tel­e­scope at La Silla, Chil­e, ac­cord­ing to the group. The in­s­tru­ment, called the High Ac­cu­ra­cy Ra­di­al Ve­loc­i­ty for Plan­e­tary Search­er, is touted as one of the most suc­cess­ful tools for de­tecting exo­pla­n­ets to date.

The in­stru­ment meas­ured wig­gles in the star’s mo­tion cor­re­spond­ing to ve­loc­i­ty changes of just two to three me­ters per sec­ond—the speed of a brisk walk, ac­cord­ing to the Ge­ne­va Ob­serv­a­to­ry’s Mi­chel May­or, prin­ci­pal in­ves­ti­ga­tor for the in­stru­ment. Giv­en the re­sults so far, “Earth-mass plan­ets around red dwarfs are with­in reach” of dis­cov­ery, he pre­dicted.


-- Sucheta Dalal