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Distant planet judged possibly habitable

April 23, 2007
Courtesy ESO 
and World Science staff
Updated April 25

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,” dubbed Gliese 581c, along with 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.

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In what would be a landmark discovery, astronomers have reported finding the most Earth-like planet outside our Solar System to date: a world just 50% wider than ours, possibly with liquid oceans. Swiss, French and Portuguese scientists found the body orbiting a so-called red dwarf star relatively close to Earth, and thought to harbor two other planets also. This exoplanet, as astronomers call planets around stars other than the Sun, is the smallest such body ever reported. Nonetheless, because large planets are easier to find, the planet is estimated to weigh as much as five Earths, partly thanks to its greater width. For the same reason it would also have more than twice Earth’s surface area. Other curious features of the 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 because it orbits its sun in that time. It’s 14 times closer to its star than we are from our Sun, researchers said. However, given that its host star, the red dwarf Gliese 581, is smaller and cooler than the Sun, the planet nevertheless would lie in the 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 Stephane Udry of the Geneva Observatory in Switzerland, 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 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 weighs as much as one third 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. Two years ago, the same team of astronomers 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 Astro physics, 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, astronomers said. The instrument, called the High Accuracy Radial Velocity for Planetary Searcher, is one of the most successful tools for detecting exoplanets to date, they added. 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, principle invest igator for the instrument. Few other detectors have such sensitivity, he added. Given the results so far, “Earth-mass planets around red dwarfs are within reach” of discovery, he predicted.