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Candidate “habitable” planet described as most promising yet

Sept. 29, 2010
Courtesy of the University of California Santa Cruz
and World Science staff

As­tro­no­mers have identified what they de­scribe as the most prom­is­ing can­di­date yet for a hab­it­a­ble plan­et out­side our so­lar sys­tem.

The newfound plan­et is in the same so­lar sys­tem where two oth­ers al­so con­sid­ered pos­sibly hab­it­a­ble have turned up. But one of these may be too cold and the oth­er too hot, re­search­ers said—while the new one seems to be right in be­tween, so that liq­uid wa­ter could very well ex­ist.

The dimly lit zone be­tween the light and dark side of a plan­et, called the "terminator," is the part con­sid­ered most like­ly to be hos­pi­ta­ble to life on a plan­et just re­ported dis­cov­ered. Ab­ove, an art­ist's con­cep­tion of the in­ner four plan­ets of the Gliese 581 sys­tem and their host star.  (Art­work by Lyn­ette Cook)


“Our find­ings of­fer a very com­pel­ling case for a po­ten­tially hab­it­a­ble plan­et,” said as­tron­o­mer Ste­ven Vogt of the Uni­vers­ity of Cal­i­for­nia San­ta Cruz, one of the re­search­ers. “The fact that we were able to de­tect this plan­et so quickly and so near­by tells us that plan­ets like this must be really com­mon... there could be tens of bil­lions of these sys­tems in our ga­laxy.”

The plan­et is be­lieved to or­bit closely by the “red dwarf” star Gliese 581. It would be in the mid­dle of the star’s “hab­it­a­ble zone”—the range of dis­tances from the star where tem­pe­r­a­tures al­low for liq­uid wa­ter.

Gliese 581, lo­cat­ed 20 light years away from Earth in the con­stella­t­ion Li­bra, has a some­what check­ered his­to­ry of hab­it­a­ble-plan­et claims. Two pre­vi­ously de­tected plan­ets in the sys­tem lie at the edges of the hab­it­a­ble zone, Vogt and col­leagues said. One, des­ig­nat­ed plan­et “c,” is on the hot side of that zone; anoth­er, “d,” is on the cold side.

While some as­tron­o­mers still think “d” may be hab­it­a­ble if it has a thick at­mos­phere that helps warm it, oth­ers are skep­ti­cal. 

The newly dis­cov­ered body is des­ig­nat­ed g.

“We had plan­ets on both sides of the hab­it­a­ble zone—one too hot and one too cold—and now we have one in the mid­dle that’s just right,” Vogt said.

To as­tron­o­mers, a po­ten­tially hab­it­a­ble plan­et is one that could sus­tain life, not nec­es­sarily one that hu­mans would con­sid­er a nice place to live. Hab­it­abil­ity de­pends on many fac­tors, but liq­uid wa­ter and an at­mos­phere are among the most im­por­tant.

The new find­ings, to be pub­lished in The As­t­ro­phys­i­cal Jour­nal and posted on­line at the data­base arX­iv.org, al­so in­clude a re­port of a sec­ond new plan­et that brings to six the to­tal num­ber of plan­ets thought to sur­round Gliese 581. All are cal­cu­lat­ed to have nearly cir­cu­lar or­bits, which tend to pro­mote sta­ble cli­mates since the dis­tance from the star nev­er changes much.

Gliese 581g, the new­found po­ten­tially hab­it­a­ble one, weighs the equiv­a­lent of three or four Earths and or­bits its star in a brisk 37 days, mean­ing the year is only that long, as­tron­o­mers said. Its rel­a­tively low weight for a plan­et sug­gests it’s probably rocky and has enough gra­vity to hold on to an at­mos­phere, ac­cord­ing to Vogt.

The plan­et is tid­ally locked to the star, mean­ing that one side is al­ways fac­ing the star and bask­ing in pe­r­pet­u­al day­light, while the side fac­ing away from the star is in perm­a­nent dark­ness. One ef­fect of this is to sta­bi­lize the plan­et’s cli­mates, ac­cord­ing to Vogt. The most hab­it­a­ble zone on the plan­et’s sur­face would be the line be­tween shad­ow and light, known as the “ter­mi­na­tor,” with sur­face tem­pe­r­a­tures de­creas­ing to­ward the dark side and in­creas­ing to­ward the light side.

“Any emerg­ing life forms would have a wide range of sta­ble cli­mates to choose from and to evolve around, de­pend­ing on their lon­gi­tude,” Vogt said.

The re­search­ers es­ti­mate that the av­er­age sur­face tem­pe­r­a­ture of the plan­et is be­tween -24 and 10 de­grees Fahr­en­heit (-31 to -12 de­grees Cel­sius). Ac­tu­al tem­pe­r­a­tures would range from blaz­ing hot on the side fac­ing the star to freez­ing cold on the dark side.

If Gliese 581g has a rocky, Earth-like com­po­si­tion, it would be about 1.2 to 1.4 times as wide as Earth. The sur­face gra­vity would be about the same or slightly high­er than Earth’s, so that a pe­r­son could easily walk up­right, Vogt said.

The find­ings are based on 11 years of ob­serva­t­ions of Gliese 581 us­ing an in­stru­ment de­signed by Vogt on the Keck I Tel­e­scope at the Keck Ob­serv­a­to­ry in Ha­waii. The de­vice, a spec­trom­e­ter, al­lows pre­cise mea­sure­ments of a star’s mo­tion along the line of sight from Earth, which can re­veal the pres­ence of plan­ets. The gravita­t­ional tug of an or­biting plan­et causes pe­r­i­od­ic changes in this mo­tion. Mul­ti­ple plan­ets in­duce com­plex wob­bles in the star’s mo­tion, and as­tron­o­mers use soph­is­t­icated anal­y­ses to de­tect plan­ets and de­ter­mine their or­bits and mass­es.

“It’s really hard to de­tect a plan­et like this,” Vogt said. “It took more than 200 ob­serva­t­ions with a pre­ci­sion of about 1.6 me­ters [yards] per sec­ond to de­tect this plan­et.” Study col­la­bo­ra­tors al­so took night-to-night bright­ness mea­sure­ments of the star to ver­i­fy that meas­ured stel­lar mo­tions were caused by out­side forc­es and not by pro­cesses with­in the star it­self.

Giv­en the rel­a­tively small num­ber of stars that have been care­fully mon­i­tored by plan­et hunters, this discovery has come sur­pris­ingly soon, Vogt said.

“If these are rare, we should­n’t have found one so quickly and so near­by,” Vogt said. “The num­ber of sys­tems with po­ten­tially hab­it­a­ble plan­ets is probably on the or­der of 10 or 20 pe­rcent, and when you mul­ti­ply that by the hun­dreds of bil­lions of stars in the Milky Way, that’s a large num­ber.”


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Astronomers say they may have detected the most promising candidate yet for a potentially habitable planet outside our solar system. The planet is in the same solar system where two others also considered possibly habitable have turned up. But one of these may be too cold and the other too hot, researchers said—while the newfound planet seems to be right in between, so that liquid water could very well exist. “Our findings offer a very compelling case for a potentially habitable planet,” said astronomer Steven Vogt of the University of California Santa Cruz, one of the researchers. “The fact that we were able to detect this planet so quickly and so nearby tells us that planets like this must be really common,” he added. “There could be tens of billions of these systems in our galaxy.” The planet is believed to orbit closely by the “red dwarf” star Gliese 581, in the middle of its “habitable zone”—the range of distances from the star where temperatures would allow liquid water. Gliese 581, located 20 light years away from Earth in the constellation Libra, has a somewhat checkered history of habitable-planet claims. Two previously detected planets in the system lie at the edges of the habitable zone, Voigt and colleagues said. One, designated planet “c,” is on the hot side of that zone; another, “d,” is on the cold side. While some astronomers still think “d” may be habitable if it has a thick atmosphere that helps warm it, others are skeptical. The newly discovered body is designated g. “We had planets on both sides of the habitable zone—one too hot and one too cold—and now we have one in the middle that’s just right,” Vogt said. To astronomers, a potentially habitable planet is one that could sustain life, not necessarily one that humans would consider a nice place to live. Habitability depends on many factors, but liquid water and an atmosphere are among the most important. The new findings, to be published in the Astrophysical Journal and posted online at arXiv.org, also include the discovery of a second new planet that brings to six the total number of known planets aroudn Gliese 581. All of them are calculated to have nearly circular orbits, which tend to promote stable climates since the distance from the star never changes much. Gliese 581g, the newfound potentially habitable one, weighs the equivalent of three or four Earths and orbits its star in a brisk 37 days, meaning the year is only that long, astronomers said. Its relatively low weight for a planet suggests it’s probably rocky and has enough gravity to hold on to an atmosphere, according to Vogt. The planet is tidally locked to the star, meaning that one side is always facing the star and basking in perpetual daylight, while the side facing away from the star is in perpetual darkness. One effect of this is to stabilize the planet’s surface climates, according to Vogt. The most habitable zone on the planet’s surface would be the line between shadow and light, known as the “terminator,” with surface temperatures decreasing toward the dark side and increasing toward the light side. “Any emerging life forms would have a wide range of stable climates to choose from and to evolve around, depending on their longitude,” Vogt said. The researchers estimate that the average surface temperature of the planet is between -24 and 10 degrees Fahrenheit (-31 to -12 degrees Celsius). Actual temperatures would range from blazing hot on the side facing the star to freezing cold on the dark side. If Gliese 581g has a rocky, Earth-like composition, it would be about 1.2 to 1.4 times as wide as Earth. The surface gravity would be about the same or slightly higher than Earth’s, so that a person could easily walk upright, Vogt said. The findings are based on 11 years of observations of Gliese 581 using an instrument designed by Vogt on the Keck I Telescope at the Keck Observatory in Hawaii. The device, a spectrometer, allows precise measurements of a star’s motion along the line of sight from Earth, which can reveal the presence of planets. The gravitational tug of an orbiting planet causes periodic changes in this motion. Multiple planets induce complex wobbles in the star’s motion, and astronomers use sophisticated analyses to detect planets and determine their orbits and masses. “It’s really hard to detect a planet like this,” Vogt said. “Every time we measure the radial velocity, that’s an evening on the telescope, and it took more than 200 observations with a precision of about 1.6 meters (yards) per second to detect this planet.” Study collaborators also took night-to-night brightness measurements of the star to verify that measured stellar motions were caused by outside forces and not by processes within the star itself. Given the relatively small number of stars that have been carefully monitored by planet hunters, this discovery has come surprisingly soon, Vogt said. “If these are rare, we shouldn’t have found one so quickly and so nearby,” Vogt said. “The number of systems with potentially habitable planets is probably on the order of 10 or 20 percent, and when you multiply that by the hundreds of billions of stars in the Milky Way, that’s a large number.”