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One in five Sun-like stars may host habitable world

Nov. 5, 2013
Courtesy of the W.M. Keck Observatory
and World Science staff

One in five Sun-like stars in our gal­axy have Earth-sized plan­ets that could host life, ac­cord­ing to a new stu­dy.

The fig­ure is an es­ti­mate of how many plan­ets are likely to be cir­cling their stars at suit­a­ble dis­tances for liq­uid wa­ter to stay on the plan­et sur­face.

Artist’s rep­re­sen­ta­ of the “hab­it­able zone,” the range of or­bits where liq­uid wa­ter is per­mit­ted on the sur­face of a plan­et. The au­thors find that 22±8 per­cent of Sun-like stars har­bor a plan­et be­tween one and two times the size of Earth in the hab­it­a­ble zone. (Cour­te­sy of Keck Ob­serv­a­to­ry)


The find­ings, by sci­en­tists at the Uni­vers­ity of Cal­i­for­nia, Berke­ley, and the Uni­vers­ity of Ha­waii, Ma­noa, ap­pear Nov. 4 in the jour­nal Pro­ceed­ings of the Na­t­ional Acad­e­my of Sci­ences.

“What this means is, when you look up at the thou­sands of stars in the night sky, the near­est sun-like star with an Earth-size plan­et in its hab­it­a­ble zone is probably only 12 light years away and can be seen with the na­ked eye. That is amaz­ing,” said UC Berke­ley grad­u­ate stu­dent Er­ik Pe­ti­gu­ra, who led the anal­y­sis of the da­ta. A light-year is the dis­tance light trav­els in a year.

The sci­en­tists used da­ta from NASA’s Kep­ler space­craft and the W. M. Keck Ob­serv­a­to­ry on the sum­mit of Mau­na Kea, Ha­waii.

For NASA, the es­ti­mate “is really im­por­tant, be­cause suc­ces­sor mis­sions to Kep­ler will try to take an ac­tu­al pic­ture of a plan­et, and the size of the tel­e­scope they have to build de­pends on how close the near­est Earth-size plan­ets are,” said An­drew How­ard, as­tron­o­mer with the In­sti­tute for As­tron­o­my at the Uni­vers­ity of Ha­waii. “An abun­dance of plan­ets or­bit­ing near­by stars sim­pli­fies such fol­low-up mis­sions.”

The team, which al­so in­clud­ed UC Berke­ley as­tron­o­mer and plan­et-hunter Geof­frey Mar­cy, cau­tioned that Earth-size plan­ets in Earth-size or­bits are not nec­es­sarily hos­pi­ta­ble to life, even if they or­bit in the “hab­it­a­ble zone” of a star, where the tem­per­a­tures are suit­a­ble for liq­uid wa­ter.

“Some may have thick at­mo­spheres, mak­ing it so hot at the sur­face that DNA-like mo­le­cules would not sur­vive. Oth­ers may have rocky sur­faces that could har­bor liq­uid wa­ter suit­a­ble for liv­ing or­gan­isms,” Mar­cy said. “We don’t know what range of plan­et types and their en­vi­ron­ments are suit­a­ble for life.”

Just last week, How­ard, Mar­cy and their col­leagues pro­vid­ed hope that many such plan­ets ac­tu­ally are rocky. They re­ported that one Earth-sized plan­et dis­cov­ered – al­be­it, one far too hot for life as we know it – is about as heavy as Earth and most likely made of rock and iron, like Earth.

“This gives us some con­fi­dence that when we look out in­to the hab­it­a­ble zone, the plan­ets Er­ik is de­scrib­ing may be Earth-size, rocky plan­ets,” How­ard said.

All of the po­ten­tially hab­it­a­ble plan­ets found in their sur­vey are around “K” stars, which are cool­er and slightly smaller than the sun, Pe­ti­gu­ra said. But the anal­y­sis in­di­cat­ed that the re­sult is al­so ap­pli­ca­ble to “G” stars like the sun. The two star types to­geth­er ac­count, them­selves, for an es­ti­mated one in five stars.

NASA launched the now crip­pled Kep­ler space tel­e­scope in 2009 to look for plan­ets that cross in front of their stars, which causes a tiny re­duc­tion in the star’s bright­ness. From among the 150,000 stars pho­tographed, NASA’s Kep­ler team re­ported more than 3,000 plan­et can­di­dates. Many of these are much larg­er than Earth – rang­ing from large plan­ets with thick at­mo­spheres, like Nep­tune, to gas gi­ants like Ju­pi­ter – or in or­bits so close to their stars that they are roasted.

To sort them out, Pe­ti­gu­ra and his col­leagues are us­ing the twin tel­e­scopes of the Keck Ob­serv­a­to­ry to an­a­lyze light from as many stars as pos­si­ble. This will help them de­ter­mine each star’s true bright­ness and cal­cu­late the width of each trans­iting plan­et, with an em­pha­sis on Earth-width plan­ets.

Al­though the Kep­ler in­stru­ment is no long­er work­ing, the new find­ings sat­is­fy its main mis­sion: to de­ter­mine how many of the 100 bil­lion stars in our gal­axy have po­ten­tially hab­it­a­ble plan­ets, the sci­en­tists said.


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One in five Sun-like stars in our galaxy have Earth-sized planets that could host life, according to a new study. The figure is an estimate of how many planets are likely to be circling their stars at suitable distances for liquid water to stay on the planet surface. The findings, by scientists at University of California, Berkeley, and University of Hawaii, Manoa, appear Nov. 4 in the journal Proceedings of the National Academy of Sciences. “What this means is, when you look up at the thousands of stars in the night sky, the nearest sun-like star with an Earth-size planet in its habitable zone is probably only 12 light years away and can be seen with the naked eye. That is amazing,” said UC Berkeley graduate student Erik Petigura, who led the analysis of the data. A light-year is the distance light travels in a year. The scientists used data from NASA’s Kepler spacecraft and the W. M. Keck Observatory on the summit of Mauna Kea, Hawaii. For NASA, the estimate “is really important, because successor missions to Kepler will try to take an actual picture of a planet, and the size of the telescope they have to build depends on how close the nearest Earth-size planets are,” said Andrew Howard, astronomer with the Institute for Astronomy at the University of Hawaii. “An abundance of planets orbiting nearby stars simplifies such follow-up missions.” The team, which also included UC Berkeley astronomer and planet-hunter Geoffrey Marcy, cautioned that Earth-size planets in Earth-size orbits are not necessarily hospitable to life, even if they orbit in the “habitable zone” of a star, where the temperatures are suitable for liquid water. “Some may have thick atmospheres, making it so hot at the surface that DNA-like molecules would not survive. Others may have rocky surfaces that could harbor liquid water suitable for living organisms,” Marcy said. “We don’t know what range of planet types and their environments are suitable for life.” Just last week, Howard, Marcy and their colleagues provided hope that many such planets actually are rocky. They reported that one Earth-sized planet discovered – albeit, a planet with a likely temperature of 2,000 Kelvin, which is far too hot for life as we know it – is about as heavy as Earth and most likely made of rock and iron, like Earth. “This gives us some confidence that when we look out into the habitable zone, the planets Erik is describing may be Earth-size, rocky planets,” Howard said. All of the potentially habitable planets found in their survey are around “K” stars, which are cooler and slightly smaller than the sun, Petigura said. But the analysis indicated that the result is also applicable to “G” stars like the sun. The two star types together account, themselves, for an estimated one in five stars. NASA launched the now crippled Kepler space telescope in 2009 to look for planets that cross in front of, or transit, their stars, which causes a slight reduction – about a hundredth of a percent – in the star’s brightness. From among the 150,000 stars photographed every 30 minutes for four years, NASA’s Kepler team reported more than 3,000 planet candidates. Many of these are much larger than Earth – ranging from large planets with thick atmospheres, like Neptune, to gas giants like Jupiter – or in orbits so close to their stars that they are roasted. To sort them out, Petigura and his colleagues are using the twin telescopes of the Keck Observatory to analyze light from as many stars as possible. This will help them determine each star’s true brightness and calculate the width of each transiting planet, with an emphasis on Earth-width planets. Although the Kepler instrument is no longer working, the new findings satisfy its main mission: to determine how many of the 100 billion stars in our galaxy have potentially habitable planets, the scientists said.