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First Earth-sized planet in star’s “habitable zone” reported found

April 17, 2014
Courtesy of the SETI Institute
and World Science staff

In a long-awaited first, as­tro­no­mers say they have found an Earth-sized plan­et, out­side our own so­lar sys­tem, or­bit­ing a star at a dis­tance suit­a­ble to al­low liq­uid wa­ter on the plan­et’s sur­face.

That means it could po­ten­tially host life—though no sig­nals have been de­tected from the body, des­ig­nat­ed Kep­ler-186f, as­tro­no­mers add. Kep­ler-186f is meas­ured to be so far away from us that even mov­ing at light speed, it would take al­most an es­ti­mat­ed five cen­turies to get there. Any radio or light signals reach­ing us from the plan­et would be that old also.

An artist's concept of what the planet Kep­ler 186f might look like if it were ha­bit­able. In its sky one would see its star and sis­ter plan­ets. (Credit: Danielle Futselaar)


Still, “this is the first de­fin­i­tive Earth-sized plan­et found in the ‘hab­it­a­ble zone’ around an­oth­er star,” said Elisa Quin­tana of the SETI In­sti­tute at NASA Ames Re­search Cen­ter in Moun­tain View, Ca­lif. She is the lead au­thor of a re­port on the find­ings pub­lished in the re­search jour­nal Sci­ence.

“Find­ing such plan­ets is a pri­ma­ry goal of the Kep­ler space tele­scope,” she added. “The star is a main-sequence M-dwarf, a very com­mon type. More than 70 per­cent of the hun­dreds of bil­lions of stars in our gal­axy are M-dwarfs.”

The finding will surely shape fu­ture in­ves­ti­ga­t­ions of exoplan­ets, or plan­ets out­side our own so­lar sys­tem, with pos­si­ble Earth-like sur­faces, sci­en­tists said. The body is the fifth and out­er­most world to be de­tected in the plan­etary sys­tem of a red dwarf star known as Kep­ler-186. The star lies ap­prox­i­mate­ly in the dir­ect­ion of the con­stell­ation Cyg­nus, the Swan, in the north­ern sky on the plane of the Mil­ky Way.

Of the nearly 1,800 con­firmed exoplan­ets found in the past two dec­ades, about 20 are thought to or­bit their host star in the hab­it­a­ble zone—a range of or­bital dis­tances at which sur­face wa­ter on a plan­et with an at­mos­phere would nei­ther freeze nor boil. But all of these are larg­er than Earth, so they might be gas­e­ous plan­ets, like the larg­er plan­ets of our own so­lar sys­tem. The au­thors es­ti­mate the new­found plan­et is less then 10 per­cent wider than Earth, based on a meas­ured dim­ming of star­light from Kep­ler-186 as the plan­et passes in front of it.

The­o­ret­i­cal mod­els sug­gest that plan­ets up to about 1.5 times as wide as Earth “are un­likely to be swathed in at­mos­pheres of hy­dro­gen and he­li­um, the fate that’s be­fall­en the gas gi­ants of our own so­lar sys­tem,” ex­plained Thom­as Barc­lay, a staff sci­ent­ist for the Kep­ler mis­sion af­fil­i­at­ed with both NASA and the Bay Ar­ea En­vi­ron­men­tal Re­search In­sti­tute.

So “Kep­ler-186f is likely a rocky world, and in that sense si­m­i­lar to Ve­nus, Earth and Mars.”

Plan­ets or­bit­ing red dwarf stars were tra­di­tion­ally con­sid­ered not very hab­it­a­ble. This is be­cause they would have to cir­cle their stars quite closely to get warmth, but this would lead to “ti­dal lock­ing,” a situa­t­ion in which the same side of the plan­et al­ways faces the star. Then one side would pre­sumably get ex­tremely hot, the oth­er frig­idly cold.

Yet more re­cent mod­el­ing stud­ies sug­gest winds or ocean cur­rents could re­duce this prob­lem by evening out tem­per­a­ture varia­t­ions. Kep­ler-186f is al­so far enough away from its host star that it’s probably not locked, the as­tro­no­mers said, and likely al­so not too vul­ner­a­ble to stel­lar flares, com­mon with dwarf stars.

As­tro­no­mers are look­ing for any sig­nals sug­gestive of life us­ing the SETI In­sti­tute’s Al­len Tel­e­scope Ar­ray, which ob­serves Kep­ler can­di­date exo­plan­ets.

Ac­cord­ing to Quin­tana, at a dis­tance of 490 light-years away from us, Kep­ler-186f may be too dim for fol­low-up sur­veys to probe its at­mos­phere, even with next-genera­t­ion tele­scopes. A light-year is the dis­tance light trav­els in a year.

But “our re­search tells us that we should be able to find plan­ets around bright stars that will be ide­al tar­gets to ob­serve with James Web­b,” a NASA space-based tel­e­scope now un­der con­struc­tion, she said. That’s ex­pected to be able to di­rectly im­age plan­ets around near­by dwarf stars, and break up their light to de­scribe their at­mos­pheres.


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In a long-awaited first, astronomers say they have found an Earth-sized planet, outside our own solar system, orbiting a star at a distance suitable to allow liquid water on the planet’s surface. That means it could potentially host life—though no signals have been detected from the body, designated Kepler-186f, astronomers add. “This is the first definitive Earth-sized planet found in the ‘habitable zone’ around another star,” said Elisa Quintana of the SETI Institute at NASA Ames Research Center in Mountain View, Calif. She is the lead author of a report on the findings published in the research journal Science. “Finding such planets is a primary goal of the Kepler space telescope,” she added. “The star is a main-sequence M-dwarf, a very common type. More than 70 percent of the hundreds of billions of stars in our galaxy are M-dwarfs.” Kepler-186f is measured to be so far away from us that even moving at light speed, it would take almost an estimated five centuries to get there. Still, the finding is believed to support the view that worlds similar to our own exist. It will surely shape future investigations of exoplanets, or planets outside our own solar system, with possible Earth-like surfaces, scientists said. The body is the fifth and outermost world to be detected in the planetary system of a red dwarf star known as Kepler-186. Of the nearly 1,800 confirmed exoplanets found in the past two decades, about 20 are thought to orbit their host star in the habitable zone—a range of orbital distances at which surface water on a planet with an atmosphere would neither freeze nor boil. But all of these are larger than Earth, so they might be gaseous planets, like the larger planets of our own solar system. The authors estimate the newfound planet is around Earth’s size, based on a measured dimming of starlight from Kepler-186 as the planet passes in front of it. Theoretical models suggest that planets up to about 1.5 times as wide as Earth “are unlikely to be swathed in atmospheres of hydrogen and helium, the fate that’s befallen the gas giants of our own solar system,” explained Thomas Barclay, a staff scientist for the Kepler mission affiliated with both NASA and the Bay Area Environmental Research Institute. So “Kepler-186f is likely a rocky world, and in that sense similar to Venus, Earth and Mars.” Planets orbiting red dwarf stars were traditionally considered not very habitable. This is because they would have to circle their stars quite closely to get warmth, but this would lead to “tidal locking,” a situation in which the same side of the planet always faces the star. Then one side would presumably get extremely hot, the other frigidly cold. Yet more recent modeling studies suggest winds or ocean currents could reduce this problem by evening out temperature variations. Kepler-186f is also far enough away from its host star that it’s probably not locked, the astronomers said, and probably also not too vulnerable to stellar flares, common with dwarf stars. No signals suggestive of life have been detected from Kepler-186f, but astronomers are looking, using the SETI Institute’s Allen Telescope Array, which observes Kepler candidate exoplanets. According to Quintana, at a distance of 490 light-years away from us, Kepler-186f may be too dim for follow-up surveys to probe its atmosphere, even with next-generation telescopes. A light-year is the distance light travels in a year. But “our research tells us that we should be able to find planets around bright stars that will be ideal targets to observe with James Webb,” a NASA space-based telescope now under construction, she said. That’s expected to be able to directly image planets around nearby dwarf stars, and break up their light to describe their atmospheres.