"Long before it's in the papers"
July 24, 2015

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Possibly Earth-like planet reported near Sun-like star, another first

July 24, 2015
Courtesy of the University of Texas at Austin
and World Science staff

As­tro­no­mers work­ing with NASA’s Kep­ler mis­sion have found what they call the first near-Earth-sized plan­et in the “hab­it­able zone” of a Sun-like star.

The hab­it­a­ble zone is the range of dis­tances from a star where tem­per­a­tures are suit­a­ble for plan­ets with liq­uid wa­ter on their sur­faces. While oth­er roughly Earth-sized plan­ets have been found in hab­it­a­ble zones, this would be the first case in which the star has been deemed si­m­i­lar to our Sun. 

Di­a­gram and artist's con­cept com­par­ing Earth and its sun, on the one hand (left), with Kepler-452b and its own sun on the oth­er (right). (Cour­te­sy of Mc­Don­ald Ob­serv­a­to­ry)


“We are push­ing to­ward Earth 2.0,” said as­tron­o­mer Mi­chael Endl of the Mc­Don­ald Ob­serv­a­to­ry at the Un­ivers­ity of Tex­as at Aus­tin, one of the re­search­ers. “This plan­et is probably the most si­m­i­lar to Earth yet found.”

The plan­et, dubbed Kep­ler-452b, lies about 1,400 light-years from Earth in the con­stella­t­ion Cyg­nus. A light-year is the dis­tance light trav­els in a year.

It’s 60 per­cent wid­er than Earth and is con­sid­ered a “super-Earth-sized” plan­et. Its weight and make­up are un­known, but pre­vi­ous re­search sug­gests that a plan­et of its size has a bet­ter than even chance of be­ing rocky, like Earth, as­tron­o­mers said. 

Its year is 385 days long, about like Earth’s.

Endl was in­volved in fol­low­up studies about the plan­et, us­ing the Mc­Don­ald Ob­serv­a­to­ry. As he ex­plained, data from Kep­ler pro­vides the ra­tio of a po­ten­tial plan­et’s size to the star’s size, but not the ac­tu­al size of ei­ther. So once Kep­ler finds a plan­et can­di­date, tele­scopes at Mc­Don­ald Ob­serv­a­to­ry and else­where get to work learn­ing the de­tails.

In many ways, “if you know the host star, you know the plan­et,” Endl said.

The fol­lowup mea­sure­ments helped pin down the plan­et’s size to be­tween 1.4 and 1.8 times the size of Earth — a size that makes the­o­riz­ing about the plan­et’s make­up a bit tricky.

“At around 1.5 times the Earth’s ra­di­us there seems to be a tran­si­tion go­ing on from pre­dom­i­nantly rocky plan­ets to plan­ets that con­tain more volatiles — ices,” Endl said, “which would make it a mini-ice gi­ant.” In the case of Kep­ler-452b, “we don’t know if it’s a big rocky plan­et or if it’s a mini-Neptune,” and thus a large, gas­e­ous plan­et.

Ground-based mea­sure­ments al­so in­di­cat­ed that the host star, Kep­ler-452, is 1.5 bil­lion years old­er than the Sun, and is 10 per­cent larg­er and 20 per­cent brighter. It has the same tem­per­a­ture as the Sun, and like the Sun, Kep­ler-452b is clas­si­fied as a G2-type star.

“Kep­ler has re­cently shown that vir­tu­ally all of the stars that we see in the sky probably host plan­etary sys­tems,” said Bill Cochran, al­so of the uni­vers­ity and a co-in­vest­iga­tor of the Kep­ler mis­sion. “Now we are dis­cov­er­ing that a sig­nif­i­cant num­ber of those sys­tems are very much like our own and may have the ca­pa­bil­ity of be­ing hab­it­a­ble.”

In their view, the find­ing—a­long with 12 new small hab­it­a­ble zone can­di­dates Kep­ler has un­cov­ered, many around Sun-like stars—marks anoth­er mile­stone in the jour­ney to un­der­stand our place in the cos­mos.

“We can think of Kep­ler-452b as an old­er, big­ger cous­in to Earth, pro­vid­ing an op­por­tun­ity to un­der­stand and re­flect up­on Earth’s evolv­ing en­vi­ron­ment,” said Jon Jen­kins, Kep­ler da­ta anal­y­sis lead at NASA’s Ames Re­search Cen­ter. “It is awe-in­spir­ing to con­sid­er that this plan­et has spent 6 bil­lion years in the hab­it­a­ble zone of its star, long­er than Earth. That’s sub­stanti­al op­por­tun­ity for life to arise, should all the nec­es­sary in­gre­di­ents and con­di­tions for life ex­ist.”

Endl said that a star’s hab­it­a­ble zone changes over its life­time. As a star be­comes old­er and brighter, the more in­tense radia­t­ion pushes its hab­it­a­ble zone far­ther out. As­tro­no­mers es­ti­mate how long Kep­ler-452b has spent in its star’s hab­it­a­ble zone by com­bin­ing the star’s bright­ness and age with their meas­ure­ment of the plan­et’s or­bit.

The find­ing has been ac­cept­ed for pub­lica­t­ion in The As­tro­nom­i­cal Jour­nal.


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Astronomers working with NASA’s Kepler mission have found what they call the first near-Earth-sized planet in the “habitable zone” of a Sun-like star. The habitable zone is the range of distances from a star where temperatures are suitable for planets with liquid water on their surfaces. While other roughly Earth-sized planets have been found in habitable zones, this would be the first case in which the star has been deemed similar to our Sun. “We are pushing toward Earth 2.0,” said astronomer Michael Endl of the McDonald Observatory at the University of Texas at Austin, one of the researchers. “This planet is probably the most similar to Earth yet found.” The planet, dubbed Kepler-452b, lies about 1,400 light-years from Earth in the constellation Cygnus. A light-year is the distance light travels in a year. It’s 60 percent wider than Earth and is considered a “super-Earth-sized” planet. Its weight and makeup are unknown, but previous research suggests that a planet of its size has a better than even chance of being rocky, like Earth, astronomers said. Its year is 385 days long, about like Earth’s. Endl was involved in followup confirmations about the planet, using the McDonald Observatory. He explained Kepler data provides the ratio of a potential planet’s size to the star’s size, but not the actual size of either. So once Kepler finds a planet candidate, telescopes at McDonald Observatory and elsewhere get to work characterizing the host star in as much detail as possible. “If you know the host star, you know the planet,” Endl said. The followup measurements helped pin down the planet’s size to between 1.4 and 1.8 times the size of Earth — a size that makes theorizing about the planet’s makeup a bit tricky. “At around 1.5 times the Earth’s radius there seems to be a transition going on from predominantly rocky planets to planets that contain more volatiles — ices,” Endl said, “which would make it a mini-ice giant.” In the case of Kepler-452b, “we don’t know if it’s a big rocky planet or if it’s a mini-Neptune,” and thus a large, gaseous planet. Ground-based measurements also indicated that the host star, Kepler-452, is 1.5 billion years older than the Sun, and is 10 percent larger and 20 percent brighter. It has the same temperature as the Sun, and like the Sun, Kepler-452b is classified as a G2-type star. “Kepler has recently shown that virtually all of the stars that we see in the sky probably host planetary systems,” said Bill Cochran, also of the university and a co-investigator of the Kepler mission. “Now we are discovering that a significant number of those systems are very much like our own and may have the capability of being habitable.” In their view, the finding—along with 12 new small habitable zone candidates Kepler has uncovered, many around Sun-like stars—marks another milestone in the journey to understand our place in the cosmos. “We can think of Kepler-452b as an older, bigger cousin to Earth, providing an opportunity to understand and reflect upon Earth’s evolving environment,” said Jon Jenkins, Kepler data analysis lead at NASA’s Ames Research Center. “It is awe-inspiring to consider that this planet has spent 6 billion years in the habitable zone of its star, longer than Earth. That’s substantial opportunity for life to arise, should all the necessary ingredients and conditions for life exist on this planet.” Endl explained that a star’s habitable zone changes over its lifetime. As a star becomes older and brighter, the more intense radiation pushes its habitable zone farther out. Astronomers estimate how long Kepler-452b has spent in its star’s habitable zone by combining the star’s brightness and age with their measurement of the planet’s orbit. The finding has been accepted for publication in The Astronomical Journal.