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Earth's 'cousin' planet lies 500 light-years away
BY STEPHEN CLARK
ASTRONOMY NOW

Posted: 17 April 2014


Astronomers using NASA's Kepler space telescope have found the first Earth-sized planet around another star that might harbor life-sustaining liquid water, raising hopes for detecting more rocky Earth-like worlds closer to home.


The artist's concept depicts Kepler-186f, the first validated Earth-size planet to orbit a distant star in the habitable zone. Photo credit: NASA Ames/SETI Institute/JPL-Caltech
 
Scientists know little about the planet, other than it is roughly the same size as Earth and is positioned just far enough from its star for water to exist in liquid form.

Named Kepler-186f, the planet is orbiting a faint red dwarf star 500 light-years away in the constellation Cygnus, where the Kepler space telescope has focused its sharp-eyed camera since launching from Earth in March 2009.

"This is the first validated Earth-sized planet in the habitable zone of another star," said Elisa Quintana of the SETI Institute at NASA Ames Research Center in California, the lead author of a paper on the discovery published in Science.

Kepler has found nearly 1,000 planets around other stars, or exoplanets, over its five-year mission. That is more than half of the total haul of exoplanets, which stands at approximately 1,800.

But only 20 of the alien worlds are in the so-called "goldilocks zone," where temperatures are not too hot or too cold, but just right for liquid water to exist on the surface, a key ingredient for life.

Planets closer to their host stars are exposed to searing heat and the bombardment of radiation from stellar flares, stripping away their atmospheres and causing water to boil off into space. Farther away from a star, planets are likely to be encrusted in ice.

Before the discovery of Kepler-186f, the potentially habitable worlds in the exoplanet catalog were all larger than Earth, leaving some doubt whether the planets were rocky or gaseous.

The difference with Kepler-186f is it has about the same diameter as Earth, so scientists are sure it is not enshrouded in a gaseous haze like Jupiter or Saturn. Its surface is probably like the planets in the inner solar system -- made up of some combination of iron, rocks and water, Quintana said.

"Theoretical models of how planets form suggest that those with diameters less than 1.5 times that of 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," said Tom Barclay, a staff scientist on the Kepler mission from the Bay Area Environmental Research Institute in California. "Consequently, Kepler-186f is likely a rocky world, and in that sense similar to Venus, Earth and Mars."

Scientists say their best estimate is the planet is about 1.1 times the diameter of Earth and has a year about 130 days long.

Scientists say the planet may be far enough from its star that it is not tidally locked, where a star's gravity causes the same side of the planet to always be in daylight.

Tidal locking could foster harsher climates with extreme temperatures.

Kepler-186f orbits much closer to its parent star than Earth does to the sun, but the region still likes with the habitable zone because the planet's red dwarf host is dimmer and cooler.

In fact, Quintana said, the planet only receives about one-third of the stellar heat from its star as Earth does from the sun.

"This places the planet near the cooler edge of the star's habitable zone," Quintana said.

Scientists do not know if Kepler-186f has an atmosphere or any of the chemical ingredients for life.

"Just because a planet is in the habitable zone doesn't mean it's habitable," Barclay said.

"We haven't mentioned anything about the temperature of this planet, and that's for a very good reason," Barclay said. "The temperature of the planet is strongly dependent on the type of atmosphere surrounding the planet ... We don't know if it has an atmosphere."


Artist's concept of the Kepler spacecraft. Photo credit: NASA
 
The Kepler telescope works by monitoring stars for dips in brightness caused by the brief passage of a planet blocking out a portion of the starlight reaching the spacecraft's camera. The ground-based Gemini Observatory and W.M. Keck Observatory confirmed the discovery of Kepler-186f through follow-up measurements.

Barclay said the observations, coupled with numerical calculations, indicate a 99.98 percent probability Kepler-186f is real.

Kepler-186f is the fifth and outermost planet found around the star by Kepler. The other worlds all orbit much closer, with temperatures too hellishly hot to support life.

The star is a main-sequence M-dwarf, a very common type of star, according to Quintana.

More than 70 percent of the hundreds of billions of stars in the galaxy are M-dwarfs, Quintana said.

"Kepler-186f orbits a cooler star than our sun, so it's perhaps more like Earth's cousin than Earth's twin," Barclay said.

Scientists are still searching for an Earth-sized planet around a star like the sun, a medium-sized yellow dwarf, which yield more promise for follow-up observations.

Astronomers fear Kepler-186f is too far away and its host star too dim for further surveys by next-generation observatories, such as the James Webb Space Telescope set for launch in 2018. JWST is powerful enough to measure the atmospheres of planets around other stars if they are relatively close and bright.

NASA's next planet-hunter, the Transiting Exoplanet Survey Satellite set for launch in 2017, will scan nearby stars to detect candidates for further observations by JWST.

Barclay said habitable zone planets around red dwarfs are easier to find than worlds around stars like the sun. The lower ratio of the diameter of a parent star to the size of a planet, plus the smaller size of red dwarf planetary systems, ease the job of ferreting out Earth-sized worlds from data noise.

"Planets like this one, albeit discovered much closer ... will likely provide our first opportunity to search for life beyond the solar system," said Victoria Meadows, professor of astronomy at the University of Washington in Seattle and principal investigator for the Virtual Planetary Laboratory at the NASA Astrobiology Institute at Ames.

Follow Stephen Clark on Twitter: @StephenClark1.

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