<iframe width="590" height="332" src="http://launch.newsinc.com/?type=VideoPlayer/Single&widgetId=1&trackingGroup=69016&siteSection=sfgate_nws_us_sty_pp&videoId=25819366" frameborder="no" scrolling="no" noresize marginwidth="0" marginheight="0"></iframe>This artist's rendering provided by NASA on Thursday, April 17, 2014 shows an Earth-sized planet dubbed Kepler-186f orbiting a star 500 light-years from Earth. Astronomers say the planet may hold water on its surface and is the best candidate yet of a habitable planet in the ongoing search for an Earth twin. Photo: T. Pyle, Associated Press
Scientists hunting for planets far beyond our solar system have discovered the first alien world whose rocky surface may hold liquid water - the essential ingredient for life as we know it.
Of all the hundreds of so-called exoplanets found by international astronomers in the past 20 years, many of those worlds are bigger than gassy Jupiter, one or two are smaller than tiny Mercury, and some are roughly Earth-sized.
But this one is different.
It is only a little larger than Earth, our own home planet. It is the outermost of five small planets orbiting a cool and extremely common kind of star. Its surface is most probably rocky. And during its 130-day orbit around its star, it flies entirely within its habitable zone - where it is just the right distance from its star for temperatures on its surface to be just right - neither too hot nor too cold - for reservoirs of liquid water to exist.
It's the kind of exoplanet that fiction writers like to speculate about as an abode for distant life, although most astronomers would never make a leap to such a subject.
The newfound planet lies some 500 light-years away in the Milky Way constellation Cygnus, and it marks another triumph for NASA's Kepler spacecraft, whose telescope is scanning more than 145,000 stars in its search for distant planets.
Kepler has now found confirmed evidence for more than 960 planets in distant solar systems since it was launched five years ago, and continues finding them in the mass of data it has collected since the telescope's main pointing system became disabled nearly a year ago.
The latest exoplanet's identity was determined from Kepler's telescopic data by an international research team led by planet hunters at the SETI Institute and NASA's Ames Research Center in Mountain View, together with scientists from a dozen other research institutions.
Leading the group was Elisa V. Quintana, 40, a SETI postdoctoral physicist at the Ames Center, where scientists are continuing to gather streams of data from the Kepler telescope in hopes of finding still more Earth-like exoplanets.
A 'historic discovery'
UC Berkeley astronomer Geoffrey Marcy, a Kepler team member and a longtime leader in the international search for far-off planets, called the Quintana team's achievement a "historic discovery." He added in an e-mail, "This is the best case for a habitable planet yet found. Their results are absolutely rock-solid."
A report on the find was published Thursday in the journal Science.
Designated as Kepler186f, the planet is within 10 percent or so of the size of Earth, Quintana said. And the relatively cool star it orbits is called an M-type red dwarf - the kind of star that makes up at least 70 percent of all the stars in the Milky Way.
The newfound planet is "more like a cousin than a twin" of Earth, said Thomas S. Barclay of the Bay Area Environmental Research Institute in Sonoma, who works on the Kepler team at the Ames Center.
The telescope aboard Kepler is so extraordinarily sensitive it detects distant planets by what is called the transit method: It captures the instant dimming of a distant star's light when a dark object passes across its face, and by painstaking analysis of such tiny periodic changes in starlight again and again, Quintana and her colleagues can determine whether or not it's a planet.
In fact, Quintana said, it has taken four years of analysis to make sure Kepler186f is indeed what it is, the most Earth-like exoplanet yet discovered.
"There must be so many small bodies close to stars like this one, and with so many stars in the galaxy, we should be finding more and more," Quintana said.
And because those red dwarf stars are so common throughout the Milky Way, said Barclay, "they'd be fantastic places to look for more planets in the habitable zone."
Dysons, Hawkings, Plancks, and Einstiens won't be as rare.
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