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Scientists find an ‘Earth twin’ or maybe a cousin
It is a bit bigger and somewhat colder, but a planet circling a star 500 light-years away is otherwise the closest match of our home world yet discovered, astronomers announced Thursday.
The planet, known as Kepler 186f, named after NASA's Kepler planet-finding mission, which found it, has a diameter of 8,700 miles, 10 percent wider than Earth, and its orbit lies within the "Goldilocks zone" of its star, Kepler 186 - not too hot, not too cold, where temperatures could allow for liquid water to flow at the surface, making it potentially hospitable for life.
"It's Earth size," said Elisa V. Quintana of the SETI Institute and NASA's Ames Research Center in Mountain View, Calif. "It's in the habitable zone. So we now know these planets do exist."
Quintana is the lead author of a scientific paper describing the findings in this week's issue of the journal Science. Kepler 186f is the latest planet to be sifted out of the voluminous data collected by Kepler, which kept watch over 150,000 stars, looking for slight dimmings of brightness when a planet passed in front.
This follows the announcement last year that another star, Kepler 62, has two planets within its habitable zone, but those two were "super Earths," with masses probably several times that of Earth. The gravity of those planets might be strong enough to pull in helium and hydrogen gases, making them more like mini-Neptunes than large Earths.
With its smaller size, Kepler 186f is more likely to have an Earth-like rocky surface.
"It's a progression," said another member of the discovery team, Thomas S. Barclay of the Bay Area Environmental Research Institute. "This is a very, very exciting milestone discovery. It has a much higher probability of being habitable. This planet really reminds us of Earth."
The researchers speculate that it is made of the same stuff as Earth. Kepler 186f is not a perfect replica, however. It is closer to its star than the Earth is, and its year, the time to complete one orbit, is 130 days, not 365.
"Perhaps it's more of an Earth cousin than an Earth twin," Barclay said.
Kepler's original mission ended last year, with the failure of equipment that kept the telescope precisely pointed, but scientists still have years of work in analyzing the data, which has so far yielded 961 confirmed planets. More than 2,800 planet candidates remain to be studied.