An ‘Earth Twin’ has been found, or maybe a cousin

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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 discovered so far, astronomers announced on Thursday.
The planet, known as Kepler 186f, named after NASA’s Kepler planet-finding mission, which detected 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.

“Kepler 186f is the first validated, Earth-size planet in the habitable zone of another star,” Elisa V. Quintana of the SETI Institute and NASA’s Ames Research Center in Mountain View, Calif., said at a news conference on Thursday. “It has the right size and is at the right distance to have properties similar to our home planet.”

Dr. 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 drops in brightness when a planet passed in front.

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She added, “It’s fun to note that if the planet is habitable, photosynthesis may be possible.”

At the wavelengths that plants need, Kepler 186f receives only about a sixth as much light as Earth does, but “there are plenty of Earth plants that would be quite happy with that,” Dr. Meadows said.

Astronomers cannot tell the exact age of the star, but such dwarfs are the longest-lived stars in the universe. If Kepler 186f is habitable, life would have had plenty of time — billions of years — to take hold.

But speculation about the planet will remain speculation for a long time, if not forever. The Kepler measurements indicated only the size of Kepler 186f. It is too far away for astronomers to discern its mass, much less whether it has an atmosphere and oceans or if it teems with living creatures.

Nonetheless, since dwarfs are the most plentiful type of star in the galaxy, astronomers are hopeful that Earth twins are plentiful, and that some will be found close by, allowing other telescopes to make temperature and mass measurements or to identify molecules in the atmosphere.

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 962 confirmed planets. More than 2,800 planet candidates remain to be studied.

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