An Earth kin turns far, far away

Kepler 186f bigger, colder, but likely habitable, scientists say

It is a bit bigger and somewhat colder, but a planet circling a star 500 light-years away is otherwise the closest match of Earth 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. 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 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, another step in astronomers’ quest for what might be called Earth 2.0.

“It’s a progression,” said another member of the discovery team, Thomas 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. “It probably has a composition made up of iron and rock and ice and some water, as Earth does,” Barclay said, though he added that “the relative combination of those things could be very different.”

The gravity on Kepler 186f, too, would be roughly the same as Earth’s.

“You could far more easily imagine someone being able to go there and walk around on the surface,” said Stephen Kane, an astronomer at San Francisco State and another member of the research team.

Kepler 186f is not a perfect replica, however. It is closer to its star - a dwarf star that is smaller and cooler than the sun - than the Earth is, and its year, the time to complete one orbit, is 130 days, not 365. It is also at the outer edge of the habitable zone, receiving less warmth, so perhaps more of its surface would freeze.

On the other hand, the researchers said that with its greater mass, Kepler 186f could conceivably have a thicker, more insulated atmosphere to compensate.

“Perhaps it’s more of an Earth cousin than an Earth twin,” Barclay said.

Astronomers cannot tell the exact age of the star, but such dwarfs are among the longest-lived stars in the universe. So if Kepler 186f is habitable, life would have plenty of time - likely 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 Kepler186f. It is too far away for astronomers to discern its mass, much less whether it has an atmosphere and oceans or whether it teems with living creatures.

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

Kepler 186 alone has four other planets of the super-Earth variety or smaller, although the others are too close and therefore too hot to be habitable.

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.

NASA is planning to launch a follow-up mission, the Transiting Exoplanet Survey Satellite, in 2017. That spacecraft will search for small planets around the closest and brightest stars.

Front Section, Pages 4 on 04/18/2014

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