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 per cent 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.
"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."
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.
This follows the announcement last year that another star, Kepler 62, has two planets in 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 S. Barclay of the Bay Area Environmental Research Institute. "This planet really reminds us of Earth."
The researchers speculate that it is made of the same stuff as Earth - iron, rock, ice liquid water, although the relative amounts could be very different.
The gravity on Kepler 186f, too, is likely to 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 University and another member of the research team.
Kepler 186f is not a perfect replica, however. It is closer to its star - a red dwarf that is smaller, cooler and fainter than our sun - than the Earth is to its. 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.
"Perhaps it's more of an Earth cousin than an Earth twin," Barclay said.
On the other hand, with its greater mass, Kepler 186f could conceivably have a thicker, insulating atmosphere to compensate.
Red dwarfs emit more of their light at the longer infrared wavelengths, which would be more readily absorbed and trapped by ice and gases like water vapor and carbon dioxide.
"This makes the planet more efficient at absorbing energy from its star to avoid freezing over," said Victoria Meadows, an astrobiologist and planetary astronomer at the University of Washington. "Which is why this planet is still considered potentially habitable, as long as it has a dense enough atmosphere, even though it receives less light from its star than Mars does from our sun."
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," 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 961 confirmed planets. More than 2,800 planet candidates remain to be studied.