For the first time, an Earth-sized planet has been found in the habitable zone of a star — the right distance away to host liquid water and possibly life.

"This one is the most Earth-like so far," said Jason Rowe, a Canadian scientist who helped discover the new planet, in an interview that airs Saturday on CBC's Quirks & Quarks.

The new planet, Kepler 186f, has a radius just 10 per cent larger than that of the Earth, reported Rowe and his colleagues in a paper published online in the journal Science on Thursday. That means that it is likely to have a solid, rocky surface, like Earth.

If Earth-like planets are common around stars like that, we have potentially habitable zone planets in our own backyard — planets that already might be receiving the first radio broadcasts that came from the Earth. - Jason Rowe, Kepler Science Office

Scientists believe planets can't become that much bigger than Earth before they start to resemble gas giants like Neptune or Jupiter rather than Earth.

Previously, Earth-sized planets and planets in the habitable zones of stars have been found, but none has met both criteria at the same time. The previous smallest planet found in the habitable zone of a star was 60 per cent bigger than Earth, said Rowe, a Canadian research scientist with the Kepler Science Office and the SETI Institute, who works at NASA's Ames Research Center in California.

The discovery of Kepler-186f confirms that Earth-size planets exist in the habitable zone of other stars and signals a significant step closer to finding a world similar to Earth, as depicted in this image, researchers say. (Danielle Futselaar)

The new planet, discovered by the Kepler space telescope, is about 500 light years (about 4,700 trillion kilometres) away from Earth in the constellation Cygnus. It is the outermost of five planets orbiting a small, cool red dwarf star or M-star, and it completes its orbit every 130 days, reported researchers led by planetary scientist Elisa Quintana at the SETI Institute and NASA Ames Research Center.

Because red dwarfs are quite different from our sun, which is a bigger, hotter kind of star called a yellow dwarf, Quintana considers the new planet to be a "cousin" of Earth, rather than a "twin," she told The Associated Press.

Rowe said the new discovery suggests us that Earth-like planets around M-stars are common. That's exciting because M-stars are the most common stars in our galaxy and there are about 100 to 200 of them within 100 light years of Earth.

"If Earth-like planets are common around stars like that," Rowe added, "we have potentially habitable zone planets in our own backyard — planets that already might be receiving the first radio broadcasts that came from the Earth."

Star is half the size of the sun

Kepler-186f's star has half the mass of the sun, and the planet receives just a third of the heat that Earth receives from the sun. That means that even though it's as close to its star as Mercury is to our sun, Kepler-186f is on the outer edge of the "habitable zone" warm enough for liquid water to exist.

"However, it is also slightly larger than the Earth, and so the hope would be that this would result in a thicker atmosphere that would provide extra insulation," Stephen Kane, a San Francisco State University astronomer who co-authored the paper, said in a statement.

Kane added that smaller stars live much longer than bigger stars, providing more time for biological evolution to take place on planets orbiting them.

The researchers cautioned, however, that just because a planet is in the habitable zone of a star doesn't mean it's habitable, since the actual surface temperature depends very heavily on what type of atmosphere — if any — the planet has.

More than 1,700 confirmed planets have been discovered outside our solar system so far, including 961 by the Kepler telescope. The telescope, launched in 2009, found planets by watching for small changes in the brightness of stars caused by planets passing in front of them. The telescope was retired from planet hunting last year, after the system that allows it to point steadily at a given star broke down. However, many new planets are still being found in the data it collected before that.