The nearest may be no more than a dozen light years away.
Researchers came to the conclusion after reviewing four years’ worth of data from the American space agency Nasa’s Kepler space telescope.
With 300 billion stars in the Milky Way, it means our galaxy could potentially be teeming with life similar to that on earth.
Dr Andrew Howard, from the Institute of Astronomy at the University of Hawaii, who co-led the US study, said: “It’s been nearly 20 years since the discovery of the first extrasolar planet around a normal star.
“Since then we have learned that most stars have planets of some size and that Earth-size planets are relatively common in close-in orbits that are too hot for life.
“With this result we’ve come home, in a sense, by showing that planets like our Earth are relatively common throughout the Milky Way galaxy.
“For Nasa, this number – that every fifth star has a planet somewhat like Earth – is really important, because successor missions to Kepler will try to take an actual picture of a planet, and the size of the telescope they have to build depends on how close the nearest Earth-size planets are.
“An abundance of planets orbiting nearby stars simplifies such follow-up missions.”
Kepler conducted a massive search for planets beyond our solar system by detecting the tiny dip in brightness that occurs when an orbiting object crosses, or “transits”, in front of a star.
From some 150,000 stars photographed over four years, astronomers have discovered more than 3,000 planet candidates.
Many are much larger than the Earth, or follow orbits so close to their parent stars that their surfaces would be roasted.
But some are roughly Earth-size and lie in the “habitable” or “Goldilocks” zone – the orbital band close enough to a star for temperatures to be “just right” for life.
By definition, habitable zone temperatures are mild enough to allow oceans and lakes of liquid water.
Dr Howard’s team focused on 42,000 stars that are slightly cooler and smaller than the Sun. They found 603 candidate planets, of which just 10 were Earth-sized, with diameters one to two times that of the Earth, and orbiting in habitable zones.
The scientists knew that even using advanced computer software, a certain number of planets would have been missed. To estimate how many, they introduced fake planets into the Kepler data to see what percentage could be detected.
After accounting for the missing planets, plus the fact that only planets orientated the right way could be spotted “transiting” their stars, the astronomers calculated that 22% of all sun-like stars in the galaxy have Earth-size planets occupying their habitable zones.