The now-defunct Kepler space telescope, launched in 2009, was designed to detect changes in a star's brightness as an orbiting planet crosses in front of it.
The Kepler scientists reported about 3000 planetary "candidates", estimated by taking photographs every 30 minutes of about 150,000 stars.
Many of these planets are much larger than Earth and unsuitable for water and life, such as gaseous giants like Jupiter or those with thick atmospheres like Neptune, or planets that orbit too close to their star.
"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," said Andrew Howard of the University of Hawaii.
The scientists focused on a tiny sample of 42,000 stars that are similar in size and temperature to our Sun. They found 603 planets orbiting these stars, but only 10 were the in the same size-range as Earth and with an orbit suitable for liquid water.
It is likely many of the Earth-size planets in the analysis are not suitable for life even though they fall in to the habitable zone. Mars and Venus, for instance, are not habitable, although liquid water may have existed on them in the past.
"Some may have thick atmospheres, making it so hot at the surface that DNA-like molecules would not survive. Others may have rocky surfaces that could harbour liquid water suitable for living organisms," said Professor Geoffrey Marcy at Berkeley.
"The primary goal of the Kepler mission was to answer the question: when you look up in the night sky, what fraction of the stars that you see have Earth-size planets at lukewarm temperatures so that water would not be frozen into ice or vapourised into steam?" Professor Marcy said.
The focus on liquid water is critical because it is widely thought that it is a prerequisite for life, said Professor Marcy, who collaborated on the study published in the journal Proceedings of the National Academy of Sciences.
"Until now, no one knew exactly how common potentially habitable planets were around Sun-like stars in the galaxy."
* Astronomers have calculated that about one in every five of the billions of Sun-like stars in our galaxy has at least one rocky planet orbiting it at a distance where water is neither frozen solid nor boiled dry.
* Many of these planets are much larger than Earth and unsuitable for water and life, such as gaseous giants like Jupiter or those with thick atmospheres like Neptune.
* The nearest Sun-like star with an Earth-size planet in its habitable zone is probably only 12 light years away and can be seen with the naked eye.
* The scientists focussed on 42,000 stars that are similar in size and temperature to our own Sun.
* They found 603 planets orbiting these stars, but only 10 of them were in the same size-range as Earth and with an orbit suitable for liquid water.
* Liquid water is critical because it is widely thought that it is a prerequisite for life.
- Independent
