Astronomers are narrowing the field in their search for a “second Earth.”

An international team of researchers has identified the 20 most Earth-like worlds among the more than 4,000 exoplanet candidates that NASA’s Kepler space telescope has detected to date, scientists report in a new study.

All 20 potential “second Earths” lie within the habitable zones of their sun-like stars meaning they should be able to harbor liquid water on their surfaces and are likely rocky, the researchers said. [The Search for Another Earth (Video)]

Identifying these Earth-like planets is important in the hunt for alien life, said study lead author Stephen Kane, an associate professor of physics and astronomy at San Francisco State University (SFSU).