While 216 of the planets found by the Kepler mission resided in the so called “habitable zone” – a region around a star where a planet’s surface could retain liquid water – only a select group of 20 planets are thought to have “second-Earth” potential. This was based on a criteria of their position in the habitable zone, rocky composition, and Earth-like size.

“This is the complete catalogue of all of the Kepler discoveries that are in the habitable zone of their host stars,” said the study’s lead author, Stephen Kane, associate professor of physics and astronomy at San Francisco State University.

“That means we can focus in on the planets in this paper and perform follow-up studies to learn more about them, including if they are indeed habitable.”