An international team of astronomers has produced the most detailed analysis of Earth-like exoplanets yet. According to their analysis, we have discovered 20 Earth-sized rocky planets located within the "habitable zone" – the region around a star where a planet's surface could hold liquid water.

The research, which is to be published in the Astrophysical Journal, was possible thanks to the latest Kepler data release, which reported on the properties of various planets and their stars. The team discovered 216 objects in the habitable zone.

"This is the complete catalog of all of the Kepler discoveries that are in the habitable zone of their host stars," said lead author Stephen Kane, an associate professor of physics and astronomy at San Francisco State University, in a statement. "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."

Establishing the boundaries where the habitable zone lies was a complex task. If the planet is too far away, the planet will freeze and become a dry world like Mars. But if it’s too close, it will experience a runaway greenhouse effect and turn into Venus. In the middle lies the habitable zone, and since it’s not too cold and not too hot, it is often called the Goldilocks zone.

The researchers divided the types of planets into two: Anything smaller than two Earth radii was Earth-like. Anything bigger was a giant planet. Then each category was given an extra condition of being either in a conservative habitable zone or in a more optimistic one.

The paper, which is available on arXiv, tells us that we have already discovered 49 potentially welcoming Earth-sized planets (20 in the conservative habitable zone and 29 in the optimistic habitable zone), and even though the rest are likely super-Earths or gas giants, their moons might have the right conditions for life.

This research will provide an important starting point for the next generation of telescopes, such as TESS or JWST, which will hunt for signatures of life in the atmosphere of these objects.

"There are a lot of planetary candidates out there, and there is a limited amount of telescope time in which we can study them," Kane said. "This study is a really big milestone toward answering the key questions of how common is life in the universe and how common are planets like the Earth."