20 potentially habitable Earth-like planets identified

Scientists, including one of Indian origin, have identified 20 potentially habitable Earth-like rocky planets from a trove of over 4,000 exoplanets discovered by NASA's Kepler mission till date.

The new research outlines 216 Kepler planets located within the "habitable zone" — the area around a star in which a planet's surface could hold liquid water.

Of those, 20 were listed as the best candidates to be habitable rocky planets like Earth, researchers said.

"This is the complete catalogue of all of the Kepler discoveries that are in the habitable zone of their host stars," said Stephen Kane, an associate professor 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," Kane said.

The research also confirms that the distribution of Kepler planets within the habitable zone is the same as the distribution of those outside of it — additional evidence that the universe is teeming with planets and moons where life could potentially exist.

The boundaries of the habitable zone are critical. If a planet is too close to its star, it will experience a runaway greenhouse gas effect, like Venus. But if it is too far, any water will freeze, as is seen on Mars.

Researchers, including Ravi Kumar Kopparapu from NASA's Goddard Space Flight Centre in the US, sorted the planets by whether they were in a conservative or a more optimistic interpretation of the habitable zone.

Then they further sorted them by planet size: smaller, rocky planets versus larger gas giants.

The 20 planets in the most restrictive category — rocky surface and a conservative habitable zone — are the most likely to be similar to Earth.

Kane has already begun gathering additional data on these planets, as well as those in the other categories.

"It's exciting to see the sheer amount of planets that are out there, which makes you think that there is zero chance of there not being another place where life could be found," said Michelle Hill, an undergraduate at San Francisco State University.

"This study is a really big milestone towards answering the key questions of how common is life in the universe and how common are planets like Earth," said Kane.

The study was published in the Astrophysical Journal.