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Scientists who study planets outside our solar system have long been hunting for Earth's twin. They want to find a small rocky planet just like our home because it would offer the right conditions for life as we know it. Well, now, a study suggests that other kinds of planets might offer even better conditions for life.

Here's NPR's Nell Greenfieldboyce.

NELL GREENFIELDBOYCE, BYLINE: Scientists know of one planet that is definitely habitable: Earth. But Rene Heller says if you're interested in finding other habitable planets that might harbor alien life, you shouldn't assume that the best strategy is to look for another Earth. Worlds that seem very different might be even more suitable for life to emerge and evolve.

RENE HELLER: Maybe there is a population of, as we term them, superhabitable planets, which are even more likely than Earth-like planets to actually be inhabited.

GREENFIELDBOYCE: Heller is an astrophysicist at McMaster University in Canada. He and a colleague have just published a new study in the journal Astrobiology that says we need to move beyond the idea that the Earth is the quintessential habitable planet. They list a bunch of features that could make a planet superhabitable. For example...

HELLER: These planets are likely more massive than Earth-like planets, and they will tend to be older than Earth.

GREENFIELDBOYCE: More massive means they'll probably have a better magnetic field to offer protection from radiation. Older means more time for life to take hold. And while the planet should have water, Heller says they wouldn't have one big deep ocean, but rather lots of shallow seas, because on Earth, shallow water is a fantastic place to live.

Rory Barnes is an astrobiologist at the University of Washington who uses computer models to explore the habitability of planets outside our solar system. He thinks the new report is a convincing list of the features that would make a planet an even more cozy home.

RORY BARNES: There are things about the Earth that we could improve upon. You know, we could imagine a world that would have more life on it or allow life to thrive for a longer period of time than our Earth.

GREENFIELDBOYCE: But being able to imagine that kind of world doesn't mean they can find it. So far, scientists have detected about a thousand planets orbiting other stars. Current technology usually can't reveal much, just a planet's size, density and how far it orbits from its host star.

BARNES: We have so little information that we can't really say that a planet is even habitable, which would mean maybe that it just has liquid water and no life. You know, we don't - we just can't get that information today.

GREENFIELDBOYCE: But Barnes says once scientists do have a list of planets that are potentially habitable, the ideas in this new report should help rank them in terms of the potential for life.

BARNES: And so it's sort of more forward-looking than I think what a lot of people are doing right now.

GREENFIELDBOYCE: Still, he does have this word of caution. It could very well be that some planets have conditions or processes that are great for life but so alien to us that we'd never think of them.

Nell Greenfieldboyce, NPR News.

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