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Scientists have been able to detect hundreds of planets outside our solar system orbiting faraway stars. Mostly, these planets seem to be gas giants, like Jupiter. But recently, astronomers have been getting closer to finding one like Earth. Earlier this year, they reported finding a small, rocky planet.

And as NPR's Nell Greenfieldboyce reports, they have now found one that's probably mostly water.

NELL GREENFIELDBOYCE: The name of this new planet is GJ 1214b. Every day and a half, it circles a small reddish star. Scientists detected the planet by watching that star. They saw the star dim slightly as the planet crossed in front of it and could see how the star wobbled as the planet's gravity pulled on it. All that told them the size of the planet and how much it weighed, letting them figure out what it's likely made of.

Professor DAVID CHARBONNEAU (Astronomy, Harvard-Smithsonian Center for Astrophysics): This is certainly the first planet around another star, which we think is mostly made of water.

GREENFIELDBOYCE: That's David Charbonneau at the Harvard-Smithsonian Center for Astrophysics. He led the team which is reporting on this planet in the journal Nature.

Prof. CHARBONNEAU: We think what we have here is a water world with a thin atmosphere surrounding it, perhaps of hydrogen and helium.

GREENFIELDBOYCE: Pretend you could get in a spaceship and go to this planet, which you can't, since it's 40 light-years away. But if you could, first, you'd see the dim red star. Then you'd see the planet, about two and a half times the size of Earth, and shrouded in an alien atmosphere.

Prof. CHARBONNEAU: And then if you imagine descending down into that atmosphere, which I would advise against because of the very high pressure, and then looking back, you would slowly see this red, faint illuminating star fade out of view and ultimately disappear from view just due to the thickness of the atmosphere.

GREENFIELDBOYCE: You'd be in darkness and then maybe splash into an ocean. Even though this planet is about 400 degrees Fahrenheit, the pressure of the atmosphere could keep water liquid.

Liquid water usually excites scientists because it supports life on Earth. But Charbonneau says completely dark, super hot water under crushing pressures doesn't seem so friendly.

Prof. CHARBONNEAU: The surface of our planet probably wouldn't be anything like the surface of the Earth and, therefore, wouldn't host life as we know it.

GREENFIELDBOYCE: He says additional telescope observations should reveal more about the planet's atmosphere.

Marc Kuchner is a planetary scientist at NASA's Goddard Space Flight Center in Maryland. He says it's possible this planet is made of something other than water, but the water world explanation is appealing since our own solar system has planets like Neptune, made of ice.

Mr. MARC KUCHNER (Astronomer, NASA's Goddard Space Flight Center, Maryland): And it's easy to imagine a world like that located much closer to its star, so it's hot, and the ice has melted into water.

GREENFIELDBOYCE: Kuchner says this is only the second time scientists have been able to figure out the likely composition of a planet not too much bigger than Earth around a distant star. That other planet is too dense to be made mostly of water. Instead, it's probably a super hot rocky planet covered in lava.

Nell Greenfieldboyce, NPR News.

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