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The apparently watery super-Earth GJ 1214b orbits a red dwarf star in an artist's conception.

The apparently watery super-Earth GJ 1214b orbits a red dwarf star in an artist's conception.

Image courtesy David A. Aguilar, CfA

Ker Than

for National Geographic News

December 16, 2009

A newfound "water world" orbiting a star just 40 light-years away is the first known Earthlike planet close enough for us to "sniff" its atmosphere, astronomers say.

Dubbed GJ 1214b, the planet is only about 2.7 times larger than Earth and about 6.5 times more massive. (Get the latest on the search for new Earths.)

Based on its density, scientists think GJ 1214b is made up of about three-quarters liquid water with a solid core of iron and nickel and an atmosphere of hydrogen and helium—not unlike Earth.

But in most other ways, the planet is a "very different beast" from our home world, the researchers say.

"It's basically one big ocean," said study leader David Charbonneau of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.

"There are no continents of rock floating on top or peaking up through the water."

(Related: "Most Earthlike Planet Yet Found May Have Liquid Oceans.")

Moreover, GJ 1214b is hotter than Earth and its atmosphere is ten times thicker than our own, the study authors say.

This would make things difficult for life as we know it. For starters, the atmospheric pressure on the planet's surface must be immense, and very little light would be able to penetrate the haze to reach the oceans.

New Planet Earthlike, but "Completely Alien"

The new super-Earth was discovered using the MEarth project, a suite of small, ground-based telescopes set to detect minute changes in the brightness of dim, red stars known as M dwarfs.

Periodic dips in starlight can be caused by planets that partially eclipse, or transit, their host stars. Because M dwarfs are dimmer than stars like our sun, it's easier to spot light reductions caused by smaller Earth-size planets.

(Explore an interactive guide to the known planets, including those outside our solar system.)

Although GJ 1214b isn't directly visible, the exact changes in starlight due to its transit allowed astronomers to measure the planet's size and mass, offering clues to its composition.

And because the water world is so close to Earth, Charbonneau added, space-based optical telescopes such as Hubble or Kepler could one day be used to "sniff out" the exact chemicals in the planet's atmosphere.

"Some of the light from the star passes through the atmosphere [on its way to Earth], and imprinted on that are features of whatever atoms and molecules are present," Charbonneau said.

Overall, the discovery is a "landmark find" that fills a knowledge gap in planetary science, said Greg Laughlin, an astrophysicist at the University of California, Santa Cruz, who was not involved in the study.

"I've always wondered what a six-Earth-mass planet would be like," Laughlin said. "Now we know. It's something completely alien to our own solar system."

Findings published in this week's issue of the journal Nature.

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