A rocky and water-rich planet, not much heftier than our own, has been discovered so close to our solar system that astronomers one day may be able to study its atmosphere.
And though astronomers are pretty certain the water exists, they don't know its state, with speculations ranging from liquid water to water ice and an exotic state called a superfluid.
The extrasolar planet, now named GJ 1214b, is about 40 light-years away. It orbits a red dwarf star. It is the only known "Super-Earth" exoplanet ? worlds that have masses between Earth and Neptune ? with a confirmed atmosphere.
"Astronomically speaking, this [planet] is on our block," meaning it's in our cosmic neighborhood, said study leader David Charbonneau of the Harvard-Smithsonian Center for Astrophysics in Mass. "For perspective, our own TV signals have already passed beyond the distance of this star."
The planet is about three times the size of Earth and about 6.5 times as massive. It is the second smallest planet discovered outside of our solar system to date, trailing behind only , which is 1.7 times Earth's size and about five times as massive.
GJ 1214b is rare among known rocky exoplanets because it partially eclipses, or transits, its star as seen from Earth.
This fortunate alignment allows astronomers to calculate the size and density of the planet, and Charbonneau's team thinks GJ 1214b is likely a water world with a solid center. Moreover, the planet has a thick surrounding atmosphere of hydrogen and helium.
Normally, a planet located at that distance from this particular type of star would be so hot that any water on its surface would be in a vapor form.
But scientists think the thick atmosphere of GJ 1214b creates a high pressure environment that keeps water on the surface in a liquid state.
That's just speculation, however.
"It really depends on how hot the planet is on the inside, and we don't know that," Sara Seager, an astrophysicist at MIT, told SPACE.com. "I think this planet doesn?t have liquid water because it is too hot on the inside. I think it goes from water ice, to a very exotic kind of water ? a superfluid ? and then it goes to vapor," added Seager, who just published a new eBook on exoplanets called "Is there Life Out There? The Search for Habitable Exoplanets."
(Seager was not involved in the new planet discovery.)
There are downsides to having such a thick atmosphere: First, the pressure is crushing, making life as we know it difficult. And secondly, the thick atmosphere blocks light from the feeble star from reaching the planet's surface.
"If you picture the sun as a 1,000-watt light bulb, this star is a 3-watt light bulb," Charbonneau said.
Whatever its exact composition, astronomers are excited about the finding. "We're really looking for a planet that's a big Earth orbiting in the Goldilocks zone of a small star, transiting," Seager said. "This one scores three out of four. We're excited because we're getting closer and closer to the thing we want to find." The missing piece: GJ 1214b doesn't orbit within the star's habitable, or Goldilocks, zone.
The planet was discovered using a suite of small, ground-based telescopes and is detailed in this week's issue of the journal Nature.