Super-Earth found close by, may host water

A relatively small planet orbiting a star not far from Earth may be made mostly of water, new observations show.

“This planet is the most Earthlike planet yet discovered,” comments Geoffrey Marcy of the University of California, Berkeley. The observations are reported in the Dec. 17 Nature.

“We’re on the eve of a new phase of exoplanets studies,” adds Sara Seager of MIT. “This planet is a harbinger of what’s to come. It’s not just that we can study this one object in more detail. It’s the torch, telling us about this new thing that’s going to happen.”

The planet, called GJ 1214b, is the second super-Earth — a planet with a mass roughly between five and 10 times Earth’s — detected as it crossed in front of its star and the first that is close enough for astronomers to study its atmosphere. It is 6.5 times more massive than Earth and 2.7 times wider.

The discovery comes on the heels of other exoplanet sightings, two of which may also be super-Earths. All three newly found possible super-Earths fall in a mass range between that of Earth and Uranus — a range not represented in the solar system. “This is completely unexpected,” says Greg Laughlin of the University of California, Santa Cruz, a coauthor of the two papers reporting the other findings. “It tells us that planets really form very easily.”

Because GJ 1214b passes in front of its star, or transits, from Earth’s point of view, it’s possible to tell what the planet is made of. David Charbonneau of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and colleagues used the planet’s size and mass to calculate density, a clue to composition. The team found the radius by measuring how much light the planet blocked as it passed in front of the star. The planet’s mass was calculated using follow-up observations from the European Southern Observatory’s HARPS (High Accuracy Radial Velocity Planet Searcher) instrument, which measures how much the planet’s gravity tugs at the parent star.

The only other known transiting super-Earth, CoRoT-7b, has a rocky composition (SN: 10/10/09, p. 8). On the basis of its density, GJ 1214b could be as much as 75 percent water.

GJ 1214b orbits its star, a red dwarf 40 light years from Earth, once every 38 hours at a distance of 1.3 million miles — about one-fortieth the distance between Mercury and the sun. Despite its close orbit, the planet reaches temperatures of only about 280º Celsius because its star is relatively cool.

“If the sun is a 1,000-watt light bulb, this star is a 3-watt light bulb,” Charbonneau says. Most of the 413 extrasolar planets found to date have surfaces that boil at thousands of degrees Celsius.

Although the planet is hot as an oven, it may have a hydrogen and helium atmosphere that could provide enough pressure at the surface to keep water liquid, even at such high temperatures.

The planet surfaced as part of the MEarth (pronounced “mirth”) survey (SN: 12/20/08, p. 16). The survey uses eight ground-based, amateur-grade telescopes that automatically follow 2,000 small, dim stars in search of transiting planets. GJ 1214b showed up in the first six months of the start of observations.

“The fact that we found it in the first six months means either we were very lucky, or these planets are very common,” Charbonneau says.

The other newly found planets also indicate that super-Earths are common, Laughlin says. Using the Keck Observatory in Hawaii and the Anglo-Australian Telescope in Australia, Laughlin and colleagues found at least four and possibly six new planets orbiting sunlike stars, the teams report online December 14. One of them, orbiting the star 61 Virginis, is at least five times Earth’s mass, and its detection is slated to be published in the Astrophysical Journal  (currently at http://arxiv.org/abs/0912.2599). The other candidate super-Earth, which orbits the star HD 1461, is at least 7.5 times Earth’s mass, and a paper detailing its detection has been submitted to the Monthly Notices of the Royal Astronomical Society (currently at http://arxiv.org/abs/0912.2566).

None of the new planets are likely to be habitable, though. The planets near sunlike stars are too hot, and the possible atmosphere of GJ 1214b would be thick enough to block all incoming light, an important ingredient for life on Earth. But Charbonneau, Laughlin and Seager all say that finding habitable — and perhaps inhabited — planets is “only a matter of time.”

“These planets are like mileposts on the road rather than a destination itself,” Laughlin says. “It’s very exciting to know that we’re getting close.”