Meet GJ 1214b, the most Earth-like planet ever found outside our solar system.

It's not exactly Earth's twin: It's about six times bigger, a whole lot hotter and made mostly of water. But compared to the giant gas balls that account for nearly every other extrasolar planet ever found, it's pretty darn close. And through a fortunate happenstance of cosmic geometry, astronomers will be able to study GJ 1214b in great detail.

"If you want to describe in one sentence what this planet is, it's a big, hot ocean," said Harvard University astronomer David Charbonneau. "We can even study its atmosphere. This planet will occupy us for years. That's part of what's so exciting about it."

Described by Charbonneau and 17 other astronomers in a paper published Wednesday in Nature, GJ 1214b is the latest of roughly 400 planets detected by earthly telescopes. Of these, 28 are considered "super-Earths" — planets with a mass roughly comparable to our own.

The super-Earths themselves are too distant to be seen. Instead, astronomers infer their presence from subtle distortions in starlight, caused when photons travel through the super-Earths' gravitational fields. Depending on the degree of distortion, astronomers can even calculate a planet's mass.

That's how Corot-7b, a rocky planet with roughly twice the heft of Earth, was spotted in February. Ditto Gliese 581c, identified two months later, and orbiting its star at a distance consistent with human notions of habitability.

Unfortunately, not much more will ever be known about those planets. Corot-7b is 500 light-years away, too distant for our telescopes to discern more detail. And from our viewing angle, Gliese 581c never quite crosses directly in front of its sun, causing photons to warp in ways that would reveal its atmospheric character.

GJ 1214b does pass in front of its sun. Separated from Earth by a distance of just 42 light years, it's close enough to be studied. Scientists will finally get to look at another Earth-like world.

"Only rarely does a long-sought scientific frontier loom so prominently just beyond the horizon, that the next generation of instruments seems sure to reach it," wrote Geoffrey Marcey, a University of California, Berkeley astronomer, in a commentary accompanying the findings. "They provide the most-watertight evidence so far for a planet that is something like our own Earth, outside our solar system."

Based on its radius and mass — about 2.7 and 6.6 times that of Earth's — Charbonneau and the other astronomers have calculated GJ 1214b's density. It appears to be composed of extraordinarily deep oceans, surrounding a rocky core.

The planet's atmosphere and precise composition remain a mystery, but it's likely composed of many of the same elements found elsewhere at sites of planetary formation, in swirling disks of dust and gas that have yet to accrete: hydrogen, helium, nitrogen, magnesium, oxygen, carbon.

That list of ingredients raises at least the possibility of life. With an estimated temperature of 370 degrees Fahrenheit, GJ 1214b is an unlikely incubator (Earth's toughest extremophile, a microbe that lives in deep-sea volcanic vents, maxes out at 284 degrees) but it's not impossible.

"I don't want to imply that there's any indication of life as we know it. It might have life, but it would have to be a strange kind of life," said Charbonneau.

The telescopes sure to be trained on GJ 1214b in the near future will try to answer that question. But even if it proves barren, other planets await. The telescopes that spotted GJ 1214b were custom designed to find Earth-like planets around nearby stars, and had only operated for a few months before striking water.

"We only look at a handful of stars before finding this planet, said Charbonneau. "Either we got lucky, or the planets are very common."

Image: Nature

Help give GJ 1214b a better name.

See Also:

Citations: "A super-Earth transiting a nearby low-mass star." By David Charbonneau, Zachory K. Berta, Jonathan Irwin, Christopher J. Burke, Philip Nutzman, Lars A. Buchhave, Christophe Lovis, Xavier Bonfils, David W. Latham, Stephane Udry, Ruth A. Murray-Clay, Matthew J. Holman, Emilio E. Falco, Joshua N. Winn, Didier Queloz, Francesco Pepe, Michel Mayor, Xavier Delfosse & Thierry Forveille. Nature*, Vol. 462 No. 7275. Dec. 16, 2009.*

"Water world larger than Earth." By Geoffrey Marcy. Nature*, Vol. 462 No. 7275. Dec. 16, 2009.*

Brandon Keim's Twitter stream and reportorial outtakes; Wired Science on Twitter. Brandon is currently working on a book about ecosystem and planetary tipping points.