The tl;dr version: Astronomers have found a planet which might have liquid water, although it's probably not very Earth-like.Our collection of exoplanets continues to expand and, in recent years, some dedicated hardware like CoRoT and Kepler have joined the search in space. But the latest discovery comes from some pretty mundane hardware—a collection of 40cm telescopes—and has some very compelling properties: a super earth that's likely to harbor liquid water, and orbits a star that's close enough to allow current observatories to image its atmosphere.
The results come courtesy of the MEarth project (a description is available via the arXiv), which is based on Mount Hopkins in Arizona. Instead of exotic, high-end optics, MEarth relies on eight 0.4m telescopes that can be pointed independently. The project works because the hardware is pointed at a very carefully chosen collection of stars: about 2,000 nearby M-dwarfs, which, as the name implies, are relatively small stars. That means that even a moderate-sized planet orbiting one will occlude a significant fraction of its surface during transit. The MEarth scopes are able to spot anything that blocks more than a half of a percent of the light as it transits in front of its host star.
Based on the first paper to come out of the project, everything worked precisely as planned. Every 1.58 days, the light from the M-dwarf GJ 1214 displayed a dip in its luminosity of about 1.3 percent. The team apparently alerted the HARPS project, which detects exoplanets by looking for red and blue shifts that their gravitational pull creates in the light from their host stars.
The HARPS instrument was able to confirm the presence of a planet and, based on how it tugged the host star around, estimated its mass: 6.55 times that of the Earth. We've also got a good sense of the radius of GJ 1214, which allows us to estimate the planet's radius, based on how much of the star's light it's occluding. Combine that radius (2.7 Earth radii) with the mass, and you have a rough estimate of the planet's density. It turns out to be less than half as dense as Earth.
There are a number of ways this could come about, but the two most likely ones are a rocky core surrounded by hydrogen gas, or planet with roughly equal amounts of rocky material and water. The scientists involved lean heavily towards the latter explanation, since it's highly unlikely that there would be much free hydrogen floating around the inner part of a planetary disk that's rich enough in iron and rocky material to produce a solid core. So, the conclusion is that GJ 1214b probably has a thin shell of hydrogen and helium for an atmosphere, and then a massive ocean above a silicate and iron core.
Depending on how reflective the planet's atmosphere is, it may have temperatures as high as 555K, or as low as 393K—the latter figure is only 20°C above the boiling point of water. That's far and away the coolest planet we've yet spotted, and a far cry from the only other super earth we know much about, which is hot enough that its atmosphere probably contains vaporized titanium oxides. Given the gravitational differences with Earth, which will cause pressure to build up rapidly, and it's apparent that at least some of the water will almost certainly be liquid.
Because of the proximity of GJ 1214b to its host star, some of this ocean will almost certainly boil into the atmosphere (which an accompanying perspective compares to a sauna), where it will be split by UV radiation into hydrogen and oxygen. The calculated radius of the planet is a bit thicker than the one predicted by planetary models, which provides a further indication that an atmosphere is present. The planet is hot enough, however, that some fraction of those gasses will inevitably escape its pull. All indications are that the host star is old, though, so it appears that GJ 1214b is at a stable, or at least long-lived, point in its evolution.
Assuming that the planet has an atmosphere dominated by hydrogen, the authors calculate that 0.16 percent of the light from GJ 1214 will pass through the atmosphere during transit events. As such, GJ 1214b may provide one of the best opportunities we have to study the contents of an exoplanet's atmosphere in the near future.