From the News and Views in this week's Nature:
The hunt for Earth-like worlds has taken a major step forward with the discovery of a planet only 2.7 times larger than Earth. Its mass and size are just as theorists would expect for a water-rich super-Earth...
Charbonneau's team1 has found that the small, faint star GJ 1214 undergoes repeated dimming of 1.3% for 52 minutes every 1.6 days. The only plausible interpretation is that a planet orbits the star with an orbital period of 1.6 days and that it has a radius that is 12% that of the star. Good estimates of the star's radius (21% that of the Sun) put the planet's radius at only 2.7 Earth radii. Such a small planet orbiting a star other than the Sun is an extraordinary find. With the tools currently available, only one other extrasolar planet has been reported that is thought to be close in size to Earth, namely CoRoT-7b, at 1.7 Earth radii. The new planet, which is only about 13 parsecs away, is named GJ 1214b. Importantly, it pulls gravitationally on its host star, causing the star to move with a speed of 12 m s−1, which the team has detected through measurements of wavelength shifts in the star's light (the Doppler effect). The planet's inferred mass is a mere 6.6 Earth masses, which, when combined with its radius, leads to a density of 1.9 g cm−3. By contrast, Earth's average density is much higher, at 5.5 g cm−3. Because water has a low density of about 1 g cm−3, the chemical composition of the new planet is probably some admixture of rock and water, with perhaps a small atmosphere of hydrogen and helium...
That solid material forms the building blocks of large planets such as Saturn and Neptune, and perhaps smaller planets as well, such as the new one1. But the density of 1.9 g cm−3 for this new planet imposes a constraint on the relative amounts of each constituent. To keep the planet's density that low requires that it contains large amounts of water. If the planet were pure Fe and silicates, its density would be similar to Earth's. It must contain a huge amount of water, roughly 50% by mass.
The actual article can be found here.
This planet would NOT be just like earth. The atmosphere is likely to be very different and it is not clear whether there could be solid continents to support terrestrial life. However, I am of the opinion that anywhere you find liquid water, there is a good shot of at least simple (bacteria-like) life if not more complex life.
GJ 1214b as it is now called is quite a landmark discovery. It is hard to see it that way now, since it is unlikely that any kind of exploratory mission can be launched any time soon, so we have to rely on indirect methods of studying it. But I suspect a lot of smart people are thinking right now of how we can get a good look at what seems like a big, very watery, possibly (probably?) life-bearing world in our general galactic neighborhood.