So Water World is real, and it was discovered in 2009 by the MEarth Project. What's oh-so-special about this exoplanet is that it's larger than Earth, hotter than Earth, but less dense than Earth, which would suggest a much higher percentage of water, and less rock.
Water World!
The creatively named GJ 1214b was found while searching for Earth-like planets outside our solar system (in Star Trek lingo, this would be Class M planet). GJ 1214b was nice enough to pass in front of its star within our plane of view, and has a transit of 38 hours. And since astronomers are so clever, they were able to figure out the size of the planet, which has a radius 2.7 times that of Earth's, and a mass of 6.7 Earths. Due to the very short transit, this would mean a very close orbit. And due the the very close orbit, this planet is HOT! About 230 C! So while life is not likely happening now, it IS very interesting to find a planet with such a large amount of water.
Astronomers were able to detect water on this planet due to the wavelength absorption of light as it passes in front of the star. The absorption rates fall in line the best with water, and not only that, but its atmosphere could be up to 50% water. Since the size and mass is known, the density computed for GJ 1214b is about 2 grams per cubic centimeter. This supports the idea that it is made up of mostly water, as it is much less dense than Earth (5.5 g/c^3). Water of course has a density of 1 g/c^3.
Due to it's large mass, and thick atmosphere, pressure on GJ 1214b would be pretty high. In space where there is no pressure, water can boil even at temperatures that would mean freezing on Earth. With enough pressure, it was suggested that there may even be materials like "hot ice". How weird is that?
It is theorized that GJ 1214b was formed much farther out from its star and then migrated closer, passing through the habitable zone. This would have meant the melting of the water ice, and temperatures very similar to Earth. It is not known, however, how long it was in that magical zone. Could life has started? Maybe.
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