David Charbonneau and his team at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. managed to deduce that this super-Earth could be made up of as much as 75 percent water by measuring how much the red dwarf's light dimmed when GJ 1214b passed in front of it. By knowing how much the light dimmed, the exoplanet's size could be worked out.
Then, after measuring the exoplanet's orbital period of 38 hours (i.e. a year on Earth is 365 days, a year on GJ 1214b is a short 1.6 days!), orbital distance and the amount of "wobble" the exoplanet causes on its parent star, they were able to calculate the exoplanet's mass. Knowing its size and mass led the astronomers to calculate its density.
Knowing the density of this alien world helps us gauge what it is made out of, and in this case GJ 1214b is made up primarily of water.
Although GJ 1214b isn't thought to be habitable, its surface temperature is around 530 degrees Fahrenheit (280 degrees Celsius). Usually on Earth, water would boil at these temperatures, but the atmospheric conditions on GJ 1214b could maintain hot water oceans in a liquid state.