Back in 2009 the planet GJ 1214b was discovered by the ground-based MEarth Project. At the time the astronomers were able to identify the diameter of the planet to be 2.7 times Earth’s diameter (roughly the size of Uranus), while being seven times heavier. At just 2 million kilometers away from its Red Dwarf star, the estimated temperature on GJ 1214b is around 230 C.
Shortly after its discovery, further study found its atmosphere contained large amounts of water. Water is a simple molecule and not uncommon in the Universe, but researchers have been wondering if the water in the atmosphere represented a haze or steam. The Hubble Space Telescope, operated by both NASA and ESA, the European Space Agency, was called in to determine which it was. A hazy atmosphere looks different to infrared light than a steamy atmosphere, and Hubble has limited infrared capability.
The observations tip in favor of it being a steamy atmosphere, which leads to an interesting conclusion. This planet must be mostly made of water, and a lot of it. Earth is sometimes referred to as the Blue Marble because of the amount of water covering the surface, but with an overall density of 5.5 grams per cubic centimeter, compared to 1 gm/cm^3 for water, we are mostly denser materials. The density of GJ 1214b is roughly 2 gm/ cm^3 which requires a great amount of water to achieve. (The atmospheric analysis already ruled out it being a gas giant.)
No other planet has been found like this before. Gas giants and terrestrial planets, like Jupiter and Earth respectively, have been found orbiting many stars but never one with so much water. The researchers believe the planet actually formed further from its star, where water would not have been blown away by solar winds, and then shifted to a nearer orbit.