Planets orbiting two stars, giant super-Earths and worlds racked with boiling rock storms - these are just some of the galactic oddities among the more than 700 exoplanets discovered in the past 20 years. Now there's another new class of extrasolar planet to add to the list - the waterworld.

The planet known as GJ 1214b was originally discovered in 2009 as part of the ground-based MEarth Project led by Harvard-Smithsonian Center for Astrophysics (CfA) scientist David Charbonneau.

Located 40 light-years away (making it a relatively close neighbor), GJ 1214b has a diameter approximately 2.7 times that of Earth, but weighs almost seven times as much as Earth and has an estimated temperature of 230 degrees Celsius (446°F) .

Harvard-Smithsonian Center for Astrophysics (CfA) scientist Jacob Bean and colleagues measured the atmosphere of the planet in 2010, concluding that it was mostly made up of water and a recent study using observations from the Hubble Space Telescope supports this theory.

To establish what the haze surrounding the planet was made up of, an international team of astronomers led by Zachory Berta used Hubble's Wide Field Camera 3 (WFC3) to study light filtered through the planet's atmosphere during a transit of its parent star. This enabled the scientists to differentiate between a steamy and a hazy atmosphere - in this case it was concluded that GJ 1214b has a dense atmosphere of water vapor.

The planet's density has been calculated at about 2 grams per cubic centimeter, making it less than half as dense as the Earth (5.5 grams per cubic centimeter) which suggests that GJ 1214b has much more water, and less rock, than Earth.

"GJ 1214b is like no planet we know of," Berta said. "A huge fraction of its mass is made up of water."

But this doesn't mean that GJ 1214b is just a watery version of our own world - it's a little weirder than that.

"The high temperatures and high pressures would form exotic materials like 'hot ice' or 'superfluid water', substances that are completely alien to our everyday experience," Berta said.

A Paper [PDF] reporting these results has been accepted for publication in the Astrophysical Journal and is available online.

And no, there have not been any reported sightings of Kevin Costner on GJ 1214b.

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