GJ 1214b, a planet some 40 light-years from Earth, is a water world. It's almost entirely made of liquid, has an estimated temperature of 230C and is enshrouded by a steamy atmosphere.
"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," explains Zachory Berta of the Harvard-Smithsonian Center for Astrophysics.
It's classified as a super-Earth, as the planet is about 2.7 times Earth's diameter and weighs almost seven times as much. It orbits its star every 38 hours at a distance of just two million kilometres, resulting in those super-hot temperatures.
The planet was first discovered in 2009, by the ground-based MEarth Project. The next year, astronomers from the Harvard-Smithsonian Center for Astrophysics (CfA) measured the exoplanet's atmosphere, and found that it could be composed mainly of water.
But those observations could also be explained if the planet just had a thick, hazy atmosphere. So, the team waited for GJ 1214b to cross in front of its host star (a red dwarf). When that happens, the star's light is filtered through the planet's atmosphere, giving hints to the mix of gases it contains.
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"We're using Hubble to measure the infrared colour of sunset on this world," explained the CfA team leader.
The team used Hubble's Wide Field Camera 3 (WFC3) to watch the planet's transit, and found the spectrum of GJ 1214b to be featureless over a wide range of wavelengths. That's consistent with a dense atmosphere of water vapour, suggesting the planet is covered by steam -- not haze.
Because the team knows the planet's mass and size, they can calculate the density. It comes out at about about two grams per cubic centimetre -- seeing as water has a density of one gram per cubic centimetre, and the Earth's average density is 5.5 grams per cubic centimetre, GJ 1214b has much more water and much less rock than our so-called "Blue Marble".
The liquid planet has been flagged as a "prime candidate" for study by Hubble's successor: the James Webb Space Telescope, when it launches around 2018.
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