I know, another exoplanet. But just because hundreds of these non-solar planets have been found doesn’t mean we’re not learning interesting things about them. Take the planet GJ1214b, which was discovered in 2009 by the MEarth project. Data from the new Wide Field Camera 3 on the Hubble telescope has now confirmed that the planet is mostly water.
At about seven times the mass of the Earth, GJ121b is classed as a ‘super-Earth’. It was discovered by the transit method. That is, astronomers caught it passing in front of its star as it whips around its red dwarf host every 38 hours. More importantly, the position of the transit and the energy output of the star make it possible to detect the planet’s atmosphere as it obscures the light coming from the star behind it.
Although a few models could have been consistent with the data, the most likely scenario is that GJ121b contains an atmosphere that is at least 50% water vapor. The planet also likely has a rock and ice core surrounded by a thick envelope of hydrogen, helium and water. So, GJ121b is watery world with a steamy atmosphere. Another first in the assembly of exoplanets.
GJ1214b, shown in this artist's conception, is a super-Earth orbiting a red dwarf star 40 light-years from Earth. New observations from NASA's Hubble Space Telescope show that it is a waterworld enshrouded by a thick, steamy atmosphere. GJ1214b therefore represents a new type of world, like nothing seen in our solar system or any other planetary system currently known.
Credit: David A. Aguilar (CfA) 
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