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Astronomers using observations made by the Hubble Space telescope have announced the discovery of a new type of exoplanet. The planet, named GJ 1214b, was discovered in 2009 by ground-based telescopes orbiting a red-dwarf star 40 light-years from Earth. New observations using the Hubble suggest that a large fraction of the mass of the planet is water.

This waterworld is larger than the Earth but smaller then the planet Uranus and is enshrouded by a thick, steamy atmosphere. An international team of astronomers led by Zachory Berta of the Harvard-Smithsonian Center for Astrophysics (CfA) made the observations. Berta said "GJ 1214b is like no planet we know of. A huge fraction of its mass is made up of water."

GJ 1214b shown in this artist's view, is a new type of waterworld planet enshrouded by a thick, steamy atmosphere. (NASA/ESA/D. Aguilar (Harvard-Smithsonian Center for Astrophysics))

In 2010, CfA scientist Jacob Bean and colleagues reported that they had measured the atmosphere of GJ 1214b, finding it likely composed mainly of water. However, their observations could also be explained by the presence of a planet-enshrouding haze in GJ 1214b's atmosphere, similar to Saturn's moon Titan.

Now the new measurements made using Hubble tip the balance in favour of a dense steamy water vapour atmosphere, rather than one with a haze. Hence GJ 1214b should have much more water than Earth does but much less rock. As a result, the internal structure of GJ 1214b would be greatly different from that of the Earth.

This super-Earth is about 2.7 times Earth's diameter and weighs almost seven times as much. It orbits close to its parent star, requiring only 38 hours to complete one orbit at a distance of 2 million kilometres. This is about 75 times closer than the Earth is to the Sun. The estimated temperature of GJ 1214b is 230 degrees Celsius and the high temperature combined with high pressure suggests that exotic materials such as "hot ice" or "superfluid water" might exist there.

The planet's short distance from Earth makes it a likely candidate for follow-up observations with the Hubble Space telescope replacement, James Webb Space Telescope (JWST). Despite funding issues and delays it is hoped that the JWST will launch by the end of this decade.

See also

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Kepler space telescope spots first Earth sized planets
Astronomers announce discovery of 50 new exoplanets
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