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Thursday, February 23, 2012 / Duncan Forgan

PlanetWatch: GJ1214b confirmed by Hubble as a Waterworld


It’s difficult to blink without seeing a press release on an unusual exoplanet.  This time it’s GJ1214b who’s back in the headlines.  It was discovered by the Mearth project in 2009, using the transit method.  Mearth purposefully looks at M type stars, i.e. very low mass stars, because smaller planets will be able to block out a larger fraction of the host starlight.

The Mearth Project, composed of 8 16" telescopes controlled robotically. Credit: Cfa

When GJ1214b was discovered, the team were able to constrain its mass between one and ten Earth masses from the period of the planet’s orbit. Combined with the direct measurement of the planet’s radius (2.7 Earth radii), the Mearth team were able to infer that the planet’s density was close to that of water.

This is more than just a big quivering blob of liquid.  The core of this planet would presumably have some very exotic forms of water that exist only at very high pressures, such as Ice VII, which has a bizarre crystalline structure compared to regular ice, or superfluid water, which acts as if it has no friction or viscosity.

Even regular, old-fashioned steam would seem to exist here.  In 2009, theoretical models underestimated the apparent radius of GJ1214b, which the Mearth team interpreted as the presence of a steamy atmosphere above the surface.

Now, 3 years later, the Hubble Space Telescope has been brought to bear on GJ1214b.  Hubble’s Wide Field Camera 3 was trained on the planet as it went through a transit.  As the planet obscures its host star, starlight shines through the planet’s atmosphere towards Earth.  The atmosphere preferentially absorbs at specific wavelengths, leaving a chemical fingerprint in the light’s spectrum.

An artist's impression of GJ1214b transiting its host star. Credit: CfA

The fingerprints might still be a little smudgy, but data suggests the atmosphere is at least 20% water, probably much higher.  The best guesses for the composition of the planet would appear to be an icy, rocky core with a very watery atmosphere and a healthy pinch of hydrogen and helium.  How exactly the planet ended up quite so watery is unclear – it would almost certainly have had to form quite far from its host star, beyond the distance at which ices melt (usually called the snow line), and migrated inwards.

Could it be inhabited? A large surplus of water in a variety of phases is certainly a boon for life on Earth, and we know how diverse and teeming the oceans are.  But bear in mind the temperature on the surface was estimated to be around 200 degrees C.  This would certainly lead us to say that terrestrial life would struggle to live here, but the oceans of GJ1214b will be weird to say the least, containing bizarre ice and superfluid water…if there was life, it would have to be pretty weird too.

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