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55 Cancri A is a Sun-like star some 40 light years away. It has an apparent magnitude of about 6 and so is visible to the naked eye in the constellation of Cancer.

This star is unusual in that it is just one of a handful that are known to have at least 5 planets. The innermost of these planets–55 Cancri e–was discovered in 2004 and has since had plenty of attention from astronomers. Various groups have observed the the changes in radial velocity that it causes its parent star. This tells them about that it orbits its star every 18 hours and that its mass is about 8 times Earth’s or about half Neptune’s.

But without a measurement of the planet’s radius, it’s not possible to determine the planet’s density. So 55 Cancri e could be an ice giant like Neptune or a terrestrial planet more like our world.

Today, Michael Gillon at the University of Liege in Belgium and a few pals reveal some interesting new data about this exoplanet. These guys have observed 55 Cancri e in a different way, by watching it pass in front its parent star using both NASA’s Spitzer space telescope and Canada’s MOST space telescope.

These kinds of observations are important because the amount of light the planet blocks during each transit is essentially a measure of its radius. Consequently, Gillon and co are able to say that the radius of 55 Cancri e is about twice Earth’s. That makes it almost certain that this is a rocky planet.

But it’s possible to make a few more deductions. A rocky planet is likely to be made from a combination of iron and magnesium-silicon-oxides, like the rocky planets in our Solar System. The density of these materials is well known and this raises a problem. “We find that 55 Cnc e is too large to be made out of just rocks,” say Gillon and co. “Therefore, it has to have an envelope of volatiles.”

These guys look at two possibilities. The first is an atmosphere of hydrogen and helium, rather like the atmospheres of our ice giants. But they rule this out because such an atmosphere would escape into space in just a few million years.

The second possibility is an envelope of water with a mass some 20 per cent of the planet’s total. (By contrast the water on Earth makes up only 0.023 per cent of its mass.)

This, say Gillon and co, is more likely because the water is less likely to escape into space and so would hang around for billions of years.

So 55 Cancer e must be a waterworld.

However, this waterworld is nothing like the planet envisioned in the Kevin Costner movie. 55 Cancer e is so close to its sun that the water is likely to be in a supercritical state, when the liquid and gas phases become one.

The planet may also be tidally locked so that one side is in permanent sunshine while the other is in permanent darkness.

That should make for some interesting weather, not to mention some interesting chemistry too. We should know more in a few years. Gillon and co say the planet’s envelope should be directly visible to the next generation of space telescopes.

Ref: arxiv.org/abs/1110.4783: Improved precision on the radius of the nearby super-Earth 55 Cnc e

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