Super-Earth Atmospheres Can Be Ripped Away By Host Star Radiation

Stars will ultimately betray their planets. Whether they end in a cataclysmic supernova or swell up before casting off their outer shell, the planets nearest to them will often be obliterated as a result. Our Earth, sadly, is no exception to this.

Now, astronomers have confirmed that there is another way stars can destroy their host worlds at least partly. Reporting their findings in Nature Communications, it turns out that large rocky worlds can have their atmospheres violently stripped away by their host stars.

For these planets it is like standing next to a hairdryer turned up to its hottest setting, Dr. Guy Davies, a helioseismologist at University of Birmingham’s School of Physics and Astronomy, said in a statement.

Using data from NASAs Kepler telescope, the exoplanet-hunting mechanical wunderkind, the team carefully examined numerous super-Earths, rocky planetsbetween two and 20 times the mass of our own pale blue dot.

Super-Earths have been found before, with one, named 55 Cancri e, recently revealed to be a tidallylocked world that is half-magma ocean and half-perpetual night. Significantly, researchers have theorized that much of its atmosphere on its dayside, which is always facing its host star, has been stripped away by powerful waves of radiation.

Comparing the radius of exoplanets with the level of radiation given off by them, which is strongly controlled by how close they are to their host star. Earth (E), Venus (V), Mars (M) and Mercury (Me) are all plotted in green. The gray shaded area lacks exoplanets of a certain radius, and the authors think that this is because they’ve been shrunken by powerful stellar radiation. Lundkvist et al./Nature Communications

This new study all but confirms that this mechanism, known as planetary photoevaporation, is indeed real.After surveying the night sky and looking at the atmospheres of super-Earths around 102 host stars, they find that those that drift too close to their stars are smaller than they otherwise would be. This can only be because their atmospheric envelope has been boiled off and torn away.

There has been much theoretical speculation that such planets might be stripped of their atmospheres, Davies added. We now have the observational evidence to confirm this, which removes any lingering doubts over the theory.

Photoevaporation has been suggested in the past to explain the strange nature of some other exoplanets that may have once been gas giants, rather than rocky worlds. CoRoT-7b, another molten hell 480 light-years from our own Solar System, orbits 60 times closer to its own star than Mercury does to our own Sun. As a result, its atmosphere has been completely removed.

This exoplanet is thought to have once had the mass of 100 Earths, making it more akin to the mass of a gas giant rather than a super-Earth. So its possible that what we can see now is the solid remnant core of a gas giant whose gargantuan atmospheric envelope was obliterated, through photoevaporation, as it moved towards its host star.

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Read more: http://www.iflscience.com/space/super-earths-atmospheres-can-be-completely-ripped-away-nearby-host-stars-radiation

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