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Some planets stand stark naked. For the first time, we have observational evidence that some super-Earths orbit so close to their host stars that the puffy atmospheres that clothed them have been ripped away.

Planets of many sizes orbit dangerously close to their host stars, but that doesn鈥檛 seem to be the case for the most common planet throughout the galaxy: super-Earths. These rocky worlds with puffy atmospheres are bigger than Earth and smaller than Neptune, and simply don鈥檛 snuggle up to their hosts.

At least that , but no studies had been able to prove it. That鈥檚 because determining the radius of a planet around another star is tough. One of the best ways to spot one of these exoplanets is to detect it as it passes in front of its host star and blocks some of the starlight. But this transit method, used by the prolific Kepler space telescope, can only give the ratio of the planet鈥檚 radius to the star鈥檚 radius. To pin down the planet鈥檚 true size, astronomers need to know the star鈥檚 size.

Burned away

Now at Aarhus University in Denmark and her colleagues have used asteroseismology 鈥� the study of sound waves generated by turbulence on the surface of stars 鈥� to measure the radii of 102 stars, and therefore better estimate the size of their planets. They then looked at how much light each planet received from its host star to see how badly it might be fried.

鈥淲e did not get super-Earths in a region where the amount of radiation is more than about 600 times what we get on Earth today,鈥� says co-author at Yale University.

That isn鈥檛 necessarily because they don鈥檛 exist in that region, it鈥檚 because the star鈥檚 intense radiation has boiled away the atmosphere of any super-Earth that鈥檚 got that close. They鈥檝e lost so much of their bulk that they appear as small as hot Earths 鈥� no longer 鈥渟uper鈥� at all.

鈥淭his study shows that you can鈥檛 ignore the radiation from the star once you鈥檙e close enough to it,鈥� says at the University of Nevada, Las Vegas. 鈥淵ou have to understand how the star interacts with the planet鈥檚 atmosphere in order to interpret properly what you鈥檝e observing.鈥�

In fact, you could say a star plays even more of a starring role in a planet鈥檚 final form than we thought, influencing its formation, evolution, structure and whether it can be habitable.

Journal reference:聽