Stars burn away the atmospheres of close-in super-Earths

 作者:北宫赆艴     |      日期:2019-03-15 04:06:00
NASA By Shannon Hall 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’t 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’t snuggle up to their hosts. At least that appeared to be the case, but no studies had been able to prove it. That’s 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’s radius to the star’s radius. To pin down the planet’s true size, astronomers need to know the star’s size. Now Mia Lundkvist at Aarhus University in Denmark and her colleagues have used asteroseismology – the study of sound waves generated by turbulence on the surface of stars – 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. “We 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,” says co-author Sarbani Basu at Yale University. That isn’t necessarily because they don’t exist in that region, it’s because the star’s intense radiation has boiled away the atmosphere of any super-Earth that’s got that close. They’ve lost so much of their bulk that they appear as small as hot Earths – no longer “super” at all. “This study shows that you can’t ignore the radiation from the star once you’re close enough to it,” says Jason Steffen at the University of Nevada, Las Vegas. “You have to understand how the star interacts with the planet’s atmosphere in order to interpret properly what you’ve observing.” In fact, you could say a star plays even more of a starring role in a planet’s final form than we thought, influencing its formation, evolution, structure and whether it can be habitable. Journal reference: More on these topics: