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X-rays may sterilize otherwise habitable exoplanets
"Red dwarfs are far and away the most common type of star. These slow and steady burners are thought to account for roughly 75 percent of the stars in the Milky Way, and for the most part, astronomers agree that red dwarfs are prevalent throughout the entire universe. Furthermore, many exoplanets — including the seven Earth-size planets found in the TRAPPIST-1 system — have been detected around red dwarfs.
Because these stable stars are relatively cool (around 7,000 degree Fahrenheit) and live exceptionally long (trillions of years), it may seem like they would be the perfect places to hunt for habitable exoplanets. However, according to new research presented on April 3 at the European Week of Astronomy and Space Science in Liverpool, red dwarfs may be much more inhospitable to life than we previously thought.
The problem lies in the fact that red dwarfs are so darn cool. For any exoplanet to get enough heat to be in a red dwarf’s habitable zone — the region around a star where liquid water can exists — the planet must sit extremely close to the star itself. And since red dwarfs often emit large radiation flares, spew out charged particles, and undergo coronal mass ejections (CMEs; think plasma torpedoes), being near one is a risky proposition.
[...]the X-ray radiation associated with the flare-up would have cut through an Earth-sized planet’s atmosphere like a hot knife through butter. This means that copious amounts of harmful X-rays would penetrate straight down to the surface of any potential exoplanet in the habitable zone around AD Leo, effectively sterilizing the entire planet."
http://www.astronomy.com/news/2018/0...ble-exoplanets
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