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Tour: Assessing The Habitability of Planets Around Old Red Dwarfs

Planets orbiting close to the most abundant and longest-lasting stars in our Milky Way may be less hospitable to life than previously thought.

This conclusion comes from a new study using NASA's Chandra X-ray Observatory and Hubble Space Telescope that examined the red dwarf called Barnard's Star, which is one of the closest stars to Earth at a distance of only 6 light years. Red dwarf stars are much less massive than the Sun, and are expected to live much longer lives. Barnard's Star is about 10 billion years old — or over twice the age of our Sun.

Astronomers already knew that young red dwarfs, with ages less than a few billion years, blast out strong bursts of high-energy radiation like X-rays and ultraviolet light. However, scientists have known less about how much damaging radiation red dwarfs give off later in their lifetimes.

The Chandra and Hubble observations were taken to help find that out. In March 2019, Hubble observed Barnard's Star for about 7 hours and detected two ultraviolet high-energy flares. Three months later, Chandra looked at Barnard's Star for about the same amount of time and picked up a powerful X-ray flare. Based on the length of the flares and of the observations, the researchers concluded that Barnard's Star unleashes potentially destructive flares about 25% of the time.

If these snapshots are representative of how active Barnard's Star is, then it is pumping out a lot of harmful high-energy radiation into its surroundings even at its advanced age. The team then studied what these results mean for rocky planets orbiting in the habitable zone — where liquid water could exist on their surface — of a red dwarf like Barnard's Star. They concluded the high-energy bombardment from a red dwarf of this age would erode the atmospheres of closely orbiting planets, reducing the chance of these worlds supporting life.

This research contributes to the ongoing and complex discussion that scientists have been having about surrounding whether or not planets are habitable or not. The team is currently studying high-energy radiation from many more red dwarfs to determine whether the behavior they observed from Barnard's Star is typical — or not.

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