Destructive space weather associated with the “goldilocks” zone of red dwarfs means it’s unlikely planets in such environments could be habitable.
New Atlas takes up the story:
Last year, astronomers announced the discovery of Proxima b, a potentially habitable planet orbiting Proxima Centauri, which at just four light years away is the closest star to our solar system. But a team of NASA scientists is looking to change the way habitable zones are defined by taking into account the impacts of space weather. The proposed change would likely mean Proxima b, and other exoplanets orbiting red dwarf stars, probably aren’t habitable after all.
The habitable zone is the region surrounding a star in which a planetary surface can support liquid water. This is traditionally calculated based on the how much light and heat the star emits. Stars more massive than our sun that produce more light and heat have a habitable zone that is further away from the star than the Earth is from our sun, and vice versa.
But as the interdisciplinary team of NASA scientists points out, this doesn’t take into account emissions, such as X-rays and ultraviolet radiation, or stellar activity, such as flares and coronal mass ejections. The scientists say such space weather can erode the exoplanet’s atmosphere, stripping it of hydrogen and oxygen molecules, the two ingredients of water.
Such effects are particularly pronounced for exoplanets orbiting red dwarfs, which are often prime targets for potentially habitable exoplanets because they are the coolest, smallest and most numerous stars in the universe. Because they are cooler than yellow dwarfs like our sun, traditional thinking puts the habitable zone much closer to the star than in our solar system. But red dwarfs are also more active than our sun, with stellar eruptions that are more frequent and more powerful than we experience on Earth.
“When we look at young red dwarfs in our galaxy, we see they’re much less luminous than our sun today,” says Vladimir Airapetian, lead author of the paper and a solar scientist at NASA’s Goddard Space Flight Center. “By the classical definition, the habitable zone around red dwarfs must be 10 to 20 times closer-in than Earth is to the sun. Now we know these red dwarf stars generate a lot of X-ray and extreme ultraviolet emissions at the habitable zones of exoplanets through frequent flares and stellar storms.”