Most of the potentially habitable exoplanets we've discovered orbit small red dwarf stars. Red dwarfs make up about 75% of the stars in our galaxy. Only about 7.5% of the stars are of type g, like our sun. If we are looking for life on other worlds, red dwarfs seem their most likely home. However, red dwarfs pose a serious problem for habitable worlds.
The habitable zones of the stars compared. Photo credit: NASA
Red dwarfs are small and cool stars, so planets would have to orbit close to the star to be warm enough for life. This means that the worlds are at greater risk from star activity. We have long known that red dwarfs can be very active in their youth. Not only do they emit powerful solar flares, but they can also trigger powerful X-rays. A red dwarf could strip the atmosphere and fry the barren surface of a planet so close that it is in its habitable zone.
Red dwarfs also have an extraordinarily long lifespan. Our sun is in the middle of its ten billion year life, but red dwarfs can burn for billions of years. Given that the universe is only 14 billion years old, red dwarfs have plenty of time to see the life form on their planets.
Illustration of a solar flare on a red dwarf. Photo credit: Casey Reed / NASA
But a new study shatters those hopes a little. While young red dwarfs are hostile to potentially habitable worlds, they are main sequence stars so it was believed that they settled in time. So if a dwarf red planet had a magnetic field strong enough to hold onto its atmosphere, life could emerge once its star's angry youth transitions into a comfortable middle ages.
To test that idea, this new paper looked at the pre-eminent activity of Barnard's Star. It is a red dwarf only 6 light years from Earth and roughly 10 to 12 billion years old. It took a lot of time to settle down. But it turns out that Barnard's star is still pretty much active. The team observed that about 25% of the time, it is exposed to large flares and emits ultraviolet light and X-rays. This activity is so intense that it could strip the atmosphere of planets forming in the habitable zone, as well as planets that later migrate further out into the habitable zone.
If Barnard's Star activity is typical of red dwarf stars, it is very likely that red dwarfs are far too hostile for potentially habitable worlds. It would mean that life in the universe could be a lot rarer than we once hoped it would be.
Reference: France, Kevin et al. "The high-energy radiation environment around a 10-Gyr-M dwarf: finally habitable?" The Astronomical Journal 160.5 (2020): 237.