A star is like a lamp hanging over a planet. If the lamp is too bright in X-rays, the planet's atmosphere heats up and escapes. Astronomers have now compiled a passport for twelve nearby stars, detailing how much each emits in X-rays and ultraviolet. Previously, such data existed only for red dwarfs; now we have it for a variety of stars.
Why does this matter? X-rays and ultraviolet heat the upper layers of the atmosphere. Over a billion years, strong radiation can strip away an entire ocean. For instance, the star Lalande 21185 became 27 times brighter in X-rays over three years. Such variability is common among red dwarfs and complicates our understanding of whether their planets could support life.
Now scientists have passports for the first twelve stars. These passports will aid future telescopes: knowing a star's radiation allows us to calculate how long we need to observe to spot a planet next to it.
The star Lalande 21185 changed its X-ray flux by a factor of 27 over three years, as if the Sun suddenly became dozens of times brighter in X-rays for no apparent reason. Such variability is typical of red dwarfs and greatly complicates assessing the habitability of their planets.