Twelve stars have received X-ray passports. Now, for each of them, we know the exact radiation dose received by a planet in the habitable zone, the region where liquid water is possible. The passports were assembled piece by piece: archival observations from the space telescopes XMM-Newton, Chandra, and Hubble were stitched together with stellar atmosphere models. This even allowed recovering the ultraviolet that is not directly visible.
X-rays and extreme ultraviolet heat the upper layers of the atmosphere and trigger chemical reactions. Over a billion years, such radiation can strip away an entire ocean. For the star Lalande 21185, the X-ray flux varied 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.
Previously, data existed only for red dwarfs. Now the sample covers stars from hot F9 to cool M5.5, including those that future telescopes will see directly. It turned out that the sample is biased toward active stars; quiet candidates have yet to be found.
The X-ray passport is the first page. The next step is to expand the sample: out of 164 priority targets, only 19 have quality X-ray spectra. New observations with Hubble are already planned. The passports will help calculate the observing time for coronagraphs, instruments that block the star's light to reveal the planet.
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.