Stellar winds and evaporating exoplanet atmospheres

Most stars including the sun generate magnetic activity that drives a fast-moving, ionized wind and also produces X-ray and ultraviolet emission (often referred to as XUV radiation). XUV radiation from a star can be absorbed in the upper atmosphere of an orbiting planet, where it is capable of heating the gas enough for it to escape from the planet's atmosphere. M-dwarf stars, the most common type of star by far, are smaller and cooler than the sun, and they can have very active magnetic fields. Their cool surface temperatures result in their habitable zones (HZ) being close to the star (the HZ is the range of distances within which an orbiting planet's surface water can remain liquid). Any rocky exoplanets that orbit an M-dwarf in its HZ, because they are close to the star, are especially vulnerable to the effects of photoevaporation which can result in partial or even total removal of the atmosphere. Some theorists argue that planets with substantial hydrogen or helium envelopes might actually become more habitable if photoevaporation removes enough of the gas blanket.