section_path: Atmosphere and climate/Evolution
section_name: Evolution
paragraph_text: Evolution
  
  High-energy radiation from the star, such as hard UV radiation and X-rays, is expected to heat the upper atmosphere and fill it with hydrogen formed through the photodissociation of water, thus forming an extended hydrogen-rich exosphere that can escape from the planet. The X-ray and UV fluxes that K2-18b receives from K2-18 are considerably higher than the equivalent fluxes from the Sun; the hard UV radiation flux provides enough energy to drive this exosphere to escape at a rate of about 350 +400 −290 tons per second, too slow to remove the planet's atmosphere during its lifespan. Observations of decreases of Lyman alpha radiation emissions during transits of the planet may show the presence of such an exosphere; this discovery requires confirmation.