section_path: Atmosphere and climate
section_name: Atmosphere and climate
paragraph_text: Atmosphere and climate
  
  Observations with the Hubble Space Telescope have found that K2-18b has an atmosphere consisting of hydrogen. The presence of water vapour is likely but with uncertainty, as James Webb Space Telescope observations indicating concentrations of less than 0.1%; this may be due to the JWST seeing a dry stratosphere as the atmosphere is thought to have an efficient cold trap. Ammonia concentrations appear to be unmeasurably low. JWST observations indicate that methane and carbon dioxide each make up about 1% of the atmosphere. Other carbon oxides were not reported; only an upper limit to their concentrations (a few percent) has been established. The atmosphere makes up at most 6.2% of the planet's mass, and its composition probably resembles that of Uranus and Neptune.
  
  There is little evidence of hazes in the atmosphere, while evidence for water clouds, the only kind of clouds likely to form at K2-18b, is conflicting. If they exist, the clouds are most likely icy but liquid water is possible. Apart from water, ammonium chloride, sodium sulfide, potassium chloride and zinc sulfide could form clouds in the atmosphere of K2-18b, depending on the planet's properties. Most computer models expect that a temperature inversion will form at high elevation, yielding a stratosphere.