section_path: Atmospheric models
section_name: Atmospheric models
paragraph_text: Atmospheric models
  
  To allow surface water to be liquid for long periods of time, a planet—or moon—must orbit within the habitable zone (HZ), possess a protective magnetic field, and have the gravitational pull needed to retain an ample amount of atmospheric pressure. If the planet's gravity cannot sustain that, then all the water will eventually evaporate into outer space. A strong planetary magnetosphere, maintained by internal dynamo action in an electrically conducting fluid layer, is helpful for shielding the upper atmosphere from stellar wind mass loss and retaining water over long geological time scales.
  
  A planet's atmosphere forms from outgassing during planet formation or is gravitationally captured from the surrounding protoplanetary nebula. The surface temperature on an exoplanet is governed by the atmosphere's greenhouse gases (or lack thereof), so an atmosphere can be detectable in the form of upwelling infrared radiation because the greenhouse gases absorb and re-radiate energy from the host star. Ice-rich planets that have migrated inward into orbit too close to their host stars may develop thick steamy atmospheres but still retain their volatiles for billions of years, even if their atmospheres undergo slow hydrodynamic escape. Ultraviolet photons are not only biologically harmful but can drive fast atmospheric escape that leads to the erosion of planetary atmospheres; photolysis of water vapor, and hydrogen/oxygen escape to space can lead to the loss of several Earth oceans of water from planets throughout the habitable zone, regardless of whether the escape is energy-limited or diffusion-limited. The amount of water lost seems proportional with the planet mass, since the diffusion-limited hydrogen escape flux is proportional to the planet surface gravity.
  
  During a runaway greenhouse effect, water vapor reaches the stratosphere, where it is easily broken down (photolyzed) by ultraviolet radiation (UV). Heating of the upper atmosphere by UV radiation can then drive a hydrodynamic wind that carries the hydrogen (and potentially some of the oxygen) to space, leading to the irreversible loss of a planet's surface water, oxidation of the surface, and possible accumulation of oxygen in the atmosphere. The fate of a given planet's atmosphere strongly depends on the extreme ultraviolet flux, the duration of the runaway regime, the initial water content, and the rate at which oxygen is absorbed by the surface. Volatile-rich planets should be more common in the habitable zones of young stars and M-type stars.
  
  Scientists have proposed Hycean planets, ocean planets with a thick atmosphere made mainly of hydrogen. Those planets would have a wide range area around their star where they could orbit and have liquid water. However, those models worked on rather simplistic approaches to the planetary atmosphere. More complex studies showed that hydrogen reacts differently to starlight's wavelengths than heavier elements like nitrogen and oxygen. If such a planet, with an atmospheric pressure 10 to 20 heavier than Earth's, was located at 1 astronomical unit (AU) from their star their water bodies would boil. Those studies now place the habitable zone of such worlds at 3.85 AU, and 1.6 AU if it had a similar atmospheric pressure to Earth.