Revisiting the inner boundary of the Habitable Zone: Criteria and Feedback processes

The Habitable Zone (HZ) is generally defined as the orbital region around a star, in which life-supporting (habitable) planets can exist. Taking into account that liquid water is a fundamental requirement for life as we know it, the HZ boundaries depend on conditions, such as the amount of stellar insolation, which may limit the potential existence of liquid water on the planetary surface. We investigate two scientific key questions concerning the inner boundary of the HZ: First, which are the criteria important for the determination of the inner boundary of the HZ in general (i.e. critical point of water, water loss limit, runaway greenhouse effect), and second, where is the inner boundary of the HZ located in the Solar System. We address these questions by applying a one-dimensional radiative-convective atmospheric model to an Earth-like planet orbiting the Sun. The model includes updated treatments of the radiative transfer for the thermal infrared and solar wavelengths to account for absorption by water vapor at high temperatures and pressures. Our calculations are performed self-consistently taking into account the feedback between surface heating, water evaporation and energy transport in the atmosphere. The inner boundary of the HZ is evaluated in terms of key criteria for the Solar System. The results of our calculation of the inner boundary of the HZ for the Solar System are comparable to previous studies for the water loss limit and the critical point of water criterion. However, we find that the runaway greenhouse is not the limiting process for habitability for the particular example of the Earth-like planet orbiting the Sun. The critical point of water is reached before the entire ocean is evaporated. We show that a radiation limit of the outgoing infrared flux is a necessary but not sufficient condition for the runaway greenhouse effect. This is in contrast to results of previous studies. Rather the radiation limit is a phenomenon occurring in water dominated convective atmospheres and is not generally linked to the occurrence of a runaway greenhouse process.