Testing the Boundaries of Planetary Habitability: The Critical Need For In-Situ Data

The prime focus of astrobiology research is the search for life elsewhere in the universe, and this proceeds with the pragmatic methodology of looking for water and Earth-like conditions. In our solar system, Venus is the most Earth-like planet, yet at some point in planetary history there was a bifurcation between the two: Earth has been continually habitable since the end-Hadean, whereas Venus became uninhabitable. Indeed, Venus is the type-planet for a world that has transitioned from habitable and Earth-like conditions through the inner edge of the Habitable Zone (HZ); thus it provides a natural laboratory to study the evolution of habitability.

At the present time, exoplanet detection methods are increasingly sensitive to terrestrial planets, resulting in a much needed collaboration between the exoplanetary science and planetary science communities to leverage the terrestrial body data within the solar system. Unlike the solar system, exoplanetary science does not contain in situ data for surface conditions and thus exoplanet environments may only be inferred indirectly from other measurables, such as planetary mass, radius, orbital information, and atmospheric composition. The inference of those environments in turn are derived from detailed models constructed using the direct measurables obtained from observations of and missions to solar system bodies. In particular, understanding habitability boundaries requires exploration of both habitable and uninhabitable environments. Furthermore, current and near-future exoplanet detection missions are biased towards close-in planets, so the most suitable targets for detailed follow-up observations are more likely to be Venus-like than Earth-like planets. The further study and understanding of the evolution of Venus’ atmosphere and its present state provides a unique opportunity to complement the interpretation of these exoplanet observations.

we describe how the current limitations in our knowledge of Venus are impacting present and future exoplanetary science, including remote sensing techniques that are being or will be employed in the search for and characterization of exoplanets. We discuss Venus in the context of defining the boundaries of habitability, and how exoplanets are enabling tests of potential runaway greenhouse regimes where Venus analogs may reside. We discuss specific outstanding questions regarding the Venus environment and the relevance of those issues to understanding the atmospheres and interior structure of exoplanets.