Abstract

The surface climate of rocky exoplanets is influenced by a large number of stellar, and planetary properties most of which are currently unconstrained. Our empirical knowledge of small planets around small stars is limited to the stellar properties, planet radius, and incoming stellar flux. Although this information is necessary to constrain the surface climate, they alone are insufficient to do so. My goal is to quantify what we know about liquid water bearing planets (the Habitable Zone) based on our current observational knowledge. To do that, I use a statistical approach to synthesize a large number of planet scenarios and I study the subset of the population that harbors liquid water on the surface. The approach treats stellar and planetary properties as random variables each with their respective probability density function (PDF). Some variables are observationally constrained (e.g., incoming stellar flux and planet radius). Others remain unconstrained to date (e.g., atmospheric composition, surface albedo) and I adopt various PDFs for such variables. The method allows me to investigate what fraction of planets could harbor liquid water as a function of planet radius and incoming stellar flux. I find that based on our current observational knowledge, the probability that an Earth-like exoplanet (a planet with one Earth radius and an incoming stellar flux of one solar constant) harbors liquid water is between 0% and 70%. The probability is strongly influenced by the PDFs of surface pressure and atmospheric composition. Thus, the constraints will be stronger once these properties are better known.