A POPULATION-BASED HABITABLE ZONE PERSPECTIVE
What can we tell about exoplanet habitability if currently only the stellar properties, planet radius, and the incoming stellar flux are known? A planet is in the habitable zone (HZ) if it harbors liquid water on its surface. The HZ is traditionally conceived as a sharp region around stars because it is calculated for one planet with specific properties. Such an approach is limiting because the planet's atmospheric and geophysical properties, which influence the presence of liquid water on the surface, are currently unknown but expected to be diverse. A statistical HZ description is outlined that does not favor one planet type. Instead, the stellar and planet properties are treated as random variables, and a continuous range of planet scenarios is considered. Various probability density functions are assigned to each random variable, and a combination of Monte Carlo sampling and climate modeling is used to generate synthetic exoplanet populations with known surface climates. Then, the properties of the subpopulation bearing liquid water is analyzed. Given our current observational knowledge, the HZ takes the form of a weakly constrained but smooth probability function. The HZ has an inner edge, but a clear outer edge is not seen. Currently only optimistic upper limits can be derived for the potentially observable HZ occurrence rate. Finally, we illustrate through an example how future data on exoplanet atmospheres will help to narrow down the probability that an exoplanet harbors liquid water, and we identify the greatest observational challenge in the way of finding a habitable exoplanet.