  Provided that sufficient resources are deployed, we can look forward to an
extraordinary future in which we will characterize potentially habitable
planets. Until now, we have had to base interpretations of observations on
habitability hypotheses that have remained untested. To test these theories
observationally, we propose a statistical comparative planetology approach to
questions of planetary habitability. The key objective of this approach will be
to make quick and cheap measurements of critical planetary characteristics on a
large sample of exoplanets, exploiting statistical marginalization to answer
broad habitability questions. This relaxes the requirement of obtaining
multiple types of data for a given planet, as it allows us to test a given
hypothesis from only one type of measurement using the power of an ensemble.
This approach contrasts with a "systems science" approach, where a few planets
would be extensively studied with many types of measurements. A systems science
approach is associated with a number of difficulties which may limit overall
scientific return, including: the limited spectral coverage and noise of
instruments, the diversity of exoplanets, and the extensive list of potential
false negatives and false positives. A statistical approach could also be
complementary to a systems science framework by providing context to interpret
extensive measurements on planets of particular interest. We strongly recommend
future missions with a focus on exoplanet characterization, and with the
capability to study large numbers of planets in a homogenous way, rather than
exclusively small, intense studies directed at a small sample of planets.
