  The search for habitable exoplanets and life beyond the Solar System is one
of the most compelling scientific opportunities of our time. Nevertheless, the
high cost of building facilities that can address this topic and the keen
public interest in the results of such research requires the rigorous
development of experiments that can deliver a definitive advance in our
understanding. Most work to date in this area has focused on a "systems
science" approach of obtaining and interpreting comprehensive data for
individual planets to make statements about their habitability and the
possibility that they harbor life. This strategy is challenging because of the
diversity of exoplanets, both observed and expected, and the limited
information that can be obtained with astronomical instruments. Here we propose
a complementary approach that is based on performing surveys of key planetary
characteristics and using statistical marginalization to answer broader
questions than can be addressed with a small sample of objects. The fundamental
principle of this comparative planetology approach is maximizing what can be
learned from each type of measurement by applying it widely rather than
requiring that multiple kinds of observations be brought to bear on a single
object. As a proof of concept, we outline a survey of terrestrial exoplanet
atmospheric water and carbon dioxide abundances that would test the habitable
zone hypothesis and lead to a deeper understanding of the frequency of
habitable planets. We also discuss ideas for additional surveys that could be
developed to test other foundational hypotheses is this area.
