  Recently, many Earth-sized planets have been discovered around stars other
than the Sun that might possess appropriate conditions for life. The
development of theoretical methods for assessing the putative habitability of
these worlds is of paramount importance, since it serves the dual purpose of
identifying and quantifying what types of biosignatures may exist and
determining the selection of optimal target stars and planets for subsequent
observations. This Colloquium discusses how a multitude of physical factors act
in tandem to regulate the propensity of worlds for hosting detectable
biospheres. The focus is primarily on planets around low-mass stars, as they
are most readily accessible to searches for biosignatures. This Colloquium
outlines how factors such as stellar winds, the availability of ultraviolet and
visible light, the surface water and land fractions, stellar flares, and
associated phenomena place potential constraints on the evolution of life on
these planets.
