  The search for life on exoplanets is motivated by the universal ways in which
life could modify its planetary environment. Atmospheric gases such as oxygen
and methane are promising candidates for such environmental modification due to
the evolutionary benefits their production would confer. However, confirming
that these gases are produced by life, rather than by geochemical or
astrophysical processes, will require a thorough understanding of planetary
context, including the expected counterfactual atmospheric evolution for
lifeless planets. Here, we evaluate current understanding of planetary context
for several candidate biosignatures and their upcoming observability. We review
the contextual framework for oxygen and describe how conjectured abiotic oxygen
scenarios may be testable. In contrast to oxygen, current understanding of how
planetary context controls non-biological methane (CH$_4$) production is
limited, even though CH$_4$ biosignatures in anoxic atmospheres may be readily
detectable with the James Webb Space Telescope. We assess environmental context
for CH$_4$ biosignatures and conclude that abundant atmospheric CH$_4$
coexisting with CO$_2$, and CO:CH$_4$ << 1 is suggestive of biological
production, although precise thresholds are dependent on stellar context and
sparsely characterized abiotic CH$_4$ scenarios. A planetary context framework
is also considered for alternative or agnostic biosignatures. Whatever the
distribution of life in the Universe, observations of terrestrial exoplanets in
coming decades will provide a quantitative understanding of the atmospheric
evolution of lifeless worlds. This knowledge will inform future instrument
requirements to either corroborate the presence of life elsewhere or confirm
its apparent absence.
