  The search for habitable exoplanets inspires the question - how do habitable
planets form? Planet habitability models traditionally focus on abiotic
processes and neglect a biotic response to changing conditions on an inhabited
planet. The Gaia hypothesis postulates that life influences the Earth's
feedback mechanisms to form a self-regulating system, and hence that life can
maintain habitable conditions on its host planet. If life has a strong
influence, it will have a role in determining a planet's habitability over
time. We present the ExoGaia model - a model of simple 'planets' host to
evolving microbial biospheres. Microbes interact with their host planet via
consumption and excretion of atmospheric chemicals. Model planets orbit a
'star' which provides incoming radiation, and atmospheric chemicals have either
an albedo, or a heat-trapping property. Planetary temperatures can therefore be
altered by microbes via their metabolisms. We seed multiple model planets with
life while their atmospheres are still forming and find that the microbial
biospheres are, under suitable conditions, generally able to prevent the host
planets from reaching inhospitable temperatures, as would happen on a lifeless
planet. We find that the underlying geochemistry plays a strong role in
determining long-term habitability prospects of a planet. We find five distinct
classes of model planets, including clear examples of 'Gaian bottlenecks' - a
phenomenon whereby life either rapidly goes extinct leaving an inhospitable
planet, or survives indefinitely maintaining planetary habitability. These
results suggest that life might play a crucial role in determining the
long-term habitability of planets.
