  The "liquid water habitable zone" (HZ) concept is predicated on the ability
of the silicate weathering feedback to stabilize climate across a wide range of
instellations. However, representations of silicate weathering used in current
estimates of the effective outer edge of the HZ do not account for the
thermodynamic limit on concentration of weathering products in runoff set by
clay precipitation, nor for the energetic limit on precipitation set by
planetary instellation. We find that when the thermodynamic limit is included
in an idealized coupled climate/weathering model, steady-state planetary
climate loses sensitivity to silicate dissolution kinetics, becoming sensitive
to temperature primarily through the effect of temperature on runoff and to
pCO$_2$ through an effect on solute concentration mediated by pH. This
increases sensitivity to land fraction, CO$_2$ outgassing, and geological
factors such as soil age and lithology, all of which are found to have a
profound effect on the position of the effective outer edge of the HZ. The
interplay between runoff sensitivity and the energetic limit on precipitation
leads to novel warm states in the outer reaches of the HZ, owing to the
decoupling of temperature and precipitation. We discuss strategies for
detecting the signature of silicate weathering feedback through exoplanet
observations in light of insights derived from the revised picture of
weathering.
