  Recognizing whether a planet can support life is a primary goal of future
exoplanet spectral characterization missions, but past research on habitability
assessment has largely ignored the vastly different conditions that have
existed in our planet's long habitable history. This study presents simulations
of a habitable yet dramatically different phase of Earth's history, when the
atmosphere contained a Titan-like organic-rich haze. Prior work has claimed a
haze-rich Archean Earth (3.8-2.5 billion years ago) would be frozen due to the
haze's cooling effects. However, no previous studies have self-consistently
taken into account climate, photochemistry, and fractal hazes. Here, we
demonstrate using coupled climate-photochemical-microphysical simulations that
hazes can cool the planet's surface by about 20 K, but habitable conditions
with liquid surface water could be maintained with a relatively thick haze
layer (tau ~ 5 at 200 nm) even with the fainter young sun. We find that
optically thicker hazes are self-limiting due to their self-shielding
properties, preventing catastrophic cooling of the planet. Hazes may even
enhance planetary habitability through UV shielding, reducing surface UV flux
by about 97% compared to a haze-free planet, and potentially allowing survival
of land-based organisms 2.6.2.7 billion years ago. The broad UV absorption
signature produced by this haze may be visible across interstellar distances,
allowing characterization of similar hazy exoplanets. The haze in Archean
Earth's atmosphere was strongly dependent on biologically-produced methane, and
we propose hydrocarbon haze may be a novel type of spectral biosignature on
planets with substantial levels of CO2. Hazy Archean Earth is the most alien
world for which we have geochemical constraints on environmental conditions,
providing a useful analog for similar habitable, anoxic exoplanets.
