  Terrestrial exoplanets in habitable zones are ubiquitous. It is, however,
unknown which have Earth-like or Venus-like climates. Distinguishing different
planet-types is crucial for determining whether a planet could be habitable. We
investigate the potential of polarimetry for distinguishing exo-Earths from
exo-Venuses. We present computed fluxes and polarisation of starlight that is
reflected by exoplanets with atmospheres in evolutionary states from current
Earth to current Venus, with cloud compositions ranging from pure water to 0.75
sulfuric acid solution, for wavelengths between 0.3 and 2.5 microns. The
polarisation of the reflected light shows larger variations with the planetary
phase angle than the total flux. Across the visible, the largest polarisation
is reached for an Earth-like atmosphere with water clouds, due to Rayleigh
scattering above the clouds and the rainbow near 40 deg phase angle. In the
near-infrared, the planet with a Venus-like CO2 atmosphere and thin water
clouds shows the most prominent polarisation features due to scattering by the
small cloud droplets. A planet around Alpha Centauri A would leave temporal
variations on the order of 10E-13 W/m3 in the reflected flux and 10E-11 in the
degree of polarisation along the planet's orbit for a spatially unresolved
star-planet system. Star-planet contrasts are on the order of 10E-10. Current
polarimeters cannot distinguish between the possible evolutionary phases of
spatially unresolved terrestrial exoplanets, as a sensitivity near 10E-10 is
required to discern the planet signal on the background of unpolarised
starlight. Telescopes capable of reaching planet-star contrasts lower than
10E-9 should be able to observe the variation of the planet's resolved degree
of polarisation as a function of its phase angle and thus to discern an
exo-Earth from an exo-Venus based on its clouds' unique polarisation
signatures.
