  Rocky planets with temperate conditions provide the best chance for
discovering habitable worlds and life outside the Solar System. In the last
decades, new instrumental facilities and large observational campaigns have
been driven by the quest for habitable worlds. Climate models aimed at studying
the habitability of rocky planets are essential tools to pay off these
technological and observational endeavours. In this context, we present
EOS-ESTM, a fast and flexible model aimed at exploring the impact on
habitability of multiple climate factors, including those unconstrained by
observations. EOS-ESTM is built on ESTM, a seasonal-latitudinal energy balance
model featuring an advanced treatment of the meridional and vertical transport.
The novel features of EOS-ESTM include: (1) parameterizations for simulating
the climate impact of oceans, land, ice, and clouds as a function of
temperature and stellar zenith distance; (2) a procedure (EOS) for calculating
the radiative transfer in atmospheres with terrestrial and non-terrestrial
compositions illuminated by solar- and non-solar-type stars. By feeding
EOS-ESTM with Earth's stellar, orbital and planetary parameters we derive a
reference model that satisfies a large number of observational constraints of
the Earth's climate system. Validation tests of non-terrestrial conditions
yield predictions that are in line with comparable results obtained with a
hierarchy of climate models. The application of EOS-ESTM to planetary
atmospheres in maximum greenhouse conditions demonstrates the possibility of
tracking the snowball transition at the outer edge of the HZ for a variety of
planetary parameters, paving the road for multi-parametric studies of the HZ.
