  The discovery of a large number of terrestrial exoplanets in the habitable
zones of their stars, many of which are qualitatively different from Earth, has
led to a growing need for fast and flexible 3D climate models, which could
model such planets and explore multiple possible climate states and surface
conditions. We respond to that need by creating ExoPlaSim, a modified version
of the Planet Simulator (PlaSim) that is designed to be applicable to
synchronously rotating terrestrial planets, planets orbiting stars with
non-solar spectra, and planets with non-Earth-like surface pressures. In this
paper we describe our modifications, present validation tests of ExoPlaSim's
performance against other GCMs, and demonstrate its utility by performing two
simple experiments involving hundreds of models. We find that ExoPlaSim agrees
qualitatively with more-sophisticated GCMs such as ExoCAM, LMDG, and ROCKE-3D,
falling within the ensemble distribution on multiple measures. The model is
fast enough that it enables large parameter surveys with hundreds to thousands
of models, potentially enabling the efficient use of a 3D climate model in
retrievals of future exoplanet observations. We describe our efforts to make
ExoPlaSim accessible to non-modellers, including observers, non-computational
theorists, students, and educators through a new Python API and streamlined
installation through pip, along with online documentation.
