  A new generation of exoplanet research beckons and with it the need for
simulation tools that accurately predict signal and noise in transit
spectroscopy observations. We developed ExoSim: an end-to-end simulator that
models noise and systematics in a dynamical simulation. ExoSim improves on
previous simulators in the complexity of its simulation, versatility of use and
its ability to be generically applied to different instruments. It performs a
dynamical simulation that can capture temporal effects, such as correlated
noise and systematics on the light curve. It has also been extensively
validated, including against real results from the Hubble WFC3 instrument. We
find ExoSim is accurate to within 5% in most comparisons. ExoSim can interact
with other models which simulate specific time-dependent processes. A dedicated
star spot simulator allows ExoSim to produce simulated observations that
include spot and facula contamination. ExoSim has been used extensively in the
Phase A and B design studies of the ARIEL mission, and has many potential
applications in the field of transit spectroscopy.
