POSEIDON: A Multidimensional Atmospheric Retrieval Code for Exoplanet Spectra

This is the software archive for the initial v1.0 release of POSEIDON - as published in the Journal of Open Source Software. Summary Exoplanet atmospheres are a dynamic and fast-changing field at the frontier of modern astronomy. Telescope observations can reveal the chemical composition, temperature, cloud properties, and (potentially) the habitability of these remote worlds. Astronomers can measure these atmospheric properties by observing how the fraction of starlight blocked by a planet passing in front of its host star changes with wavelength - a technique called transmission spectroscopy. Since the wavelengths where different atoms and molecules absorb are already known (from laboratory measurements or quantum mechanics), astronomers can compare models of exoplanet spectra to observations to infer the chemical composition of exoplanets. POSEIDON is a Python package for the modelling and analysis of exoplanet spectra. POSEIDON has two main functions: (i) computation of model spectra for 1D, 2D, or 3D exoplanet atmospheres; and (ii) a Bayesian fitting routine ('atmospheric retrieval') that can infer the range of atmospheric properties consistent with an observed exoplanet spectrum