chromatic and chromatic_fitting: New open-source Python tools for precise atmospheric spectroscopy with JWST and beyond

As we enter a new era of extremely precise exoplanet atmosphere spectroscopy, it is crucial that the tools we use for extracting planetary spectra are fast, flexible, and user-friendly. Here we present two new open-source Python packages for the community. chromatic is a friendly program to read and write spectroscopic light-curves from/to a variety of formats, simplify many common calculations, and provide publication-ready visuals. chromatic_fitting, built on chromatic, can perform efficient model fits to spectroscopic light-curve data and produce transmission (or emission) spectra. By combining any number of transit, eclipse, polynomial (in time, x/y, etc.), Gaussian Process, or user-defined models, we can simultaneously account for the spectral signatures imprinted by planets, stellar activity and instrumental systematics. chromatic_fitting also has the flexibility to carry out both 'white light’ and multi-wavelength fitting, so can fully exploit the impressive wavelength coverage of facilities like JWST and spectrophotometric synergies between instruments. The aim of both tools is to easily and efficiently compare data reduction techniques, standardize the light-curve-fitting stage and understand the impact of model and parameter assumptions on transmission spectra. We hope these tools can help push the limit for extracting reliable exoplanet spectra and reduce uncertainties on atmospheric retrievals. As part of the ERS program, we successfully applied these tools to spectrophotometry from several of JWST’s instruments. We demonstrate their unique versatility and future applicability to, not only, JWST, but other instruments both in space and on the ground.