NEMESISPY: a Python package for simulating and retrieving exoplanetary spectra

Journal article

Spectra of exoplanets allow us to probe their atmospheres’ composition and thermal structure and, when applicable, their surface conditions (Burrows, 2014). Spectroscopic characterisation of a large population of exoplanets may help us understand the origin and evolution of planetary systems (Chachan et al., 2023; Madhusudhan et al., 2017; Mordasini et al., 2016). The extraction of information from spectral data is known as atmospheric retrievals (e.g., Irwin et al., 2008; Line et al., 2013; Madhusudhan & Seager, 2009), which can be divided into two steps: forward modelling and model fitting. At a minimum, the forward modelling step requires an atmospheric model for the observed planet and a radiative transfer pipeline that can calculate model spectra given some input atmospheric model. The model fitting step typically requires a Bayesian parameter inference algorithm that can constrain the free parameters of the forward model by fitting the observed spectra. Atmospheric retrieval pipelines have long been applied to the spectral analysis of the Earth and other solar system planets, and the discovery of exoplanets further ignited the development of new retrieval pipelines with varying focus and functionalities (MacDonald & Batalha, 2023).

NEMESISPY is a Python package developed to perform parametric atmospheric modelling and radiative transfer calculation for the retrievals of exoplanetary spectra. It is a recent development of the well-established Fortran NEMESIS library (Irwin et al., 2008), which has been applied to the atmospheric retrievals of both solar system planets and exoplanets employing numerous different observing geometries (Barstow et al., 2014, 2016; Barstow, 2020; Irwin et al., 2020; James et al., 2023; Krissansen-Totton et al., 2018; Lee et al., 2012; Teanby et al., 2012). NEMESISPY can be easily interfaced with Bayesian inference algorithms to retrieve atmospheric properties from spectroscopic observations. Recently, NEMESISPY has been applied to the retrievals of Hubble and Spitzer data of a hot Jupiter (Yang et al., 2023), as well as to JWST/Mid-Infrared Instrument (JWST/MIRI) data of a hot Jupiter (Yang et al., 2024).

Authors: Yang, J; Alday, J; Irwin, P

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: Open Journals
• Journal: Journal of Open Source Software
• Volume: 9
• Issue: 101
• Article number: 6874
• Publication date: 2024-09-08
• Acceptance date: 2024-09-08
• DOI: 10.21105/joss.06874
• EISSN: 2475-9066
• Language: English
• Keywords: astronomy; exoplanets; spectroscopy; radiative transfer; atmospheric retrieval