A comparison of exoplanet spectroscopic retrieval tools

Abstract

Over the last several years, spectroscopic observations of transiting exoplanets have begun to uncover information about their atmospheres, including atmospheric composition and indications of the presence of clouds and hazes. Spectral retrieval is the leading technique for interpretation of transmission spectra and is employed by several teams using a variety of forward models and parameter estimation algorithms. However, different model suites have mostly been used in isolation and so it is unknown whether the results from each are comparable. As we approach the launch of the James Webb Space Telescope we anticipate advances in wavelength coverage, precision, and resolution of transit spectroscopic data, so it is important that the tools that will be used to interpret these information rich spectra are validated. To this end, we present an inter-model comparison of three retrieval suites: TauREx , NEME-SIS and CHIMERA. We demonstrate that the forward model spectra are in good agreement (residual deviations on the order of 20−40 ppm), and discuss the results of cross retrievals between the three tools. Generally, the constraints from the crossretrievals are consistent with each other and with input values to within 1σ. However, for high precision scenarios with error envelopes of order 30 ppm, subtle differences in the simulated spectra result in discrepancies between the different retrieval suites, and inaccuracies in retrieved values of several σ. This can be considered analogous to substantial systematic/astrophysical noise in a real observation, or errors/omissions in a forward model such as molecular linelist incompleteness or missing absorbers.

TauREx 3: A Fast, Dynamic, and Extendable Framework for Retrievals

“…We do not intend to perform a comprehensive study or reinterpretation of the available HD 209458b data as this task would be beyond the scope of this paper. We highlight that the previous version of TauREx 2 was cross-compared with other retrieval codes in Barstow et al (2020), showing agreement between the TauREx 2, NEMESIS (Irwin et al 2008), and CHIMERA (Line et al 2013) retrieval codes. For the optimizer, we use MultiNest (Feroz et al 2009;Buchner et al 2014) compiled with an MPI.…”

“…In summary, benchmarking shows the results of TauREx 3 are consistent with those of TauREx 2 (which was benchmarked against both NEMESIS and CHIMERA; Barstow et al 2020), while showing a sixfold improvement in retrieval runtime. This result demonstrates that retrievals of higherresolution spectra from missions such as JWST and Ariel are feasible within a couple of hours.…”

“…Recently, TauREx 2 forward model and retrieval results have been verified against the NEMESIS and CHIMERA retrieval codes by Barstow et al (2020). The study benchmarked retrievals from each code on mock forward models generated by the other codes.…”

The ExoMolOP database: Cross sections andk-tables for molecules of interest in high-temperature exoplanet atmospheres

“…from various aspects of computed cross sections have been explored by Hedges & Madhusudhan (2016), Rocchetto (2017), Gharib-Nezhad & Line (2019) and Barstow et al (2020). Differences largely arise from either the choice of line list for a particular species, or from the different treatments of the broadening of the lines, including line-wing cut-offs.…”

“…The broadening is considered to be from H 2 only in this example. Barstow et al (2020) have highlighted the differences in forward models and retrievals caused by the choice of molecular broadening parameters. It is important to note that although there is a known difference between different parameters, there is still much work required in order to determine the 'true' parameters, or even a good approximation, for many species.…”