Direct emission spectroscopy of exoplanets with the medium resolution imaging spectrometer on board JWST MIRI: I. Molecular mapping and sensitivity to instrumental effects

TL;DR

This study demonstrates that JWST's MIRI Medium Resolution Spectrometer can effectively perform molecular mapping to detect and characterize atmospheres of directly imaged giant exoplanets in the mid-infrared, despite some limitations.

Contribution

The paper assesses the feasibility of molecular mapping with JWST/MIRI MRS for exoplanet atmospheres using simulations and introduces methods to detect multiple molecules.

Findings

MIRI MRS can detect multiple atmospheric molecules in exoplanets.

Two approaches effectively identify key molecules like H2O, CO, CH4, and NH3.

Simulations of HR8799 and GJ504 systems show successful atmospheric characterization.

Abstract

The Medium Resolution Spectrometer on board JWST/MIRI will give access to mid-IR spectra while retaining spatial information. With the unparalleled sensitivity of JWST and the MIRI detectors, the MRS has the potential to revolutionise our understanding of giant exoplanet atmospheres. Molecular mapping is a promising detection and characterisation technique used to study the spectra of directly imaged exoplanets. We aim to examine the feasibility and application of this technique to MRS observations. We used the instrument simulator MIRISIM to create mock observations of resolved star and exoplanet systems. As an input for the simulator, we used stellar and planet parameters from literature, with the planet spectrum being modelled with the radiative transfer code petitRADTRANS. After processing the raw data with the JWST pipeline, we high pass filter the data to account for the stellar point spread function, and used a forward modelling approach to detect the companions and constrain the chemical composition of their atmospheres through their molecular signatures. We identified limiting factors in spectroscopic characterisation of directly imaged exoplanets with the MRS and simulated observations of two representative systems, HR8799 and GJ504. In both systems, we could detect the presence of multiple molecules that were present in the input model of their atmospheres. We used two different approaches with single molecule forward models, used in literature, that are sensitive to detecting mainly H$_2$O, CO, CH$_4$, and NH$_3$, and a log-likelihood ratio test that uses full atmosphere forward models and is sensitive to a larger number of less dominant molecular species. We show that the MIRI MRS can be used to characterise widely separated giant exoplanets in the mid-IR using molecular mapping.