The effect of stellar contamination on low-resolution transmission spectroscopy: needs identified by NASA’s Exoplanet Exploration Program Study Analysis Group 21

Abstract

Study Analysis Group 21 (SAG21) of NASA’s Exoplanet Exploration Program Analysis Group (ExoPAG) was organized to study the effect of stellar contamination on space-based transmission spectroscopy, a method for studying exoplanetary atmospheres by measuring the wavelength-dependent radius of a planet as it transits its star. Transmission spectroscopy relies on a precise understanding of the spectrum of the star being occulted. However, stars are not homogeneous, constant light sources but have temporally evolving photospheres and chromospheres with inhomogeneities like spots, faculae, plages, granules, and flares. This SAG brought together an interdisciplinary team of more than 100 scientists, with observers and theorists from the heliophysics, stellar astrophysics, planetary science, and exoplanetary atmosphere research communities, to study the current research needs that can be addressed in this context to make the most of transit studies from current NASA facilities like HST and JWST. The analysis produced 14 findings, which fall into three Science Themes encompassing (1) how the Sun is used as our best laboratory to calibrate our understanding of stellar heterogeneities (‘The Sun as the Stellar Benchmark’), (2) how stars other than the Sun extend our knowledge of heterogeneities (‘Surface Heterogeneities of Other Stars’) and (3) how to incorporate information gathered for the Sun and other stars into transit studies (‘Mapping Stellar Knowledge to Transit Studies’). In this invited review, we largely reproduce the final report of SAG21 as a contribution to the peer-reviewed literature.

High Tide or Riptide on the Cosmic Shoreline? A Water-rich Atmosphere or Stellar Contamination for the Warm Super-Earth GJ 486b from JWST Observations

“…We stress that, while our present observations cannot distinguish between the water-rich atmosphere scenario and unocculted starspots, these two scenarios deviate substantially at shorter wavelengths (see Figure 4). Consequently, even in the case of aerosol-laden atmospheres (Rackham et al 2023), future observations at shorter wavelengths can readily distinguish which scenario is correct.…”

Starspot Mapping with Adaptive Parallel Tempering. II. Application to TESS Data for M-dwarf Flare Stars AU Microscopii, YZ Canis Minoris, and EV Lacertae

“…It will also be helpful to scan the stellar surface for spot occultations by multiple transits of exoplanets (e.g., Morris et al 2017;Bruno & Deleuil 2021). Surface mapping on the stellar surface plays an important role in quantifying stellar effects on exoplanet characterizations (Rackham et al 2018(Rackham et al , 2023. spot parameters are of the same degree as those in the two-spot model (Table 6).…”

An In-depth Look at TOI-3884b: A Super-Neptune Transiting an M4Dwarf with Persistent Starspot Crossings

“…This is of particular concern with M dwarfs, whose spots are cool enough to host their own water absorption features (Jones et al 1995). Rackham et al (2023) emphasize the need for future in-depth studies into untangling star and planetary spectra, especially as we begin to probe the atmospheres of terrestrial worlds with JWST and future instruments. With half of its transit covered by a spot and the other over the photosphere, comparing the resulting transmission spectrum from the first half of the transit to that from the second may yield an unprecedented probe into the effects of cooler spots.…”