JWST COMPASS: NIRSpec/G395H Transmission Observations of TOI-776 c, a 2 R ⊕ M Dwarf Planet

The atmospheres of planets between the size of Earth and Neptune at short orbital periods have been under intense scrutiny. Of the approximately dozen planets in this regime with atmospheres studied so far, a few appear to have prominent molecular features, while others appear relatively void of detectable atmospheres. Further work is therefore needed to understand the atmospheres of these planets, starting with observing a larger sample. To this end, we present the 3–5 μm transmission spectrum of TOI-776 c, a warm (Teq ∼ 420 K), ∼2 R⊕, ∼7 M⊕ planet orbiting an M1V star, measured with JWST NIRSpec/G395H. By combining two visits, we measure a median transit precision of ∼18 and ∼32 ppm in the NRS1 and NRS2 detectors, respectively. We compare the transmission spectrum to both nonphysical and physical models and find no strong evidence for molecular features. For cloud-top pressures larger than 10−3 bars, we rule out atmospheric metallicities less than 180–240× solar (depending on the reduction and modeling technique), which corresponds to a mean molecular weight of ∼6–8 g mol−1. However, we find that simple atmosphere mixture models (H2O + H2/He or CO2 + H2/He) give more pessimistic constraints, and we caution that mean molecular weight inferences are model dependent. We compare TOI-776 c to the similar planet TOI-270 d and discuss possible options for further constraining TOI-776 c’s atmospheric composition. Overall, we suggest that these TOI-776 c observations may represent a combination of planetary and stellar parameters that fall just below the threshold of detectable features in small-planet spectra; finding this boundary is one of the main goals of the COMPASS program.