JWST COMPASS: NIRSpec/G395H Transmission Observations of the Super-Earth TOI-836b

We present two transit observations of the ∼870 K, 1.7 R _⊕ super-Earth TOI-836b with JWST NIRSpec/G395H, resulting in a 2.8–5.2 μm transmission spectrum. Using two different reduction pipelines, we obtain a median transit depth precision of 34 ppm for Visit 1 and 36 ppm for Visit 2, leading to a combined precision of 25 ppm in spectroscopic channels 30 pixels wide (∼0.02 μm). We find that the transmission spectrum from both visits is well fit by a zero-sloped line, by fitting zero-sloped and sloped lines as well as step functions to our data. Combining both visits, we are able to rule out atmospheres with metallicities <250 times solar for an opaque pressure level of 0.1 bar, corresponding to mean molecular weights of ≲6 g mol^‑1. We therefore conclude that TOI-836b does not have an H₂-dominated atmosphere, in possible contrast with its larger, exterior sibling planet, TOI-836 c. We recommend that future proposals to observe small planets exercise caution when requiring specific numbers of transits to rule out physical scenarios, particularly for high metallicities and planets around bright host stars, as PandExo predictions appear to be more optimistic than what the gains from additional transits implied by our data suggest.