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Arxiv: JWST COMPASS: A NIRSpec/G395H Transmission Spectrum of the Sub-Neptune TOI-836c Published: 4/1/2024 8:38:17 PM Updated: 4/1/2024 8:38:17 PM

Paper abstract: Planets between the sizes of Earth and Neptune are the most common in theGalaxy, bridging the gap between the terrestrial and giant planets in our SolarSystem. Now that we are firmly in the era of JWST, we can begin to measure, inmore detail, the atmospheres of these ubiquitous planets to better understandtheir evolutionary trajectories. The two planets in the TOI-836 system areideal candidates for such a study, as they fall on either side of the radiusvalley, allowing for direct comparisons of the present-day atmospheres ofplanets that formed in the same environment but had different ultimate endstates. We present results from the JWST NIRSpec G395H transit observation ofthe larger and outer of the planets in this system, TOI-836c (2.587R_{\oplus}, 9.6 M_{\oplus}, T_{\rm eq}~665 K). While we measureaverage 30-pixel binned precisions of ~24ppm for NRS1 and ~43ppm forNRS2 per spectral bin, we do find residual correlated noise in the data, whichwe attempt to correct using the JWST Engineering Database. We find afeatureless transmission spectrum for this sub-Neptune planet, and are able torule out atmospheric metallicities <175\times Solar in the absence ofaerosols at <~1 millibar. We leverage microphysical models to determinethat aerosols at such low pressures are physically plausible. The resultspresented herein represent the first observation from the COMPASS (Compositionsof Mini-Planet Atmospheres for Statistical Study) JWST program, which alsoincludes TOI-836b and will ultimately compare the presence and compositions ofatmospheres for 12 super-Earths/sub-Neptunes.