JWST COMPASS: The 3-5 Micron Transmission Spectrum of the Super-Earth L 98-59 c

Abstract

We present a JWST NIRSpec transmission spectrum of the super-Earth exoplanet L 98-59 c. This small (R${}_p=1.385\pm 0.085$R${}_{\oplus }$, M${}_p=2.22\pm 0.26$R${}_{\oplus }$), warm (T${}_{\text{eq}}=553$K) planet resides in a multi-planet system around a nearby, bright (J = 7.933) M3V star. We find that the transmission spectrum of L 98-59 c is featureless at the precision of our data. We achieve precisions of 22ppm in NIRSpec G395H's NRS1 detector and 36ppm in the NRS2 detector at a resolution R$\sim$200 (30 pixel wide bins). At this level of precision, we are able rule out primordial H${}_2$-He atmospheres across a range of cloud pressure levels up to at least $\sim$0.1mbar. By comparison to atmospheric forward models, we also rule out atmospheric metallicities below $\sim$300$\times$ solar at 3$\sigma$ (or equivalently, atmospheric mean molecular weights below $\sim$10~g/mol). We also rule out pure methane atmospheres. The remaining scenarios that are compatible with our data include a planet with no atmosphere at all, or higher mean-molecular weight atmospheres, such as CO${}_2$- or H${}_2$O-rich atmospheres. This study adds to a growing body of evidence suggesting that planets $\lesssim 1.5$R${}_{\oplus }$ lack extended atmospheres.

21 Pages, 13 Figures; Accepted to AJ