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

Abstract

We present a JWST Near-InfraRed Spectrograph (NIRSpec) transmission spectrum of the super-Earth exoplanet L 98-59 c. This small ( R _p = 1.385 ± 0.085 R _⊕ , M _p = 2.22 ± 0.26 R _⊕ ), warm ( T _eq = 553 K) planet resides in a multiplanet 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 22 ppm in NIRSpec G395H’s NRS1 detector and 36 ppm in the NRS2 detector at a resolution R ∼ 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 ∼0.1 mbar. By comparison to atmospheric forward models, we also rule out atmospheric metallicities below ∼300× solar at 3 σ (or, equivalently, atmospheric mean molecular weights below ∼10 g mol ^−1 ). 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 ≲1.5 R _⊕ lack extended atmospheres.

Keywords

• Exoplanet astronomy
• Exoplanet atmospheres
• Exoplanet atmospheric composition
• Exoplanets
• Super Earths