Distinguishing Oceans of Water from Magma on Mini-Neptune K2-18b

Abstract

Mildly irradiated mini-Neptunes have densities potentially consistent with them hosting substantial liquid-water oceans (“Hycean” planets). The presence of CO _2 and simultaneous absence of ammonia (NH _3 ) in their atmospheres has been proposed as a fingerprint of such worlds. JWST observations of K2-18b, the archetypal Hycean, have found the presence of CO _2 and the depletion of NH _3 to 4 μ m region, where CO _2 and CO features dominate: magma ocean models suggest a systematically lower CO _2 /CO ratio than estimated from free-chemistry retrieval, indicating that deeper observations of this spectral region may be able to distinguish between oceans of liquid water and magma on mini-Neptunes.

Keywords

• Exoplanet atmospheres
• Exoplanet structure
• Exoplanet atmospheric composition