A secondary atmosphere on the rocky exoplanet 55 Cancri e

TL;DR

This study presents JWST thermal emission data for 55 Cancri e, indicating it likely has a secondary, volatile-rich atmosphere, distinct from a vaporized rock shroud, advancing understanding of rocky exoplanet atmospheres.

Contribution

First direct thermal emission spectrum of 55 Cancri e from JWST, providing evidence for a secondary volatile atmosphere over a vaporized rock scenario.

Findings

Rules out a vaporized rock atmosphere model.

Supports presence of a CO2 or CO-rich secondary atmosphere.

Suggests atmosphere is outgassed from a magma ocean.

Abstract

Characterizing rocky exoplanets is a central endeavor of astronomy, and yet the search for atmospheres on rocky exoplanets has hitherto resulted in either tight upper limits on the atmospheric mass or inconclusive results. The 1.95-REarth and 8.8-MEarth planet 55 Cnc e, with a predominantly rocky composition and an equilibrium temperature of ~2000 K, may have a volatile envelope (containing molecules made from a combination of C, H, O, N, S, and P elements) that accounts for up to a few percent of its radius. The planet has been observed extensively with transmission spectroscopy, and its thermal emission has been measured in broad photometric bands. These observations disfavor a primordial H2/He-dominated atmosphere but cannot conclusively determine whether the planet has a secondary atmosphere. Here we report a thermal emission spectrum of the planet obtained by JWST's NIRCam and MIRI instruments from 4 to 12 {\mu}m. The measurements rule out the scenario where the planet is a lava world shrouded by a tenuous atmosphere made of vaporized rock, and indicate a bona fide volatile atmosphere likely rich in CO2 or CO. This atmosphere can be outgassed from and sustained by a magma ocean.