Possible Carbon Dioxide Above the Thick Aerosols of GJ 1214 b

TL;DR

This study presents JWST observations of GJ 1214 b revealing potential carbon dioxide and methane in its atmosphere, suggesting high metallicity, but more data is needed for confirmation due to low signal strength.

Contribution

First detection of possible CO2 and CH4 in GJ 1214 b's atmosphere using JWST, indicating high metallicity and advancing understanding of sub-Neptune compositions.

Findings

Potential detection of CO2 and CH4 signatures.

Preference for models with these molecules over featureless spectra.

Need for further observations to confirm findings.

Abstract

Sub-Neptune planets with radii smaller than Neptune (3.9 Re) are the most common type of planet known to exist in The Milky Way, even though they are absent in the Solar System. These planets can potentially have a large diversity of compositions as a result of different mixtures of rocky material, icy material and gas accreted from a protoplanetary disk. However, the bulk density of a sub-Neptune, informed by its mass and radius alone, cannot uniquely constrain its composition; atmospheric spectroscopy is necessary. GJ 1214 b, which hosts an atmosphere that is potentially the most favorable for spectroscopic detection of any sub-Neptune, is instead enshrouded in aerosols (thus showing no spectroscopic features), hiding its composition from view at previously observed wavelengths in its terminator. Here, we present a JWST NIRSpec transmission spectrum from 2.8 to 5.1 um that shows signatures of carbon dioxide and methane, expected at high metallicity. A model containing both these molecules is preferred by 3.3 and 3.6 sigma as compared to a featureless spectrum for two different data analysis pipelines, respectively. Given the low signal-to-noise of the features compared to the continuum, however, more observations are needed to confirm the carbon dioxide and methane signatures and better constrain other diagnostic features in the near-infrared. Further modeling of the planet's atmosphere, interior structure and origins will provide valuable insights about how sub-Neptunes like GJ 1214 b form and evolve.