Possible Carbon Dioxide above the Thick Aerosols of GJ 1214 b
Fortney, Jonathan J.; Wheatley, Peter J.; Henry, Gregory W.; Bell, Taylor J.; Kreidberg, Laura; Parmentier, Vivien; Rieke, Marcia J.; Gill, Samuel; Anderson, David R.; Beatty, Thomas G.; Welbanks, Luis; Greene, Thomas P.; Schlawin, Everett; Ohno, Kazumasa; Murphy, Matthew M.; Edelman, Isaac R.; Mehta, Nishil
United States, Japan, France, United Kingdom, Chile, Germany
Abstract
Sub-Neptune planets with radii smaller than Neptune (3.9 R ⊕) 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 μm that shows signatures of CO2 and CH4, expected at high metallicity. A model containing both these molecules is preferred by 3.3σ and 3.6σ 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 CO2 and CH4 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.