A warm Neptune's methane reveals core mass and vigorous atmospheric mixing

David K Sing; Zafar Rustamkulov; Daniel P Thorngren; Joanna K Barstow; Pascal Tremblin; Catarina Alves de Oliveira; Tracy L Beck; Stephan M Birkmann; Ryan C Challener; Nicolas Crouzet; Néstor Espinoza; Pierre Ferruit; Giovanna Giardino; Amélie Gressier; Elspeth K H Lee; Nikole K Lewis; Roberto Maiolino; Elena Manjavacas; Bernard J Rauscher; Marco Sirianni; Jeff A Valenti

doi:10.1038/s41586-024-07395-z

A warm Neptune's methane reveals core mass and vigorous atmospheric mixing

Nature. 2024 May 20. doi: 10.1038/s41586-024-07395-z. Online ahead of print.

Authors

David K Sing^{1

2}, Zafar Rustamkulov³, Daniel P Thorngren⁴, Joanna K Barstow⁵, Pascal Tremblin^{6

7}, Catarina Alves de Oliveira⁸, Tracy L Beck⁹, Stephan M Birkmann⁸, Ryan C Challener¹⁰, Nicolas Crouzet¹¹, Néstor Espinoza⁹, Pierre Ferruit⁸, Giovanna Giardino¹², Amélie Gressier⁹, Elspeth K H Lee¹³, Nikole K Lewis¹⁰, Roberto Maiolino¹⁴, Elena Manjavacas^{4

15}, Bernard J Rauscher¹⁶, Marco Sirianni¹⁷, Jeff A Valenti⁹

Affiliations

¹ Department of Earth & Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA. dsing@jhu.edu.
² Department of Physics & Astronomy, Johns Hopkins University, Baltimore, MD, USA. dsing@jhu.edu.
³ Department of Earth & Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA.
⁴ Department of Physics & Astronomy, Johns Hopkins University, Baltimore, MD, USA.
⁵ School of Physical Sciences, The Open University, Milton Keynes, UK.
⁶ Université Paris-Saclay, UVSQ, CNRS, CEA, Maison de la Simulation, Gif-sur-Yvette, France.
⁷ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, Gif-sur-Yvette, France.
⁸ European Space Agency, European Space Astronomy Centre, 28692 Villanueva de la Canada, Madrid, Spain.
⁹ Space Telescope Science Institute, Baltimore, MD, USA.
¹⁰ Department of Astronomy and Carl Sagan Institute, Cornell University, Ithaca, NY, USA.
¹¹ Leiden Observatory, Leiden University, P.O. Box 9513, Leiden, The Netherlands.
¹² ATG Europe for the European Space Agency, ESTEC, Noordwijk, The Netherlands.
¹³ Center for Space and Habitability, University of Bern, Bern, Switzerland.
¹⁴ University of Cambridge, The Old Schools, Trinity Ln, Cambridge, UK.
¹⁵ AURA for the European Space Agency (ESA), ESA Office, Space Telescope Science Institute, Baltimore, MD, USA.
¹⁶ NASA Goddard Space Flight Center, Greenbelt, MD, USA.
¹⁷ European Space Agency (ESA), ESA Office, Space Telescope Science Institute, Baltimore, MD, USA.

PMID: 38768633
DOI: 10.1038/s41586-024-07395-z

Abstract

Observations of transiting gas giant exoplanets have revealed a pervasive depletion of methane^1,2,3,4, which has only recently been identified atmospherically^5,6. The depletion is thought to be maintained by disequilibrium processes such as photochemistry or mixing from a hotter interior^7,8,9. However, the interiors are largely unconstrained along with the vertical mixing strength and only upper limits on the CH₄ depletion have been available. The warm Neptune WASP-107 b stands out among exoplanets with an unusually low density, reported low core mass¹⁰, and temperatures amenable to CH₄ though previous observations have yet to find the molecule^2,4. Here we present a JWST NIRSpec transmission spectrum of WASP-107 b which shows features from both SO₂ and CH₄ along with H₂O, CO₂, and CO. We detect methane with 4.2σ significance at an abundance of 1.0±0.5 ppm, which is depleted by 3 orders of magnitude relative to equilibrium expectations. Our results are highly constraining for the atmosphere and interior, which indicate the envelope has a super-solar metallicity of 43±8× solar, a hot interior with an intrinsic temperature of T_int=460±40 K, and vigorous vertical mixing which depletes CH₄ with a diffusion coefficient of K_zz = 10^11.6±0.1 cm²/s. Photochemistry has a negligible effect on the CH₄ abundance, but is needed to account for the SO₂. We infer a core mass of ${11.5}_{- 3.6}^{+ 3.0}$ M_⊕, which is much higher than previous upper limits¹⁰, releasing a tension with core-accretion models¹¹.