Exoplanets & Exomoons

JWST Reveals CH4, CO2, and H2O in a Metal-rich Miscible Atmosphere on a Two-Earth-Radius Exoplanet

By Keith Cowing
Status Report
March 7, 2024
Filed under , , , , , , , ,
JWST Reveals CH4, CO2, and H2O in a Metal-rich Miscible Atmosphere on a Two-Earth-Radius Exoplanet
Broadband and spectroscopic light-curve fits of the NIRISS/SOSS (left) and NIRSpec/G395H (right) transits of TOI-270 d. For both instruments, examples of 10 normalized and systematics-corrected light curves are shown (colored points) along with their best-fitting transit models (black lines) and labelled with the central wavelength of the spectroscopic channel. Broadband light-curve fits (white) are also shown at the top for Order 1 in the NIRISS/SOSS case, and for NRS1 in the NIRSpec/G395H case. For clarity, the spectroscopic light curves are plotted with a relative flux offset. For NIRSpec/G395H, light curves with wavelengths below 3.8 µm are from the NRS1 detector and the remaining from NRS2. For visual purposes, the light curves are also binned in time in groups of 10 (NIRISS/SOSS) and 20 (NIRSpec/G395H) data points, resulting in 3.6 minute bins. A full transit of planet b occurs at the start of the NIRSpec/G395H exposure, with both transits overlapping shortly as planet d starts transiting. All light curves are well-behaved, with no signs of spot-crossing events or Hα flares. — astro-ph.EP

Even though sub-Neptunes likely represent the most common outcome of planet formation, their natures remain poorly understood.

In particular, planets near 1.5-2.5R often have bulk densities that can be explained equally well with widely different compositions and interior structures, resulting in grossly divergent implications for their formation.

Here, we present the full 0.6-5.2μm JWST NIRISS/SOSS+NIRSpec/G395H transmission spectrum of the 2.2R TOI-270d (4.78M, Teq=350-380 K), delivering unprecedented sensitivity for atmospheric characterization in the sub-Neptune regime. We detect five vibrational bands of CH4 at 1.15, 1.4, 1.7, 2.3, and 3.3μm (9.4σ), the signature of CO2 at 4.3μm (4.8σ), water vapor (2.5σ), and potential signatures of SO2 at 4.0μm and CS2 at 4.6μm. Intriguingly, we find an overall highly metal-rich atmosphere, with a mean molecular weight of 5.47+1.25−1.14.

We infer an atmospheric metal mass fraction of 58+8−12% and a C/O of 0.47+0.16−0.19, indicating that approximately half the mass of the outer envelope is in high-molecular-weight volatiles (H2O, CH4, CO, CO2) rather than H2/He. We introduce a sub-Neptune classification scheme and identify TOI-270d as a “miscible-envelope sub-Neptune” in which H2/He is well-mixed with the high-molecular-weight volatiles in a miscible supercritical metal-rich envelope.

For a fully miscible envelope, we conclude that TOI-270d’s interior is 90+3−4wt% rock/iron, indicating that it formed as a rocky planet that accreted a few wt % of H2/He, with the overall envelope metal content explained by magma-ocean/envelope reactions without the need for significant ice accretion. TOI-270d may well be an archetype of the overall population of sub-Neptunes.

Björn Benneke, Pierre-Alexis Roy, Louis-Philippe Coulombe, Michael Radica, Caroline Piaulet, Eva-Maria Ahrer, Raymond Pierrehumbert, Joshua Krissansen-Totton, Hilke E. Schlichting, Renyu Hu, Jeehyun Yang, Duncan Christie, Daniel Thorngren, Edward D. Young, Stefan Pelletier, Heather A. Knutson, Yamila Miguel, Thomas M. Evans-Soma, Caroline Dorn, Anna Gagnebin, Jonathan J. Fortney, Thaddeus Komacek, Ryan MacDonald, Eshan Raul, Ryan Cloutier, Lorena Acuna, David Lafrenière, Charles Cadieux, René Doyon, Luis Welbanks, Romain Allart

Comments: 25 pages, 12 figures
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2403.03325 [astro-ph.EP] (or arXiv:2403.03325v1 [astro-ph.EP] for this version)
Submission history
From: Björn Benneke
[v1] Tue, 5 Mar 2024 21:07:43 UTC (11,681 KB)

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻