Atmospheres & Climate

JWST/NIRISS Reveals The Water-rich Steam World Atmosphere of GJ 9827 d

By Keith Cowing

Status Report

astro-ph.EP

October 7, 2024

Filed under astro-ph.EP, exoplanet, GJ 9827, GJ 9827 d, HST/WFC3, ice giant, imaging, JWST, NIRISS/SOSS, spectorscopy, steam world, sub-Neptune

JWST/NIRISS Reveals The Water-rich Steam World Atmosphere of GJ 9827 d — Top panels: Results from the white light curve fit of the supreme-SPOON reduction for Visits 1 (left) and 2 (right), and fitted systematics models. The order 1 systematics-corrected light-curves are shown (teal points), binned for visual purposes, along with the best-fit transit model (black) and the systematics model including the GP component (red, with the GP scaled for visualization purposes). The vertical blue shaded regions indicate the three candidate flares with the highest signal-to-noise spectra and following typical flare evolution profiles (see Section C). Bottom panels: Spectroscopic light-curve fits to the two NIRISS/SOSS transits of GJ 9827 d from the supreme-SPOON reduction. The left panel corresponds to the first visit and the right panel to the second visit. We show the detrended broadband light-curve for order 2, and seven spectroscopic light-curves from order 1, from top to bottom. The best-fit astrophysical model is shown for each light-curve (black) and light-curves are offset relative to each other for clarity. — astro-ph.EP

With sizable volatile envelopes but smaller radii than the solar system ice giants, sub-Neptunes have been revealed as one of the most common types of planet in the galaxy.

While the spectroscopic characterization of larger sub-Neptunes (2.5-4R_⊕) has revealed hydrogen-dominated atmospheres, smaller sub-Neptunes (1.6–2.5R_⊕) could either host thin, rapidly evaporating hydrogen-rich atmospheres or be stable metal-rich “water worlds” with high mean molecular weight atmospheres and a fundamentally different formation and evolutionary history.

Here, we present the 0.6–2.8μm JWST NIRISS/SOSS transmission spectrum of GJ 9827 d, the smallest (1.98 R_⊕) warm (T_eq,AB=0.3∼620K) sub-Neptune where atmospheric absorbers have been detected to date. Our two transit observations with NIRISS/SOSS, combined with the existing HST/WFC3 spectrum, enable us to break the clouds-metallicity degeneracy. We detect water in a highly metal-enriched “steam world” atmosphere (O/H of ∼4 by mass and H2O found to be the background gas with a volume mixing ratio of >31%).

We further show that these results are robust to stellar contamination through the transit light source effect. We do not detect escaping metastable He, which, combined with previous nondetections of escaping He and H, supports the steam atmosphere scenario. In water-rich atmospheres, hydrogen loss driven by water photolysis happens predominantly in the ionized form which eludes observational constraints.

We also detect several flares in the NIRISS/SOSS light-curves with far-UV energies of the order of 1030 erg, highlighting the active nature of the star. Further atmospheric characterization of GJ 9827 d probing carbon or sulfur species could reveal the origin of its high metal enrichment.

Caroline Piaulet-Ghorayeb, Bjorn Benneke, Michael Radica, Eshan Raul, Louis-Philippe Coulombe, Eva-Maria Ahrer, Daria Kubyshkina, Ward S. Howard, Joshua Krissansen-Totton, Ryan MacDonald, Pierre-Alexis Roy, Amy Louca, Duncan Christie, Marylou Fournier-Tondreau, Romain Allart, Yamila Miguel, Hilke E. Schlichting, Luis Welbanks, Charles Cadieux, Caroline Dorn, Thomas M. Evans-Soma, Jonathan J. Fortney, Raymond Pierrehumbert, David Lafreniere, Lorena Acuna, Thaddeus Komacek, Hamish Innes, Thomas G. Beatty, Ryan Cloutier, Rene Doyon, Anna Gagnebin, Cyril Gapp, Heather A. Knutson

Comments: 37 pages, 18 figures, Accepted for publication in ApJL
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2410.03527 [astro-ph.EP] (or arXiv:2410.03527v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2410.03527
Focus to learn more
Related DOI:
https://doi.org/10.3847/2041-8213/ad6f00
Focus to learn more
Submission history
From: Caroline Piaulet
[v1] Fri, 4 Oct 2024 15:48:22 UTC (4,656 KB)
https://arxiv.org/abs/2410.03527

Astrobiology

Keith Cowing

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) 🖖🏻

Follow on Twitter