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

Authors :
Caroline Piaulet-Ghorayeb
Björn Benneke
Michael Radica
Eshan Raul
Louis-Philippe Coulombe
Eva-Maria Ahrer
Daria Kubyshkina
Ward S. Howard
Joshua Krissansen-Totton
Ryan J. 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 Lafrenière
Lorena Acuña
Thaddeus Komacek
Hamish Innes
Thomas G. Beatty
Ryan Cloutier
René Doyon
Anna Gagnebin
Cyril Gapp
Heather A. Knutson
Source :
The Astrophysical Journal Letters, Vol 974, Iss 1, p L10 (2024)
Publication Year :
2024
Publisher :
IOP Publishing, 2024.

Abstract

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 (2.5–4 R _⊕ ) has revealed hydrogen-dominated atmospheres, smaller sub-Neptunes (1.6–2.5 R _⊕ ) 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 0.6–2.8 μ m JWST/NIRISS/SOSS transmission spectrum of GJ 9827 d, the smallest (1.98 R _⊕ ) warm ( T _eq,A=0.3 ∼ 620 K) 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 H _2 O 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 10 ^30 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.

Details

Language :
English
ISSN :
20418213 and 20418205
Volume :
974
Issue :
1
Database :
Directory of Open Access Journals
Journal :
The Astrophysical Journal Letters
Publication Type :
Academic Journal
Accession number :
edsdoj.bafb1eec668c4c80b09ceaf693c9125e
Document Type :
article
Full Text :
https://doi.org/10.3847/2041-8213/ad6f00