Exoplanetology: Exoplanets & Exomoons

JWST COMPASS: NIRSpec/G395H Transmission Observations of TOI-776 c, a 2 Rearth [Earth radius] M Dwarf Planet

By Keith Cowing
Status Report
astro-ph.EP
March 4, 2025
Filed under , , , , , , , , ,
JWST COMPASS: NIRSpec/G395H Transmission Observations of TOI-776 c, a 2 Rearth [Earth radius] M Dwarf Planet
Masses and radii of well-constrained small planets (0.2 ≤ Mp ≤ 30 M⊕). This catalog is a subset of the NASA Exoplanet Archive showing planets with consistently measured masses (RV or TTV) and measured radii (transit). The points are color-coded by composition: On the left, as bulk water mass fraction (Aguichine et al. 2021) and on the right, as bulk H2/He mass fraction (Lopez & Fortney 2014). Composition lines are shown for an equilibrium temperature of 500 K; rocky and Earth-like predictions are from Zeng et al. (2016). Bold and labeled circle symbols show planets with published JWST transmission spectra, and bold and labeled star symbols show other published COMPASS targets. TOI-776 c (the subject of this paper) is shown in larger font/symbol. — astro-ph.EP

The atmospheres of planets between the size of Earth and Neptune at short orbital periods have been under intense scrutiny.

Of the ~dozen planets in this regime with atmospheres studied so far, a few appear to have prominent molecular features while others appear relatively void of detectable atmospheres.

Further work is therefore needed to understand the atmospheres of these planets, starting with observing a larger sample. To this end, we present the 3-5 micron transmission spectrum of TOI-776 c, a warm (Teq ~420 K), ~2 Rearth, ~7 Mearth planet orbiting an M1V star, measured with JWST NIRSpec/G395H. By combining two visits, we measure a median transit precision of ~18 ppm and ~32 ppm in the NRS1 and NRS2 detectors, respectively.

We compare the transmission spectrum to both non-physical and physical models, and find no strong evidence for molecular features. For cloud-top pressures larger than 10^-3 bar, we rule out atmospheric metallicities less than 180-240x solar (depending on the reduction and modeling technique), which corresponds to a mean molecular weight of ~6-8 g/mol.

However, we find simple atmosphere mixture models (H2O+H2/He or CO2+H2/He) give more pessimistic constraints, and caution that mean molecular weight inferences are model dependent. We compare TOI-776 c to the similar planet TOI-270 d, and discuss possible options for further constraining TOI-776 c’s atmospheric composition.

Overall, we suggest these TOI-776 c observations may represent a combination of planetary and stellar parameters that fall just below the threshold of detectable features in small planet spectra; finding this boundary is one of the main goals of the COMPASS program.

Johanna Teske, Natasha E. Batalha, Nicole L. Wallack, James Kirk, Nicholas F. Wogan, Tyler A. Gordon, Munazza K. Alam, Artyom Aguichine, Angie Wolfgang, Hannah R. Wakeford, Nicholas Scarsdale, Jea Adams Redai, Sarah E. Moran, Mercedes López-Morales, Annabella Meech, Peter Gao, Natalie M. Batalha, Lili Alderson, Anna Gagnebin

Comments: 21 pages, 15 figures, 4 tables. Accepted for publication in AJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2502.20501 [astro-ph.EP] (or arXiv:2502.20501v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2502.20501
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Submission history
From: Johanna Teske
[v1] Thu, 27 Feb 2025 20:19:21 UTC (5,581 KB)
https://arxiv.org/abs/2502.20501
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) 🖖🏻

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