Exoplanetology: Exoplanets & Exomoons

TOI-421 b: A Hot Sub-Neptune with a Haze-Free, Low Mean Molecular Weight Atmosphere

By Keith Cowing
Status Report
astro-ph.EP
January 10, 2025
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TOI-421 b: A Hot Sub-Neptune with a Haze-Free, Low Mean Molecular Weight Atmosphere
Comparison of the combined exoTEDRF + Eureka! transmission spectrum to Aurora retrievals (top) and various PLATON forward models (bottom). The best-fit retrieval model is shown in the top panel with the darkness of the shading indicating the 1- and 2-σ limits, and is re-plotted in the bottom panel for comparison against the other forward models. The middle panel shows the absorption cross-sections for key molecules, which demonstrates how the spectrum is a poor fit both for CH4 and CO2. The plotted forward models include different metallicity and C/O scenarios, as well as a water-rich composition, and the inclusion of disequilibrium SO2. Forward models incorporating equilibrium chemistry are shown with dashed lines. The ‘10× Solar + SO2’ model has 108 times the equilibrium SO2 abundance as its counterpart. Offsets have been applied to the forward models at 2.8 µm to account for the best-fit offset between the NIRISS/SOSS and NIRSpec/G395M data. — astro-ph.EP

Common features of sub-Neptunes atmospheres observed to date include signatures of aerosols at moderate equilibrium temperatures (~500-800 K), and a prevalence of high mean molecular weight atmospheres, perhaps indicating novel classes of planets such as water worlds.

Here we present a 0.83-5 micron JWST transmission spectrum of the sub-Neptune TOI-421 b. This planet is unique among previously observed counterparts in its high equilibrium temperature (Teq≈920) and its Sun-like host star.

We find marked differences between the atmosphere of TOI-421 b and those of sub-Neptunes previously characterized with JWST, which all orbit M stars. Specifically, water features in the NIRISS/SOSS bandpass indicate a low mean molecular weight atmosphere consistent with solar metallicity, and no appreciable aerosol coverage. Hints of SO2 and CO (but not CO2 or CH4) also exist in our NIRSpec/G395M observations, but not at sufficient signal-to-noise to draw firm conclusions.

Our results support a picture in which sub-Neptunes hotter than ~850 K do not form hydrocarbon hazes due to a lack of methane to photolyze. TOI-421 b additionally fits the paradigm of the radius valley for planets orbiting FGK stars being sculpted by mass loss processes, which would leave behind primordial atmospheres overlying rock/iron interiors.

Further observations of TOI-421 b and similar hot sub-Neptunes will confirm whether haze-free atmospheres and low mean molecular weights are universal characteristics of such objects.

Brian Davenport, Eliza M.-R. Kempton, Matthew C. Nixon, Jegug Ih, Drake Deming, Guangwei Fu, E. M. May, Jacob L. Bean, Peter Gao, Leslie Rogers, Matej Malik

Comments: Submitted to ApJ Letters, comments welcome
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2501.01498 [astro-ph.EP] (or arXiv:2501.01498v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2501.01498
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Submission history
From: Eliza M.-R. Kempton
[v1] Thu, 2 Jan 2025 19:00:18 UTC (2,944 KB)
https://arxiv.org/abs/2501.01498
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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