TOI-421 b: A Hot Sub-Neptune with a Haze-Free, Low Mean Molecular Weight Atmosphere
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| Main Authors: | , , , , , , , , , , |
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| Format: | Preprint |
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2025
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| _version_ | 1866929656985288704 |
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| author | Davenport, Brian Kempton, Eliza M. -R. Nixon, Matthew C. Ih, Jegug Deming, Drake Fu, Guangwei May, E. M. Bean, Jacob L. Gao, Peter Rogers, Leslie Malik, Matej |
| author_facet | Davenport, Brian Kempton, Eliza M. -R. Nixon, Matthew C. Ih, Jegug Deming, Drake Fu, Guangwei May, E. M. Bean, Jacob L. Gao, Peter Rogers, Leslie Malik, Matej |
| contents | 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 ($T_{eq} \approx 920$) 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 SO$_2$ and CO (but not CO$_2$ or CH$_4$) 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. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2501_01498 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | TOI-421 b: A Hot Sub-Neptune with a Haze-Free, Low Mean Molecular Weight Atmosphere Davenport, Brian Kempton, Eliza M. -R. Nixon, Matthew C. Ih, Jegug Deming, Drake Fu, Guangwei May, E. M. Bean, Jacob L. Gao, Peter Rogers, Leslie Malik, Matej Earth and Planetary Astrophysics 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 ($T_{eq} \approx 920$) 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 SO$_2$ and CO (but not CO$_2$ or CH$_4$) 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. |
| title | TOI-421 b: A Hot Sub-Neptune with a Haze-Free, Low Mean Molecular Weight Atmosphere |
| topic | Earth and Planetary Astrophysics |
| url | https://arxiv.org/abs/2501.01498 |