Exoplanetology: Exoplanets & Exomoons

Possible Hycean Conditions In The sub-Neptune TOI-270 d

By Keith Cowing

Status Report

astro-ph.EP

March 7, 2024

Filed under astrochemistry, atmosphere, exoplanet, Hycean exoplanet, JWST, K2-18 b, NIRSpec G395H, Ocean world, Spectroscopy, Sub-Neptunes, super-Earth, TOI-270 d

Possible Hycean Conditions In The sub-Neptune TOI-270 d — Transmission spectrum of TOI-270 d observed with JWST/NIRSpec G395H and HST/WFC3. The NIRSpec G395H spectrum between 2.7 – 5.2 µm, from this work, is shown in dark red, binned to R ≈ 55 for visual clarity. The WFC3 spectrum is shown in orange and spans 1.1 – 1.6 µm, reported by Mikal-Evans et al. (2023). The retrieval is performed using the native resolution NIRSpec G395H spectrum (R ∼ 2700) and the WFC3 spectrum. The NIRSpec spectrum is vertically offset by −84 ppm, corresponding to the median retrieved offset in the canonical one-offset case. The blue curve shows the median retrieved model spectrum, while the medium- and lighter-blue contours denote the 1σ and 2σ intervals, respectively. Yellow points correspond to the median spectrum binned to match the observations. The prominent molecules responsible for the features in different spectral regions are labelled. — astro-ph.EP

The JWST has ushered in a new era in atmospheric characterisations of temperate low-mass exoplanets with recent detections of carbon-bearing molecules in the candidate Hycean world K2-18 b.

We investigated JWST observations of the TOI-270 system, with two sub-Neptunes simultaneously transiting the nearby M dwarf during the visit. We report our atmospheric characterisation of the outer planet TOI-270 d, a candidate Hycean world, with JWST transmission spectroscopy using the NIRSpec G395H instrument in the 2.7-5.2 μm range, combined with previous observations obtained with the HST WFC3 spectrograph (1.1-1.6 μm).

The spectrum reveals strong signatures of CH₄ and CO₂ at 3.8-4.9σ and 2.9-3.9σ confidence, respectively, and no evidence of NH₃. The abundant CH₄ and CO₂, at ~0.1-1% mixing ratios, and the non-detection of NH₃ are similar to the findings reported for K2-18 b and consistent with predictions for a Hycean world with a planet-wide ocean under a H₂-rich atmosphere.

We also report evidence of CS2 at a 2.3-3.0σ confidence and a potential inference of H₂O at 1.6-4.4σ, depending on the data analysis approach, and discuss possible interpretations of these results. The spectrum does not provide strong constraints on the presence of clouds or hazes in the observable atmosphere, nor any evidence for the effects of stellar heterogeneities, which is consistent with previous studies.

For the smaller inner planet TOI-270 b, we find that the spectrum is inconsistent with a featureless spectrum at ~3σ, showing some preference for an H₂-rich atmosphere in a super-Earth. We discuss the implications of our findings and future prospects.

Måns Holmberg, Nikku Madhusudhan

Comments: Accepted for publication in Astronomy & Astrophysics Letters
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2403.03244 [astro-ph.EP] (or arXiv:2403.03244v1 [astro-ph.EP] for this version)
Submission history
From: Madhusudhan Nikku
[v1] Tue, 5 Mar 2024 19:00:00 UTC (5,993 KB)
https://arxiv.org/abs/2403.03244
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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