Possible Hycean conditions in the sub-Neptune TOI-270 d

Abstract

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.

Multiple Clues for Dayside Aerosols and Temperature Gradients in WASP-69 b from a Panchromatic JWST Emission Spectrum

“…Recently, methane has been definitively detected for the first time via low-resolution, space-based spectroscopy with JWST, which opens up this molecule as a tracer of dynamical and chemical processes in exoplanets. Methane was found in the moderately irradiated Jupiter WASP-80 b (T eq = 825 K; Bell et al 2023b) as well as the lowertemperature sub-Neptunes K2-18 b (T eq = 270 K; Madhusudhan et al 2023) and TOI-270 d (T eq = 354 K; Benneke et al 2024;Holmberg & Madhusudhan 2024). The moderately irradiated Jupiter HAT-P-18 b (T eq = 850 K) may also have methane, as found by retrieval modeling, but there are no strong, high signal-to-noise ratio absorption features in the SOSS bandpass from 0.6 to 2.8 μm (Fu et al 2022) and a subsequent reanalysis did not find evidence for methane in HAT-P-18 b with the same data (Fournier-Tondreau et al 2024).…”

Accurate Infrared Line Lists for 20 Isotopologues of CS2 at Room Temperature

“…Even more recently, a new global effective Hamiltonian model (Tashkun 2022) was published for 12 C 32 S 2 , the main isotopologue. The addition of this molecule to HITRAN has resulted in its first detection in the atmosphere of Venus (Mahieux et al 2023) and the sub-Neptune exoplanet TOI-270 d (Holmberg & Madhusudhan 2024) and has enabled more CS 2 combustion studies (Peng et al 2022).…”

Sulfur Dioxide and Other Molecular Species in the Atmosphere of the Sub-Neptune GJ 3470 b

“…Existing observations of these atmospheres from the Hubble Space Telescope (HST) provide some water abundance measurements (Morley et al 2017;Wakeford et al 2017;Benneke et al 2019b), but the difficulty in making these measurements and the lack of detections of carbon-bearing molecules has made comparing to planet formation theories difficult (Welbanks et al 2019). Recent JWST observations of K2-18 b (Madhusudhan et al 2023) and TOI-270 d (Benneke et al 2024;Holmberg & Madhusudhan 2024) have begun to reveal spectra with strong molecular features in this regime, though many other sub-Neptunes have appeared featureless (Lustig-Yaeger et al 2023;May et al 2023;Kirk et al 2024).…”