A JWST Transmission Spectrum of the Temperate Sub-Neptune TOI-732 c

TL;DR

This study presents the first JWST transmission spectrum of TOI-732 c, a temperate sub-Neptune, across 0.9–12 μm to probe atmospheric composition and interior structure. Using NIRISS, NIRSpec, and MIRI data with two retrieval frameworks (AURA and POSEIDON), the authors robustly detect CH$_4$ in a hydrogen-rich atmosphere and place stringent upper limits on NH$_3$ and HCN, while uncovering evidence for excess absorption from one or more complex trace species. A comprehensive survey of up to 250 molecules reveals moderate to strong evidence for several candidate absorbers, with two compounds—isobutylene and 1-pentene—showing consistent, albeit degenerate, signals in both NIR and MIR analyses. Clouds/hazes at the terminator are inferred, and a photospheric temperature around ~310 K at 10 mbar is retrieved, suggesting a cold trap is possible but not required. The results illuminate the degeneracies between atmospheric composition and interior structure for temperate sub-Neptunes and motivate further observations and theoretical work to robustly characterize such worlds and their potential habitability.

Abstract

In recent years, JWST has facilitated detections of carbon-bearing molecules in the atmospheres of temperate sub-Neptunes orbiting M dwarfs, ushering in a new era in the characterization of this intriguing planetary regime. We report the transmission spectrum of the temperate sub-Neptune TOI-732 c, observed with JWST NIRISS, NIRSpec G395H and MIRI LRS between 0.9-12 $μ$m. The observations provide evidence for methane (CH$_4$) in a H$_2$-rich atmosphere, at a volume mixing ratio of $\sim$1\%, and non-detection of NH$_3$ and HCN, along with nominal constraints on other prominent molecules H$_2$O, CO and CO$_2$, which are typically expected in H$_2$-rich atmospheres. We conduct a comprehensive survey of 250 chemical species and find moderate to strong evidence (up to $\ln B\sim 5.9$, $3.9σ$) for additional absorption due to one or more complex molecules including higher-order hydrocarbons and/or sulfur-bearing molecules. The spectral features are strongly degenerate among these molecules and with methane, which we find at $\ln B=3.2-8.8$ (up to $3.0-4.6$$σ$) significance. Two complex molecules are preferred with at least moderate evidence ($\ln B \gtrsim 2.5$) in both the near- and mid-infrared, while several others show such evidence in at least one of the two wavelength ranges. The preferred molecules are found in trace quantities on Earth, with no significant sources identified in other planetary atmospheres, requiring future work to assess their physical plausibility in this planet. Future observations are required to resolve the degeneracies and place more robust constraints on these species. We highlight the need for further theoretical and experimental work to robustly characterize the atmospheric and internal composition of TOI-732 c and similar sub-Neptunes.

Figures (5)

• Figure 1: White light curves for the primary transit of TOI-732 c obtained with the three different instruments: NIRISS SOSS (top), NIRSpec G395H (middle) and MIRI LRS (bottom). In each case, the white light curve obtained using the JExoRES pipeline is shown along with the residuals from subtracting a median model fit. For NIRSpec, the combined white light curve from the two detectors NRS1 and NRS2 is shown, while for NIRISS we show the white light curve for Order 1. The standard deviation of the residuals for NIRISS, NIRSpec and are MIRI are 138 ppm, 119 ppm and 547 ppm, respectively. These are $\sim$2.2, $\sim$1.2, and $\sim$1.3 times the expected level, respectively. The white light curves for NIRISS and NIRSpec show planet crossing of stellar heterogeneities during transit, as discussed in section \ref{['sec:obs']}.
• Figure 2: The transmission spectrum of TOI-732 c in the NIR. We show the spectrum and retrieved model fits for the one-offset retrieval, as discussed in Section \ref{['sec:finalretrieval']}. This model includes the 6 key CNO molecules (CH$_4$, CO$_2$, CO, H$_2$O, NH$_3$ and HCN) and the 2 species found to exhibit moderate preference across our retrieval cases, referred to as X, as discussed in Section \ref{['sec:tracespecies']}. The data in orange and red show the NIRISS and NIRSpec spectra respectively, covering 0.9-2.8 $\mu$m and 2.8-5.2 $\mu$m. The retrievals were carried out using the native resolution data. For visual clarity, these are shown binned to resolutions of $R\approx25$ and $R\approx55$ for the NIRISS and NIRSpec data respectively. The NIRSpec spectrum has been vertically offset by the median retrieved value for the one-offset retrieval, by $\approx 112$ ppm. In blue we show the median retrieved spectrum, with the lighter blue contours indicating the $1\sigma$ and $2\sigma$ intervals. The points in yellow show this median spectrum binned to the resolution of the observations.
• Figure 3: Contributions of several molecular species to the transmission spectrum in the 0.9-5.2 $\mu$m range (top) and 5-12 $\mu$m range (bottom). The individual contributions from each molecule is shown. This nominal model assumes an isothermal P-T profile at 350 K, and mixing ratios consistent with our retrievals: 10$^{-2}$ for CH$_4$, 10$^{-4}$ for all the other species. The contributions for each molecule shows the transmission spectrum from this molecule, in addition to collision-induced absorption of H$_2$-H$_2$ and H$_2$-He. The molecular species shown are those which exhibit moderate evidence across at least three of our four retrieval cases, as shown in Table \ref{['tab:poseidonevidence']}.
• Figure 4: The MIRI JexoPipe and JExoRES data with retrieved spectral fits to the JexoPipe data. The spectral fits, represented by the solid curves, show the median retrieved spectra, obtained with POSEIDON retrievals on the MIRI JexoPipe data. In this set-up, the model includes the 4 baseline molecules (CH$_4$, CO$_2$, CO, H$_2$O) in addition to, in turn, either of the 2 molecules showing moderate evidence across all retrieval cases, 1-pentene and isobutylene.
• Figure 5: The posterior probability distributions retrieved for the mixing ratios of key molecules for the retrieval cases described in Section \ref{['sec:finalretrieval']}. Horizontal error bars denote the median and corresponding 1$\sigma$ interval for the molecules with significant evidence while the arrows denote the 2$\sigma$ upper limits. "6+4X" is the model described in Section \ref{['sec:finalretrieval']}, consisting of the 6 key CNO molecules (CH$_4$, CO$_2$, CO, H$_2$O, NH$_3$, and HCN) and the 4 chemical species X that exhibit moderate preference across the retrieval cases in this work (1-pentene, isobutylene, diisobutylene, and diethyl sulfide). "4+4X" is the baseline model (CH$_4$, CO$_2$, CO, H$_2$O) used in Section \ref{['sec:tracespecies']} along with the 4 moderate-preference chemical species. The remaining models ("4+X") show the baseline model with one of isobutylene and 1-pentene. All models consider one offset between the NIRISS and NIRSpec data. Evidence for CH$_4$ is found at a significance of $\ln B \sim 3.1$ ($\sim$3$\sigma$). Furthermore, evidence for excess absorption beyond the 6 expected CNO species is also present in the near infrared. As discussed in Section \ref{['sec:tracespecies']}, this may be due to a number of degenerate species. These include isobutylene and 1-pentene, which we include here. As shown, while each of these species presents a distinct peak in the respective "4+X" model, only one (isobutylene, in this case) does when multiple complex species are considered at once. The abundance estimates and detection significances are shown in Table \ref{['tab:abundances']}.