Diversity in the haziness and chemistry of temperate sub-Neptunes

TL;DR

This study reveals substantial diversity among temperate sub-Neptunes by presenting JWST/NIRSpec PRISM transmission spectroscopy of LP 791-18 c, which shows a haze-dominated, methane-rich, metal-enriched atmosphere with no CO$_2$ detection, contrasting with earlier temperate sub-Neptunes that appear relatively clear and CO$_2$-rich. Using SCARLET atmosphere retrievals across free-chemistry and chemically consistent frameworks, the authors infer a metallicity of roughly 250–400× solar and a CH$_4$/CO$_2$ ratio exceeding unity, while also exploring a potential miscible-envelope scenario via coupled SCARLET–VULCAN models. They carefully address instrumental systematics (NIN, saturation), spot-crossing events, and stellar contamination, concluding that hazes plus methane dominate LP 791-18 c’s spectrum and that the atmosphere is not uniquely determined by temperature alone. The results demonstrate intrinsic diversity among sub-Neptunes and highlight the role of aerosols and interior-derived chemistry in shaping observable upper atmospheres, informing formation histories and guiding future JWST observations. $

Abstract

Recent transit observations of K2-18b and TOI-270d revealed strong molecular absorption signatures, lending credence to the idea that temperate sub-Neptunes (T$_\mathrm{eq}$=250-400K) have upper atmospheres mostly free of aerosols. These observations also indicated higher-than-expected CO$_2$ abundances on both planets, implying bulk compositions with high water mass fractions. However, it remains unclear whether these findings hold true for all temperate sub-Neptunes. Here, we present the JWST NIRSpec/PRISM 0.7-5.4$\mathbfμ$m transmission spectrum of a third temperate sub-Neptune, the 2.4R$_\oplus$ planet LP 791-18c (T$_\mathrm{eq}$=355K), which is even more favorable for atmospheric characterization thanks to its small M6 host star. Intriguingly, despite LP 791-18c's radius, mass, and equilibrium temperature being in between those of K2-18b and TOI-270d, we find a drastically different transmission spectrum. While we also detect methane on LP 791-18c, its transit spectrum is dominated by strong haze scattering and there is no discernible CO$_2$ absorption. Overall, we infer a deep metal-enriched atmosphere (246-415$\times$solar) for LP 791-18c, with a CO$_2$-to-CH$_4$ ratio smaller than 0.07 (at 2$σ$), indicating less H$_2$O in the deep envelope of LP 791-18c and implying a relatively dry formation inside the water ice-line. These results show that sub-Neptunes that are near-analogues in density and temperature can show drastically different aerosols and envelope chemistry, and are intrinsically diverse beyond a simple temperature dependence.

Figures (11)

• Figure 1: JWST NIRSpec PRISM broadband and spectrophotometric light-curve fits of the LP 791-18 c transit observation.a. Normalized and systematics-corrected transit observations (points) for the broadband light curve (white) and for a sample of 14 spectroscopic bins (colored), along with their respective best-fit models (dark lines). The multiple light curves are plotted with an offset of 0.03 in relative flux. Each light curve is labelled with the edges of the corresponding spectroscopic bin. The observations are displayed in bins of 80 s for visual purposes, but the full unbinned time array is used for all analyses. b. Residuals from the best-fit model light curves. From the vertical axis of the panels, we observe a variation of the random photometric scatter in the light curves, which is due to the wavelength-dependent stellar photon flux and instrument throughput. c. Histograms of the non-binned residuals divided by the photometric scatter. The residuals follow the expected Gaussian distributions (black curves).
• Figure 1: Extraction of the stellar spectrum for a sample integration.a. Sample detector frame after ramp-fitting and calibration (Stage 2). The blue dashed lines highlight the aperture for the spectral extraction, whereas the red dashed lines highlight the pixels used for the column-by-column background subtraction. The color scale is cut at 20000 electrons so that more of the trace is visible. b. Data quality map of the frame above, with saturated pixels in orange and other bad pixels in white. c. Subtracted background from the 1/$f$ column-by-column correction at the integration level. d. Stellar spectrum of LP 791-18 extracted from the sample integration. The stellar flux is shown in electrons, has neither been flat-fielded nor throughput-corrected, and is shown for both the optimal extraction (orange) and the box extraction (blue).
• Figure 2: JWST NIRSpec/PRISM transmission spectrum of the temperate sub-Neptune LP 791-18 c.a. The transit spectrum of LP 791-18 c (black points with 1$\sigma$ error bars) binned at a resolution of R=25 is shown with our model transmission spectra constraints from the nested sampling free chemistry atmosphere retrieval (blue). The dark blue and light blue shaded regions show the 1$\sigma$ and 2$\sigma$ Bayesian credible intervals from the atmosphere retrieval. The best-fitting model is shown in red and the median of our samples is shown in blue. b. Molecular contributions to the retrieved best-fit transit spectrum of LP 791-18 c. Contributions of CH$_4$, H$_2$O, H$_2$S, SO$_2$, NH$_3$, CO$_2$ and CO are shown in purple, blue, chartreuse, yellow, brown, green and red. The brown region shows the opacity of the aerosols. c. The best-fitting transit spectrum of LP 791-18 c (black) is compared with a K2-18 b-like atmosphere model (green). The CO$_2$-rich model is produced from the best-fit model, to which we add CO$_2$ in order to reach the same CO$_2$/CH$_4$ ratio as for K2-18 b madhusudhan_carbon-bearing_2023. The transit spectrum is characterized by hazes that are opaque at short wavelengths with a fading opacity past 2 $\mu$m, by the methane absorption features at 2.3 and 3.3 $\mu$m, and by the absence of the 4.4 $\mu$m CO$_2$ absorption band.
• Figure 2: Summary diagnostics measurements for the transit observation of LP 791-18 c.a. Median-normalized white light curve of the transit observation of LP 791-18 c. The raw observations are shown in light grey with a binned version in black. b. Measured centre of the trace on the NIRSpec detector (light grey) with a binned version in blue. c. Measured width of the trace on the NIRSpec detector (light grey) with a binned version in orange. d. Measured displacement of the trace in the dispersion direction (light grey) with a binned version in green. e. Width of the cross-correlation peak for the measurement of the displacement in the dispersion direction (light grey) with a binned version in purple. f. Integrated H$\alpha$ flux over the time series (light grey) with a binned version in red. All diagnostics measurements are well-behaved.
• Figure 3: Measured atmospheric properties of LP 791-18 c.a. Retrieved posterior probability distributions for the metallicity of LP 791-18 c's atmosphere based on the suite of chemically consistent retrievals (standard in red, with H$_2$S in purple, with H$_2$S and stellar spots in brown). b-c. Retrieved posterior probability distributions for the abundance of CH$_4$ and the CH$_4$-to-CO$_2$ abundance ratio of LP 791-18 c's atmosphere based on the suite of free retrievals (standard in blue, with stellar spots in orange, and with the centred-log-ratio parameterization in green). d. Retrieved posterior probability distributions for the mean molecular weight $\mu$ of the atmosphere based on all atmosphere retrievals. The mean molecular weights of H$_2$/He (2.3 amu) and pure CH$_4$ (16 amu) atmospheres are shown as grey dashed lines. In all panels, the 1$\sigma$ Bayesian credible regions or 2$\sigma$ lower limits are shown as bold data points and arrows. While some differences exist between the measurements obtained from the different retrieval parameterizations and priors, the suite of retrievals performed on the spectrum of LP 791-18 c all depict methane-rich atmospheres.