Heat Reveals What Clouds Conceal: Global Carbon & Longitudinally Asymmetric Chemistry on LTT 9779 b

TL;DR

This study probes the atmosphere of LTT 9779 b, an ultra-hot Neptune residing in the Neptune desert, by applying JWST/NIRSpec G395H phase-curve spectroscopy to a full orbital cycle. Using POSEIDON retrievals and an energy-balance framework, the authors find a globally carbon-rich, high-metallicity atmosphere with CO2 as a dominant absorber, CO present but less well constrained, and H2O abundant on the dayside but largely obscured by nightside clouds; C/O is effectively unity and Fe/H exceeds 500× solar. The data reveal strong longitudinal cloud and chemical variation, including tentative SO2 on the western nightside, consistent with photochemical processing under intense irradiation. Energy-budget analysis shows modest heat recirculation with a Bond albedo around 0.29, implying limited day-to-night energy transport and supporting the presence of high-altitude clouds that modulate both emission and reflective properties. Collectively, the results demonstrate that LTT 9779 b can retain a carbon-rich atmosphere under extreme irradiation, offering critical constraints on atmospheric escape, cloud physics, and planetary formation scenarios near the hot Neptune desert.

Abstract

LTT-9779 b is an ultra-hot Neptune (Rp ~ 4.7 Re, Mp ~ 29 Me) orbiting its Sun-like host star in just 19 hours, placing it deep within the "hot Neptune desert," where Neptunian planets are seldom found. We present new JWST NIRSpec G395H phase-curve observations that probe its atmospheric composition in unprecedented detail. At near-infrared wavelengths, which penetrate the high-altitude clouds inferred from previous NIRISS/SOSS spectra, thermal emission reveals a carbon-rich atmosphere with opacity dominated by carbon monoxide (CO) and carbon dioxide (CO2). Both species are detected at all orbital phases, with retrieved mixing ratios of 10^-1 for CO and 10^-4 for CO2, indicating a globally well-mixed reservoir of carbon-bearing gases. We also moderately detect water vapor (H2O) and tentatively detect sulfur dioxide (SO2), providing insight into its chemistry and possible photochemical production under intense stellar irradiation. From these detections we infer a carbon-to-oxygen ratio near unity (C/O ~ 1) and a metallicity exceeding 500X Solar, consistent with equilibrium chemistry predictions for high-temperature atmospheres. This enrichment raises the mean molecular weight, reducing atmospheric escape, and likely helps LTT-9779 b retain a substantial atmosphere despite extreme irradiation. Our findings show that LTT-9779 b survives where few planets can, maintaining a carbon-rich atmosphere in a region where hot Neptune-class worlds are expected to evaporate. This makes LTT-9779 b a valuable laboratory for studying atmospheric escape and chemical processes under extreme conditions, offering new insight into the survival of planets in the hot Neptune desert.

Figures (15)

• Figure 1: Broadband light curves, phase curve models, and residuals for the Eureka! (v1) data reductions. NRS1 and NRS 2 detector data is shown on top and bottom. While NRS1 data demonstrated a significant ramp for both v1 and v2 datasets, systematic models were able to compensate and fit lightcurve data adequately.
• Figure 2: Emission spectra and best-fit free retrieved models for all six phases. The spectral contributions of CO, CO$_2$, H$_2$O and SO$_2$ abundances are shown in green, gold, blue and orange for each phase. The best-fit, full atmospheric models for the phase-resolved emission spectra are shown in black.
• Figure 3: Retrieved atmospheric P-T profiles and spectral pressure contributions for all six phases. Pressure temperature profiles are shown in purple, corresponding to the y-axes and bottom x-axes of each subplot. Spectral contributions are shown in green, corresponding to the y-axes and top x-axes of each subplot. Sharing a pressure axes, we are able to analyze how deep into the atmosphere we are probing at each phase. Notably, the intermediary phases present a lower-pressure depth during emission spectroscopy, potentially indicating high-altitude clouds.
• Figure 4: Retrieved CO, CO$_2$, H$_2$O and SO$_2$ abundances for all six orbital phases. Carbon-bearing species remain prominent through-out all phases. H$_2$O is detected in the Eastern Dayside and Dayside, but apparently absent elsewhere. Given a lack of evidence for thermal dissociation, the H$_2$O is still likely present in the atmosphere, but possibly concealed by higher altitude clouds as explained in \ref{['fig:Pressure_Contribution']}. SO$_2$ is tentatively present in the Western Nightside of the planet, hinting at possible photochemistry.
• Figure 5: Captured flux fraction estimates of LTT 9779 b assuming blackbody emission. The red and blue boxes show the calculated effective temperature range of the planet using NIRSpec and NIRSpec+NIRISS, respectively. Using the same formulation as Splinter2025, we estimate the captured flux fraction as the ratio of an instrument's band-integrated blackbody emission to the planet's bolometric emission. Black dashed lines indicate the captured flux fraction boundaries within the effective temperatures. NIRSpec/G395H captures $\sim$∼$$16–38% of the bolometric flux while the combination of NIRSpec and NIRISS captures $\sim$∼$$71–93% of the bolometric flux.