Dr Jake Taylor (he/him)

Abstract:

Hot Jupiters have temperature gradients of several hundreds of degrees between their permanent daysides and nightsides. Such a strong gradient creates winds with speeds of the order of kilometres per second, which advect chemical species over the whole planet. When this transport is faster than the time needed for chemical species to react, it holds back the chemical equilibration of the atmospheric carbon reservoir, which would otherwise transition from CO on the dayside to CH4 on the nightside. Direct evidence of this process has remained elusive so far, as it is often degenerate with other atmospheric processes, such as vertical mixing or non-stellar elemental abundances. Here we present observational evidence for such a fast day-to-night horizontal transport of chemical species by observing the full 18-h orbit of the exoplanet NGTS-10A b with the JWST/NIRSpec instrument. We show that the carbon chemistry is dominated by CO in both the dayside and the nightside of the planet, with a strong depletion of CH4 on the nightside compared with expectations from chemical equilibrium. By measuring the atmospheric abundances of all the main carbon and oxygen molecules, we further demonstrate that the lack of CH4 on the planetary nightside cannot be attributed to non-solar elemental abundances or to vertical mixing mechanisms and must, therefore, be due to fast horizontal transport. Our study shows the fundamental role that atmospheric transport plays in shaping the distribution of chemical species on exoplanet atmospheres.

Abstract:

With its expanded wavelength coverage and increased precision compared to previous space-based observatories, JWST provides the opportunity to revisit benchmark planets and view them in a new light. Here, we conduct an in-depth study of the atmosphere of the hot-Saturn WASP-96 b combining a new JWST NIRSpec/G395H transit with archival NIRISS/SOSS and Very Large Telescope/FORS2 transmission spectra. The combined spectrum shows clearly visible features from H2O, CO2, and Na. CO, though, remains unconstrained, precluding a firm metallicity derivation from free retrievals alone. However, self-consistent grids yield a broadly superstellar atmospheric metallicity of 2–6× stellar. When combined with a roughly stellar C/O ratio ( 0.41−0.09+0.10 from self-consistent grids), we find that WASP-96 b potentially formed via core-accretion beyond the H2O snowline and subsequently accreted volatile-rich material. Free retrievals also find a moderate preference ( lnB = 2.69) for models with SO2 versus without. WASP-96 b falls directly on the proposed “SO2 shoreline” and the retrieved SO2 abundance is well-matched to predictions from photochemical models. Our combined spectrum displays an optical slope, which our models fit with opacity from scattering aerosols—either small-particle condensate clouds or photochemical hazes—though we cannot completely rule out the broad wings of Na or the effects of stellar contamination. Future observations are necessary to disentangle these effects. Finally, we explore the possibility for limb asymmetry in WASP-96 b’s transmission spectrum and provide several tests to identify asymmetries in our data. We encourage the community to prioritize the development of a robust pathway to quantify the presence of limb asymmetry—particularly for low signal-to-noise cases.

Abstract:

Recent transit observations of K2-18 b and TOI-270 d revealed strong molecular absorption signatures, lending credence to the idea that temperate sub-Neptunes (equilibrium temperature Teq = 250–400 K) have upper atmospheres mostly free of aerosols. These observations also indicated higher-than-expected CO2 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-μm transmission spectrum of a third temperate sub-Neptune, the 2.4 R⊕ planet LP 791-18 c (Teq = 355 K), which is even more favourable for atmospheric characterization thanks to its small M6 host star. Intriguingly, despite the radius, mass and equilibrium temperature of LP 791-18 c being between those of K2-18 b and TOI-270 d, we find a drastically different transmission spectrum. Although we also detect methane on LP 791-18 c, its transit spectrum is dominated by strong haze scattering and there is no discernible CO2 absorption. Overall, we infer a deep metal-enriched atmosphere (246–415 times solar) for LP 791-18 c, with a CO2-to-CH4 ratio smaller than 0.07 (at 2σ), indicating less H2O in the deep envelope of LP 791-18 c 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.

Abstract:

Determining the prevalence of atmospheres on terrestrial planets is a core objective in exoplanetary science. While M dwarf systems offer a promising opportunity, conclusive observations of terrestrial atmospheres have remained elusive, with many yielding flat transmission spectra. We observe four transits of the hot terrestrial planet TOI-1685 b using James Webb Space Telescope (JWST)’s Near Infrared Spectrograph (NIRSpec) G395H instrument. Combining this with the transit from the previously observed phase curve of the planet with the same instrument, we perform a detailed analysis to determine the possibility of an atmosphere on TOI-1685 b. From our retrievals, the Bayesian evidence favours a simple flat line model, indicating no evidence for an atmosphere on TOI-1685 b, in line with results from the phase curve analysis. Our results show that hydrogen-dominated atmospheres can be confidently ruled out. For heavier, secondary atmospheres we find a lower limit on the mean molecular weight of , at a significance of ~5σ. Pure , , , and atmospheres, or a mixed secondary atmosphere () could explain the data (). However, pure atmospheres may be physically unlikely, and the pure and cases require a high-altitude cloud, which could also be interpreted as a thin cloud-free atmosphere. We discuss the theoretical possibility for different types of atmosphere on this planet, and consider the effects of atmospheric escape and stellar activity on the system. Though we find that TOI-1685 b is likely a bare rock, this study also highlights the challenges of detecting secondary atmospheres on rocky planets with JWST.

Abstract:

The formation and evolution of giant planets remain incompletely understood, with mounting evidence that many close-in giants may have migrated from their birth locations. The detection of helium escaping the atmosphere of exoplanets has provided a powerful new tracer of atmospheric escape and exoplanetary evolution. Here, using high-precision spectroscopic observations from the James Webb Space Telescope (JWST) Near Infrared Imager and Slitless Spectrograph (NIRISS) in single-object slitless spectroscopy mode (SOSS) mode, we report the detection of substantial helium absorption during the pre-transit phase of WASP-107 b (17σ), as well as in the transit and post-transit phases. This unique continuous helium absorption begins approximately 1.5 h before the planet’s ingress and reveals the presence of an extended thermosphere. The observations show a maximum transit depth of 2.395 ± 0.01% near the helium triplet (36σ; at the NIRISS-SOSS resolution of ~700). Our ellipsoidal model of the planetary thermosphere matches the measured light curve well, suggesting an outflow extending to tens of planetary radii. Furthermore, we confidently detect water absorption (log10H2O = −2.5 ± 0.6), superimposed with a short-wavelength slope that we attribute to a prominent signature from unocculted stellar spots (5.2σ), rather than a small-particle haze slope. We place an upper limit on the abundance of K (log10K < −4.86, or K/H < 75× stellar) at 2σ, which is consistent with the O/H supersolar metallicity estimate. Together with the supersolar water abundance and the evidence for vigorous atmospheric escape, these findings suggest that WASP-107 b has undergone inward migration in its recent past, probably accompanied by strong tidal heating that continues to sustain its inflated atmosphere and mass loss. This investigation underscores the transformative potential of JWST for investigating planetary evolution.