91̽»¨

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:

Ultrashort-period (USP) exoplanets—with Rp ≤ 2R⊕ and periods ≤1 day—are expected to be stripped of volatile atmospheres by intense host star irradiation, which is corroborated by their nominal bulk densities and previous eclipse observations, consistent with bare-rock surfaces. However, a few USP planets appear anomalously underdense relative to an Earth-like composition, suggesting an exotic interior structure (e.g., coreless) or a volatile-rich secondary atmosphere increasing their apparent radius. Here, we present the first dayside emission spectrum of the low-density (4.3 ± 0.4 g cm−3) USP planet TOI-561 b, which orbits an iron-poor, alpha-rich, ∼10 Gyr old thick-disk star. Our 3–5 μm JWST/NIRSpec observations demonstrate the dayside of TOI-561 b is inconsistent with a bare-rock surface at high statistical significance, suggesting instead a thick volatile envelope that is cooling the dayside to well below the ∼3000 K expected in the bare-rock or thin-atmosphere case. These results reject the popular hypothesis of complete atmospheric desiccation for highly irradiated exoplanets and 91̽»¨ predictions that planetary-scale magma oceans can retain substantial reservoirs of volatiles, opening up the geophysical study of ultrahot super-Earths through the lenses of their atmospheres.

Abstract:

In Titan’s atmosphere, the chemistry of simple hydrocarbons (e.g., CH4 and C2H2) and nitrogen bearing species (e.g., N2 and CN) represents an important link between molecular species and the ubiquitous organic haze that gives Titan its characteristic orange hue. Here we present a new search for two previously undetected molecules, triacetylene (C6H2) and the gas phase dicyanoacetylene (C4N2), using the Echelon-Cross-Echelle Spectrograph instrument on board the Stratospheric Observatory for Infrared Astronomy aircraft. We do not detect these two molecules but determine upper limits for their mixing ratios and column abundances. We find the 3σ upper limits on the uniform volume mixing ratio (VMR) above 100 km for C6H2 to be 4.3 × 10−11, which is lower than the photochemical model predictions. This new upper limit suggests that the growth of linear molecules is inhibited. We also put a strict upper limit on the uniform VMR for gas phase C4N2 above 125 km to be 1.0 × 10−10. This upper limit is well below the saturation mixing ratio at this altitude for C4N2 and greatly limits the feasibility of C4N2 forming ice from condensation.

Abstract:

Atmospheric convection behaves differently in hydrogen-rich atmospheres compared to higher mean molecular weight atmospheres due to compositional gradients of tracers. Previous 1D studies predict that when a condensable tracer exceeds a critical mixing ratio in H2-rich atmospheres, convection is inhibited, leading to the formation of radiative layers where the temperature decreases faster with height than in convective profiles. We use 3D convection-resolving simulations to test whether convection is inhibited in H2-rich atmospheres when the tracer mixing ratio exceeds the critical threshold, while including processes neglected in 1D, e.g., turbulent mixing and evaporation. We run two sets of simulations. First, we perform simulations initialized on saturated isothermal states and find that compositional gradients can destabilize isothermal atmospheres. Second, we perform simulations initialized on adiabatic profiles, which show distinct, stable inhibition layers form when the condensable tracer exceeds the critical threshold. Within the inhibition layer, only a small amount of energy is carried by latent heat flux, and turbulent mixing transports a small amount of tracer upward, but both are generally too weak to sustain substantial tracer or heat transport. The thermal profile gradually relaxes to a steep radiative state, but radiative relaxation timescales are long. Our results suggest stable layers driven by condensation-induced convective inhibition form in H2-rich atmospheres, including those of sub-Neptune exoplanets.

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.