Exoplanet atmospheres

Abstract:

Attempts to probe the atmospheres of rocky planets around M dwarfs present both promise and peril. While their favorable planet-to-star radius ratios enable searches for even thin secondary atmospheres, their high activity levels and high-energy outputs threaten atmosphere survival. Here we present the 0.6–2.85 μm transmission spectrum of the 1.1 R⊕, ∼ 340 K rocky planet TRAPPIST-1 c obtained over two JWST NIRISS/SOSS transit observations. Each of the two spectra displays 100–500 ppm signatures of stellar contamination. Despite being separated by 367 days, the retrieved spot and facula properties are consistent between the two visits, resulting in nearly identical transmission spectra. Jointly retrieving for stellar contamination and a planetary atmosphere reveals that our spectrum can rule out hydrogen-dominated, ≲300× solar metallicity atmospheres with effective surface pressures down to 10 mbar at the 3σ level. For high mean molecular weight atmospheres, where O2 or N2 is the background gas, our spectrum disfavors partial pressures of more than ∼10 mbar for H2O, CO, NH3, and CH4 at the 2σ level. Similarly, under the assumption of a 100% H2O, NH3, CO, or CH4 atmosphere, our spectrum disfavors thick, >1-bar atmospheres at the 2σ level. These nondetections of spectral features are in line with predictions that even heavier, CO2-rich atmospheres would be efficiently lost on TRAPPIST-1 c given the cumulative high-energy irradiation experienced by the planet. Our results further stress the importance of robustly accounting for stellar contamination when analyzing JWST observations of exo-Earths around M dwarfs, as well as the need for high-fidelity stellar models to search for the potential signals of thin secondary atmospheres.

Abstract:

The prevalence of atmospheres on rocky planets is one of the major questions in exoplanet astronomy, but there are currently no published unambiguous detections of atmospheres on any rocky exoplanets. The MIRI instrument on JWST can measure thermal emission from tidally locked rocky exoplanets orbiting small, cool stars. This emission is a function of their surface and atmospheric properties, potentially allowing detections of atmospheres. One way to find atmospheres is to search for lower dayside emission than would be expected for a blackbody planet. Another technique is to measure phase curves of thermal emission to search for nightside emission due to atmospheric heat redistribution. Here, we compare strategies for detecting atmospheres on rocky exoplanets. We simulate secondary eclipse and phase curve observations in the MIRI F1500W and F1280W filters for a range of surfaces (providing our open-access albedo data) and atmospheres on 30 exoplanets selected for their F1500W signal-to-noise ratio. We show that secondary eclipse observations are more degenerate between surfaces and atmospheres than suggested in previous work, and that thick atmospheres can support emission consistent with a blackbody planet in these filters. These results make it difficult to unambiguously detect or rule out atmospheres using their photometric dayside emission alone. We suggest that an F1500W phase curve could instead be observed for a similar sample of planets. While phase curves are time-consuming and their instrumental systematics can be challenging, we suggest that they allow the only unambiguous detections of atmospheres by nightside thermal emission.

Abstract:

<jats:p>Voyager-2 radio occultation measurements have revealed changes in the atmospheric refractivity within a 2-4 km layer near the 1.2-bar level in Uranus and the 1.6-bar level in Neptune. These changes were attributed to the presence of a methane cloud, consistent with the observation that methane concentration decreases with altitude above these levels, closely following the saturation vapor pressure. However, no clear spectral signatures of such a cloud have been detected thus far in the spectra acquired from both planets. We examine methane cloud properties in the atmospheres of the ice giants, including vertical ice distribution, droplet radius, precipitation rates, timescales, and total opacity, employing microphysical simulations under different scenarios. We used a one-dimensional (1D) cloud microphysical model to simulate the formation of methane clouds in the ice giants. The simulations include the processes of nucleation, condensation, coagulation, evaporation, and precipitation, with vertical mixing simulated using an eddy-diffusion profile (K_eddy). Our simulations show cloud bases close to 1.24 bars in Uranus and 1.64 bars in Neptune, with droplets up to 100 μm causing high settling velocities and precipitation rates (∼370 mm per Earth year). The high settling velocities limit the total cloud opacity, yielding values at 0.8 μm of ∼0.19 for Uranus and ∼0.35 for Neptune, using K_ eddy = 0.5 m^2 s^-1 and a deep methane mole fraction (μ_CH_4) of 0.04. In addition, lower K_ eddy or μ_CH_4 values result in smaller opacities. Methane supersaturation is promptly removed by condensation, controlling the decline in μ_CH_4 with altitude in the troposphere. However, the high settling velocities prevent the formation of a permanent thick cloud. Stratospheric hazes made of ethane or acetylene ice are expected to evaporate completely before reaching the methane condensation level. Since hazes are required for methane heterogeneous nucleation, this suggests either a change in the solid phase properties of the haze particles, inhibiting evaporation, or the presence of photochemical hazes.</jats:p>