Exoplanets and Stellar Physics

Abstract:

The atmospheric circulation of tidally locked planets is dominated by a superrotating eastward equatorial jet. We develop a predictive theory for the formation of this jet, proposing a mechanism in which the three-dimensional stationary waves induced by the day–night forcing gradient produce an equatorial acceleration. This is balanced in equilibrium by an interaction between the resulting jet and the vertical motion of the atmosphere. The three-dimensional structure of the zonal acceleration is vital to this mechanism. We demonstrate this mechanism in a hierarchy of models. We calculate the three-dimensional stationary waves induced by the forcing on these planets and show the vertical structure of the zonal acceleration produced by these waves, which we use to suggest a mechanism for how the jet forms. General circulation model simulations are used to confirm the equilibrium state predicted by this mechanism, where the acceleration from these waves is balanced by an interaction between the zonal-mean vertical velocity and the jet. We derive a simple model of this using the "Weak Temperature Gradient" approximation, which gives an estimate of the jet speed on a terrestrial tidally locked planet. We conclude that the proposed mechanism is a good description of the formation of an equatorial jet on a terrestrial tidally locked planet and should be useful for interpreting observations and simulations of these planets. The mechanism requires assumptions such as a large equatorial Rossby radius and weak acceleration due to transient waves, and a different mechanism may produce the equatorial jets on gaseous tidally locked planets.

Abstract:

Context. Having a short orbital period and being tidally locked makes WASP-43b an ideal candidate for the James Webb Space Telescope (JWST) phase curve measurements. Phase curve observations of an entire orbit will enable the mapping of the atmospheric structure across the planet, with different wavelengths of observation allowing different atmospheric depths to be seen. Aims. We provide insight into the details of the clouds that may form on WASP-43b and their impact on the remaining gas phase, in order to prepare the forthcoming interpretation of the JWST and follow-up data. Methods. We follow a hierarchical modelling strategy. We utilise 3D GCM results as input for a kinetic, non-equilibrium model for mineral cloud particles and for a kinetic model to study a photochemically-driven hydrocarbon haze component. Results. Mineral condensation seeds form throughout the atmosphere of WASP-43b. This is in stark contrast to the ultra-hot Jupiters, such as WASP-18b and HAT-P-7b. The dayside is not cloud free but it is loaded with few yet large mineral cloud particles in addition to hydrocarbon haze particles of a comparable abundance. Photochemically driven hydrocarbon haze appears on the dayside, but it does not contribute to the cloud formation on the nightside. The geometrical cloud extension differs across the globe due to the changing thermodynamic conditions. Day and night differ by 6000 km in pressure scale height. As reported for other planets, the C/O is not constant throughout the atmosphere and varies between 0.74 and 0.3. The mean molecular weight is approximately constant in a H2-dominated WASP-43b atmosphere because of the moderate day/night-temperature differences compared to the super-hot Jupiters. Conclusions. WASP-43b is expected to be fully covered in clouds which are not homogeneously distributed throughout the atmosphere. The dayside and the terminator clouds are a combination of mineral particles of locally varying size and composition as well as of hydrocarbon hazes. The optical depth of hydrocarbon hazes is considerably lower than that of mineral cloud particles such that a wavelength-dependent radius measurement of WASP-43b would be determined by the mineral cloud particles but not by hazes.