Exoplanet atmospheres

Abstract:

The James Webb Space Telescope (JWST) presents the opportunity to transform our understanding of planets and the origins of life by revealing the atmospheric compositions, structures, and dynamics of transiting exoplanets in unprecedented detail. However, the high-precision, time-series observations required for such investigations have unique technical challenges, and prior experience with other facilities indicates that there will be a steep learning curve when JWST becomes operational. In this paper we describe the science objectives and detailed plans of the Transiting Exoplanet Community Early Release Science (ERS) Program, which is a recently approved program for JWST observations early in Cycle 1. The goal of this project, for which the obtained data will have no exclusive access period, is to accelerate the acquisition and diffusion of technical expertise for transiting exoplanet observations with JWST, while also providing a compelling set of representative datasets that will enable immediate scientific breakthroughs. The Transiting Exoplanet Community ERS Program will exercise the time-series modes of all four JWST instruments that have been identified as the consensus highest priorities, observe the full suite of transiting planet characterization geometries (transits, eclipses, and phase curves), and target planets with host stars that span an illustrative range of brightnesses. The observations in this program were defined through an inclusive and transparent process that had participation from JWST instrument experts and international leaders in transiting exoplanet studies. Community engagement in the project will be centered on a two-phase Data Challenge that culminates with the delivery of planetary spectra, time-series instrument performance reports, and open-source data analysis toolkits in time to inform the agenda for Cycle 2 of the JWST mission.

Abstract:

The James Webb Space Telescope (JWST) presents the opportunity to transform our understanding of planets and the origins of life by revealing the atmospheric compositions, structures, and dynamics of transiting exoplanets in unprecedented detail. However, the high-precision, time-series observations required for such investigations have unique technical challenges, and prior experience with other facilities indicates that there will be a steep learning curve when JWST becomes operational. In this paper we describe the science objectives and detailed plans of the Transiting Exoplanet Community Early Release Science (ERS) Program, which is a recently approved program for JWST observations early in Cycle 1. The goal of this project, for which the obtained data will have no exclusive access period, is to accelerate the acquisition and diffusion of technical expertise for transiting exoplanet observations with JWST, while also providing a compelling set of representative datasets that will enable immediate scientific breakthroughs. The Transiting Exoplanet Community ERS Program will exercise the time-series modes of all four JWST instruments that have been identified as the consensus highest priorities, observe the full suite of transiting planet characterization geometries (transits, eclipses, and phase curves), and target planets with host stars that span an illustrative range of brightnesses. The observations in this program were defined through an inclusive and transparent process that had participation from JWST instrument experts and international leaders in transiting exoplanet studies. Community engagement in the project will be centered on a two-phase Data Challenge that culminates with the delivery of planetary spectra, time-series instrument performance reports, and open-source data analysis toolkits in time to inform the agenda for Cycle 2 of the JWST mission.

Abstract:

Sub-Neptunes are a subset of exoplanets that lie between the Earth and Neptune in size, have no solar system analogue and yet are one of the most common types of exoplanet in the galaxy. Some sub-Neptunes receive a similar level of stellar flux as Earth, making their atmospheres potentially cool enough to contain liquid water. The aim of this thesis is to simulate the atmospheres of these temperate sub-Neptunes and develop theories describing their atmospheric dynamics and potential habitability. I use a general circulation model to simulate the atmospheres of a range of dry, temperate sub-Neptunes. I show that their atmospheres are governed by horizontal weak temperature gradients over a broad range of parameter space. Their circulation is dominated by high-latitude jets, but heat is transported from the dayside to the nightside by a residual overturning circulation. I derive a scaling theory to link the strength of this circulation to the instellation. Next, I calculate the inner edge of the habitable zone for sub-Neptunes with a water surface – “Hycean worlds”. Using a 1D radiative-convective model, I show that compositional gradients induced by the condensation of water inhibit convection in a hydrogendominated atmosphere. The resulting temperature structures heat the surface and lead to the inner edge of the habitable zone moving outwards compared to traditional calculations. Lastly, I develop a general circulation model for use in hydrogen-dominated atmospheres with a non-dilute water vapour component. I demonstrate the model’s ability to simulate a range of sub-Neptune atmospheres with different deep water contents reaching as high as 70% of the atmosphere by mass. Future work can build on this model to understand how latent heating and compositional gradients impact the observable features and habitability of sub-Neptune exoplanets.