Probing Exoplanetary Climates through Temporal and Spatial Atmospheric Variations

Abstract

Thousands of extrasolar planets have now been discovered, many with characteristics unlike anything in our Solar System. An immediate goal of exoplanetary science is to understand the dynamics and formation of aerosols that shape each planet’s atmosphere, and their dependencies on the planet’s environment. Such advances pave the way toward the community’s ultimate goals including unraveling the diversity of planets in the Galaxy. In this thesis, I present new space-based observations of exoplanetary atmospheres as well as new theoretical frameworks that map how fundamental atmospheric properties change spatially within an atmosphere and vary over time. First, I present a set of repeated infrared phase curve observations of the canonical hot Jupiter WASP-43 b using the Spitzer Space Telescope. Phase curves probe a planet’s global atmospheric properties by measuring its emission over a full time-resolved orbit, and repeated observations affords the opportunity to see how these properties change over time. I show that WASP-43 b’s climate is relatively stable, and place new upper limits on the variability of its global temperature and cloud distribution. I also compare my observations to new 3-dimensional circulation models to show that WASP-43 b has a heterogeneous cloud distribution that is stable over time. Second, I present transmission spectroscopy of the hot Neptune HD 219666 b using the Hubble Space Telescope. The second hottest exo-Neptune known, HD 219666 b lies in an interesting chemical and aerosol transition regime. I combine two new near-infrared transmission spectra from ∼1.1 - 1.6 μm to detect water in its atmosphere, and these also show no evidence for temporal variability. I further use these spectra to infer that HD 219666 b has a relatively aerosol-free atmosphere. Third, I present the description of a systematic bias that may impede a new observational technique -– “limb-resolved transmission spectroscopy” — for characterizing spatial variations in atmospheres. In short, the effect is that uncertainty in a planet’s orbital ephemeris can lead to false positive and negative detections of asymmetry between the planet’s limbs. I show that this bias is actually two effects, one astrophysical and one numerical, working in concert together. I develop general analytic models to show how each effect arises and how they can be corrected for, and develop complementary numerical models to demonstrate this effect in practice for current potential targets for limb-resolved transmission spectroscopy. Fourth, I present the novel application of limb-resolved transmission spectroscopy on the exoplanet WASP-107 b using the James Webb Space Telescope (JWST). I make the first-ever space-based detection of evening-morning limb asymmetry. Further, using WASP-107 b’s limb transmission spectra I show there is a significant temperature difference between WASP-107 b’s terminators, which challenges expectations for the efficiency of heat redistribution in warm exoplanetary atmospheres. Fifth and finally, I expand on this discovery using additional JWST observations of WASP-107 b to measure its panchromatic (1 - 12 μm) evening and morning transmission spectra. This is the first time that individual limb spectra have been measured across multiple bands and combined. I show evidence that, in addition to a temperature contrast, there are also chemical abundance variations between WASP-107 b’s terminators. I also show evidence for a highly heterogeneous cloud distribution between terminators, and present new 3-dimensional circulation models which consistently predict such heterogeneity. Several of these observations were contaminated by starspot crossings during the transits. I leverage these to also develop the first model for how limb-resolved transmission spectra are biased by starspot crossings.