Previous measurements of heat redistribution efficiency (the ability to transport energy from a planet's highly irradiated dayside to its eternally dark nightside) show considerable variation between exoplanets. Theoretical models predict a positive correlation between heat redistribution efficiency and temperature for tidally locked planets; however, recent Hubble Space Telescope (HST) WASP-43b spectroscopic phase curve results are inconsistent with current predictions. Using the Spitzer Space Telescope, we obtained a total of three phase curve observations of WASP-43b (P = 0.813 days) at 3.6 and 4.5. mu m. The first 3.6. mu m visit exhibits spurious nightside emission that requires invoking unphysical conditions in our cloud-free atmospheric retrievals. The two other visits exhibit strong day-night contrasts that are consistent with the HST data. To reconcile the departure from theoretical predictions, WASP-43b would need to have a high-altitude, nightside cloud/haze layer blocking its thermal emission. Clouds/hazes could be produced within the planet's cool, nearly retrograde mid-latitude flows before dispersing across its nightside at high altitudes. Since mid-latitude flows only materialize in fast-rotating (less than or similar to 1 day) planets, this may explain an observed trend connecting measured day-night contrast with planet rotation rate that matches all current Spitzer phase curve results. Combining independent planetary emission measurements from multiple phases, we obtain a precise dayside hemisphere H2O abundance (2.5 x 10(-5)-1.1 x 10(-4) at 1 sigma confidence) and, assuming chemical equilibrium and a scaled solar abundance pattern, we derive a corresponding metallicity estimate that is consistent with being solar (0.4-1.7). Using the retrieved global CO+CO2 abundance under the same assumptions, we estimate a comparable metallicity of 0.3-1.7x solar. This is the first time that precise abundance and metallicity constraints have been determined from multiple molecular tracers for a transiting exoplanet.

The determination of atmospheric structure and molecular abundances of planetary atmospheres via spectroscopy involves direct comparisons between models and data. While varying in sophistication, most model spectra comparisons fundamentally assume one-dimensional (1D) model physics. However, knowledge from general circulation models and of solar system planets suggests that planetary atmospheres are inherently three-dimensional in their structure and composition. We explore the potential biases resulting from standard "1D" assumptions within a Bayesian atmospheric retrieval framework. Specifically, we show how the assumption of a single 1D thermal profile can bias our interpretation of the thermal emission spectrum of a hot Jupiter atmosphere that is composed of two thermal profiles. We retrieve spectra of unresolved model planets as observed with a combination of the Hubble Space Telescope Wide Field Camera 3 (WFC3)+Spitzer Infrared Array Camera (IRAC) as well as the James Webb Space Telescope (JWST) under varying differences in the two thermal profiles. For WFC3+IRAC, there is a significantly biased estimate of CH4 abundance using a 1D model when the contrast is 80%. For JWST, two thermal profiles are required to adequately interpret the data and estimate the abundances when contrast is greater than 40%. We also apply this preliminary concept to the recent WFC3+IRAC phase curve data of the hot Jupiter WASP-43b. We see similar behavior as present in our simulated data: while the H2O abundance determination is robust, CH4 is artificially well-constrained to incorrect values under the 1D assumption. Our work demonstrates the need to evaluate model assumptions in order to extract meaningful constraints from atmospheric spectra and motivates exploration of optimal observational setups.

Avec près de 4000 exoplanètes connues, le domaine est passé de simplement détecter des exoplanètes à étudier leurs propriétés atmosphériques. Cependant, les spectres en lumières réfléchies de ces objets sont encore mal compris. Les exoplanètes réfléchissent une partie de la lumière qu’elles reçoivent de leur étoile, selon les propriétés de l’atmosphère, ce qui affecte le budget énergétique de la planète. Les Jupiters chaudes, c’est-à-dire des planètes de types Jupiter avec des périodes orbitales très courtes, sont les cibles les plus faciles à observer par spectroscopie des éclipses. L’albédo est une mesure directe de la lumière réfléchie qui peut être mesurée pendant que la planète passe derrière l’étoile hôte. Dans leur cas spécifique, une incohérence apparente, appelée le problème d’albédo des Jupiters chaudes, reste non résolu. Alors que les géantes gazeuses du système solaire ont des albédos de Bond inférieurs aux albédos géométriques, les mesures dans le visible et l’infrarouges pour HD 189733b et HD 209458b indiquent le contraire. Ceci pourrait être expliqué par des albédos géométriques plus élevés à des longueurs d’onde UV/visibles hors de la bande passante de Kepler, mais très peu de mesures existent pour corroborer cela. Ce mémoire présente le spectre de réflexion complet de WASP-43b, incluant 3 mesures d’éclipse obtenues par le HST (290-570 nm) ainsi que 28 obtenues par la mission TESS (600-1000 nm). Lorsque combinées avec les observations Spitzer ou les observations d’éclipse du JWST à venir, ces mesures répondront à des questions-clés concernant la structure et composition atmosphérique de la planète, le budget énergétique global et sa circulation. / With nearly 4000 exoplanets known, the field has evolved from merely detecting exoplanets to actually probing atmospheric properties. However, reflected light spectra from these objects are still not fully understood. Exoplanets reflect a portion of the light that they receive from the star, the amount of which depends on the properties of the atmosphere and in turn affects the energy budget of the planet. Hot Jupiters, i.e. Jupiter-like planets giants with very short orbital periods are the easiest targets amenable to eclipse spectroscopy. Albedo is a direct measure of reflected light that can be measured while the planet eclipses behind the host star. In the specific case of these intriguing planets, an apparent inconsistency, termed as the hot Jupiter Albedo Problem, remains unsolved. While Solar System gas giants show Bond albedos lower than geometric albedos, the measurements from optical and infrared instruments for HD 189733b and HD 209458b show the opposite. This phenomenon has the potential to be explained by higher geometric albedos at UV/optical wavelengths outside the Kepler bandpass, but very few measurements exist to corroborate this. This thesis presents WASP-43b’s full reflection spectrum, including 3 eclipse measurements obtained by the HST (290-570 nm) along with 28 obtained by the TESS mission (600-1000 nm). When combined with the Spitzer or the upcoming JWST’s eclipse observations, these measurements will answer key questions about the planet’s atmospheric composition and structure, global energy budget and circulation.