Over the past few decades, short-period giant planets have been discovered in extrasolar planetary systems, allowing for new tests of planetary evolution theories. Many of these giant exoplanets have high temperatures (>1000 K) and do not directly resemble Jupiter or Saturn. Only in the past few years have exoplanets akin to the cold (~100 K) gas giants in the solar system been identified. In this dissertation, I investigated giant gaseous planets through comparative studies of Saturn and exoplanets. Saturn has been the target of numerous high-precision observations, making it the ideal candidate for comparative studies. I simulated transit observations of a Saturn-analog exoplanet and determined that cold exoplanet atmospheres are amenable to characterization via transmission spectroscopy. By casting Saturn as an exoplanet, I demonstrated the potential for exoplanets to place the solar system in a Galactic context. The transit spectrum of Saturn also highlighted the importance of atmospheric refraction in transit observations. Refraction alters the path of light propagating in an atmosphere. I showed that out-of-transit refracted light provides an opportunity to identify and characterize the atmospheres of cold transiting and non-transiting exoplanets. I searched exoplanet parameter space to locate the maximal effect and derived a criterion that predicts which atmospheres produce detectable refracted light signatures. My consideration of exoplanetary refraction also included a parallel study of Saturn's atmosphere. I developed a novel method to measure atmospheric refractivity from distorted images of the Sun. I used this method to infer Saturn's atmospheric structure for more than a dozen Saturn solar occultations and to identify seasonal variations in Saturn's stratospheric temperature. Lastly, I obtained ground-based observations of the long-period transiting exoplanet Kepler-421b to refine its transit ephemeris. Without accurate transit ephemerides, long-period exoplanet characterization with large space-based observatories cannot occur. My unique observations represent the first step toward ensuring that long-period exoplanets are characterized in the near future. In summary, this dissertation lays the foundation for investigations of cold giant exoplanets, which exist in an almost entirely unexplored regime of exoplanetary science. Using Saturn to provide context and motivation, I began confronting the challenges facing this new discipline of exoplanetary science.

Very precise photometry of transiting extrasolar planets can be used to learn about the physical structure of Jupiter-like planets in an exoplanetary systems. The fraction of light reflected from the planet (albedo) provides crucial insight into the chemical structure of atmospheres and global thermal properties of a planet, including heat dissipation and global weather patterns. Measuring the albedo of an exoplanet requires very precise photometry with high time sampling and nearly continuous time coverage spanning may orbits, which can be achieved at present only from space. We present space-based photometry of the transiting exoplanetary system HD 209458 obtained with the MOST (Microvariablity and Oscillations of STars) satellite during 2004 and 2005. The data span 14 and 44 days respectively, and have nearly complete time coverage for both spans. The HD 209458 photometry was obtained in MOST's Direct Imaging mode, not part of the original mission but implemented to make possible measurements of stars in the brightness range 6.5 < V < 13. The photometric reduction techniques developed for this thesis have become the standard pipeline for processing MOST Direct Imaging data, in particular the corrections for stray Earthshine reaching the MOST instrument focal plane. Our analysis of MOST photometry of HD 209458 sets a 1 sigma upper limit on the depth of the optical eclipse of the planet of 17 parts per million (ppm) = 17 micromagnitudes. This corresponds to a 1 sigma upper limit on the flux ratio of planet to star of 1.57 x 10^-5 and an upper limit on the geometric albedo in the MOST bandpass (400 to 700 nm) of 8%. This is the most sensitive measurement of an exoplanetary albdeo ever obtained. The limit on the albedo of the gas giant HD 209458b means it much less reflective as Jupiter. This result rules out the presence of reflective clouds in the atmosphere of HD 209458b and has already enabled theoretical modeling of far-infrared measurements to contrain the planet's equilibrium atmospheric temperature to be 1550 +/- 150 K. The MOST albedo analysis demonstrates the potential of spacebased photometry missions like CoRoT (launched in December 2006) and Kepler (due for launch in early 2009) and has already provided important lessons for both missions.

Very precise photometry of transiting extrasolar planets can be used to learn about the physical structure of Jupiter-like planets in an exoplanetary systems. The fraction of light reflected from the planet (albedo) provides crucial insight into the chemical structure of atmospheres and global thermal properties of a planet, including heat dissipation and global weather patterns. Measuring the albedo of an exoplanet requires very precise photometry with high time sampling and nearly continuous time coverage spanning may orbits, which can be achieved at present only from space. We present space-based photometry of the transiting exoplanetary system HD 209458 obtained with the MOST (Microvariablity and Oscillations of STars) satellite during 2004 and 2005. The data span 14 and 44 days respectively, and have nearly complete time coverage for both spans. The HD 209458 photometry was obtained in MOST's Direct Imaging mode, not part of the original mission but implemented to make possible measurements of stars in the brightness range 6.5 < V < 13. The photometric reduction techniques developed for this thesis have become the standard pipeline for processing MOST Direct Imaging data, in particular the corrections for stray Earthshine reaching the MOST instrument focal plane. Our analysis of MOST photometry of HD 209458 sets a 1 sigma upper limit on the depth of the optical eclipse of the planet of 17 parts per million (ppm) = 17 micromagnitudes. This corresponds to a 1 sigma upper limit on the flux ratio of planet to star of 1.57 x 10^-5 and an upper limit on the geometric albedo in the MOST bandpass (400 to 700 nm) of 8%. This is the most sensitive measurement of an exoplanetary albdeo ever obtained. The limit on the albedo of the gas giant HD 209458b means it much less reflective as Jupiter. This result rules out the presence of reflective clouds in the atmosphere of HD 209458b and has already enabled theoretical modeling of far-infrared measurements to contrain the planet's equilibrium atmospheric temperature to be 1550 +/- 150 K. The MOST albedo analysis demonstrates the potential of spacebased photometry missions like CoRoT (launched in December 2006) and Kepler (due for launch in early 2009) and has already provided important lessons for both missions.

Very precise photometry of transiting extrasolar planets can be used to learn about the physical structure of Jupiter-like planets in an exoplanetary systems. The fraction of light reflected from the planet (albedo) provides crucial insight into the chemical structure of atmospheres and global thermal properties of a planet, including heat dissipation and global weather patterns. Measuring the albedo of an exoplanet requires very precise photometry with high time sampling and nearly continuous time coverage spanning may orbits, which can be achieved at present only from space. We present space-based photometry of the transiting exoplanetary system HD 209458 obtained with the MOST (Microvariablity and Oscillations of STars) satellite during 2004 and 2005. The data span 14 and 44 days respectively, and have nearly complete time coverage for both spans. The HD 209458 photometry was obtained in MOST's Direct Imaging mode, not part of the original mission but implemented to make possible measurements of stars in the brightness range 6.5 < V < 13. The photometric reduction techniques developed for this thesis have become the standard pipeline for processing MOST Direct Imaging data, in particular the corrections for stray Earthshine reaching the MOST instrument focal plane. Our analysis of MOST photometry of HD 209458 sets a 1 sigma upper limit on the depth of the optical eclipse of the planet of 17 parts per million (ppm) = 17 micromagnitudes. This corresponds to a 1 sigma upper limit on the flux ratio of planet to star of 1.57 x 10^-5 and an upper limit on the geometric albedo in the MOST bandpass (400 to 700 nm) of 8%. This is the most sensitive measurement of an exoplanetary albdeo ever obtained. The limit on the albedo of the gas giant HD 209458b means it much less reflective as Jupiter. This result rules out the presence of reflective clouds in the atmosphere of HD 209458b and has already enabled theoretical modeling of far-infrared measurements to contrain the planet's equilibrium atmospheric temperature to be 1550 +/- 150 K. The MOST albedo analysis demonstrates the potential of spacebased photometry missions like CoRoT (launched in December 2006) and Kepler (due for launch in early 2009) and has already provided important lessons for both missions. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

abstract: The field of exoplanet science has matured over the past two decades with over 3500 confirmed exoplanets. However, many fundamental questions regarding the composition, and formation mechanism remain unanswered. Atmospheres are a window into the properties of a planet, and spectroscopic studies can help resolve many of these questions. For the first part of my dissertation, I participated in two studies of the atmospheres of brown dwarfs to search for weather variations. To understand the evolution of weather on brown dwarfs we conducted a multi-epoch study monitoring four cool brown dwarfs to search for photometric variability. These cool brown dwarfs are predicted to have salt and sulfide clouds condensing in their upper atmosphere and we detected one high amplitude variable. Combining observations for all T5 and later brown dwarfs we note a possible correlation between variability and cloud opacity. For the second half of my thesis, I focused on characterizing the atmospheres of directly imaged exoplanets. In the first study Hubble Space Telescope data on HR8799, in wavelengths unobservable from the ground, provide constraints on the presence of clouds in the outer planets. Next, I present research done in collaboration with the Gemini Planet Imager Exoplanet Survey (GPIES) team including an exploration of the instrument contrast against environmental parameters, and an examination of the environment of the planet in the HD 106906 system. By analyzing archival HST data and examining the near-infrared colors of HD 106906b, we conclude that the companion shows weak evidence of a circumplanetary dust disk or cloud. Finally, I measure the properties of the low mass directly imaged planet 51 Eridani b. We combined published J, H spectra with updated LP photometry, new K1, K2 spectra, and MS photometry. The new data confirms that the planet has redder than similar spectral type objects, which might be due to the planet still transitioning from to L-to-T. Model atmospheres indicate a cooler effective temperature best fit by a patchy cloud atmosphere making 51 Eri b an excellent candidate for future variability studies with the James Webb Space Telescope. / Dissertation/Thesis / Doctoral Dissertation Astrophysics 2017

Since the first discovery of a transiting planet in 2000, transmission spectroscopy has proved essential for characterising the rapidly increasing number of known extrasolar planets. When a planet is in a favourable alignment, it periodically passes (transits) in front of its host star, during which time it blocks a fraction of the stellar light. During a transit, the starlight passes through the planetary atmosphere, causing the signatures of atoms or molecules present in that atmosphere to imprint themselves on the stellar spectrum, allowing direct observation of a planet's atmospheric composition. At the start of this thesis, only two planets (HD 189733b and HD 209458b) had been studied in any detail, mainly from space. The two planets showed surprisingly different qualities for two objects with only a small temperature difference between them, and motivated both wider and more detailed studies of the exoplanet population. Since the start of my PhD, the amount of exoplanet knowledge has grown rapidly, with observations from the ground becoming important, and with studies branching out towards new planets. There are several contributions made by this thesis to the field. Chapter 3 details the detection of the resolved sodium D doublet in the atmosphere of HD 189733b, a planet with a featureless broad-band transmission spectrum dominated by Rayleigh scattering. The results confirmed the presence of sodium absorption as well as resolving the feature for the first time, and placing constraints on relative abundances. Furthermore, in Chapter 4, I outline a method based on earlier work which allows observers to retrieve atmospheric temperature information from resolved spectral features. This method is applied to the observations of HD 189733b, showing that the planet has a hot thermosphere similar to HD 209458b. The models are then also used in later chapters. I then present the first results from a ground-based optical long-slit spectroscopic survey in Chapter 5, and the first results from a space-based optical-near-IR spectroscopic survey in Chapter 6. From the ground, I detect absorption from sodium in the atmosphere of XO-2b, making this the first planet with sodium and potassium detected in its atmosphere. I also find that the Na I D feature lacks broad line wings, suggesting haze or cloud cover. From space, I observed the transmission spectrum of WASP-19b, finding solar abundance water features and a likely lack of predicted TiO features. WASP-19b is the first planet to have confirmed water features at solar-abundance level. In Chapter 7 I conclude and discuss future work, including a project aimed at understanding why WASP-19b lacks TiO features, and projects which move beyond the hot Jupiter class.

<p>If a distant star happens to host an orbiting exoplanet, then that planet will exert a gravitational influence on the star that may be detectable from the earth by the apparent ``stellar wobble''---regular, periodic variations in the observed radial velocities. Two complicating factors, however are ``stellar jitter'' and measurement errors, both of which generate radial velocity ``noise''. As a result, it is not always obvious whether variations in radial velocities are due to an orbiting planet or to mere noise. The problem of model selection---is that a planet or not?---may be addressed by using any number of summary statistics, and the choice may be made using frequentist hypothesis testing, Bayesian model selection, or likelihood analysis.</p><p>Here we compare the planet-detecting ability of three different statistics: the ``Maximal Periodogram Spike'', or the height of the tallest spike in the Lomb-Scargle periodogram; the Maximum Likelihood Ratio; and the Bayes Factor. We measure the usefulness of each statistic by how many observations would be required before the statistic would able to distinguish accurately the no-planet model from the one-planet model when presented with data from a star hosting a planet of one of six different types---we vary the magnitude of the radial velocity function's semi-amplitude and the eccentricity of the orbit. The data are simulated, using observations made at random times.</p><p>We find that the current practice of examining the tallest spike in the periodogram cannot easily be improved upon using either the Maximum Likelihood Ratio (which performs comparably) or the Bayes Factor (which is exceedingly difficult to estimate for eccentric orbits unless it is already plain that a planet is present). We also conclude that a planet having a semiamplitude of 3 m/s (which could potentially correspond to the smallest planet yet discovered outside of a multi-planet system) could in fact be detected using current methods, if there was an initial commitment to make at least 150 or 200 radial velocity observations on the same candidate star, even in the absence of growing evidence for planetary presence.</p> / Dissertation

This PhD thesis presents work on the atmospheres of both brown dwarfs and exoplanets from an observers viewpoint. The composition and weather of these worlds are explored starting with M-type brown dwarfs and continuing through the L, T and Y spectral sequence, before entering the planetary regime of hot-Jupiters and super-Earths. The similarities and differences between these objects such as their radii, surface gravities, pressures, temperatures and composition are discussed. This thesis presents new results from an extensive near-infrared monitoring survey of a uniform and unbiased sample of 69 L &amp; T dwarfs spanning the L0 to T8 spectral range. Results show that amongst 14 identified variables, nine of them newly identified, variable brown dwarfs are not concentrated at the L - T transition, nor are they observed in a specific colour, or preferentially in binary systems. The thesis also presents narrow-band photometric measurements of the hot-Jupiter HAT-P-1b and the super-Earth GJ~1214b using the 10.4~m Gran Telescopio Canarias (GTC) and the OSIRIS instrument. Results for HAT-P-1b show a strong presence of potassium in the atmosphere caused by a large scale height, possibly due to higher than anticipated temperatures in the upper atmosphere or the dissociation of molecular hydrogen caused by the UV flux from the host star. Results for GJ 1214b, which constitute the first tunable filter measurements of a super-Earth, find no evidence for the presence of methane showing a featureless transmission spectrum consistent with previous studies.

The brightness and variability of the atmosphere in the thermal infrared poses obstacles to precision photometry measurements. The need to remove atmospheric effects calls for the use of a comparison star, but it is usually impossible to fit both science and comparison targets on current long-wavelength (> 2 mu m) detectors. We present a new pointing mode at the Large Binocular Telescope, which has twin 8.4-m primary mirrors that can be pointed up to similar to 2 arcminutes apart and allow the placement of both targets on a small-field infrared detector. We present an observation of the primary transit of an exoplanet in front of its host star, and use it to provide preliminary constraints on the attainable photometric precision.

Most of the 500 extra-solar planets detected to date have been discovered either by radial velocity measurements or photometric transits but very few by direct techniques. Direct imaging of exoplanets, however, gives access to a wider variety of information on the planet, from its orbital position to its spectrum, but is a difficult task to achieve because of the small angular separation between the star and its planet and the large flux ratio between them. For these two reasons, direct imaging of exoplanets has up to now been limited to the most favorable cases of bright giant exoplanets orbiting at large distances from their host stars. %The present work aims at developing the high dynamic range capabilities of single- and multi-aperture imaging techniques for the detection and characterization of planetary systems. The first part of this work reports studies of adaptive optics-aided ground-based telescopes and their complementarity with space-based facilities for the detection of extra-solar planets. Results obtained with the Well-Corrected Subaperture at Palomar observatory on narrow multiple systems are then used to illustrate this study. The second part of this work is dedicated to stellar and nulling interferometry. We first present a new data reduction technique for interferometry using the statistical distributions of the noise sources to significantly improve the precision of interferometric measurements. This technique is then applied to the Palomar Fiber Nuller instrument at Palomar observatory to constrain the presence of dust and companions in the innermost regions of Vega's stellar environment and to derive stellar angular diameters with very high accuracies. Finally, we introduce an on-going survey that we are pursuing with the AMBER interferometric instrument at the Very Large Telescope (Cerro Paranal, Chile) aiming at detecting sub-stellar companions around young main sequence stars.