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The Next Generation Transit Survey (NGTS) is a new ground-based survey for transiting exoplanets. Our primary goal is to find the first statistically-significant sample of Neptunes and super-Earths that are bright enough for radial velocity confirmation. By measuring precise masses and radii we will constrain the bulk composition and internal structure of planets that span the transition between the gas giants and terrestrial planets. Our brightest exoplanets will also be suitable for atmospheric characterisation with large facilities such as the VLT, JWST and the E-ELT. NGTS construction began in June 2013, and the survey is due to commence in 2014.

The study of exoplanets has rapidly developed in the last twenty years, and the detailed characterization of planetary atmospheres has become a key area of research. For transiting planets around bright stars, atmospheric features can be detected with transmission spectroscopy. I will present a low resolution transmission spectrum of WASP-52b, and show that the most likely interpretation is that the planet is shrouded with an opaque cloud layer. By using transmission spectroscopy at much higher resolution, in this thesis I will present the first spatially resolved measurements of a weather system in an exoplanet. By modeling the absorption profile of sodium on HD189733b, I show that the planet atmosphere has an excess velocity not explained by planetary rotation. HD209458b is evaporating under intense irradiation from its star, and may lose as much as 1010 g s-1. Mechanisms of mass loss are poorly understood, in particular the efficiency. To calculate this rate for HD209458b a key component is missing - the high energy flux of the star. I will demonstrate that it is possible to recover this flux by building a coronal model for the star, using constraints for different temperatures of plasma from UV and X-ray sources. I will present commissioning data gathered with NGTS that are the most precise ever gathered with a ground based wide field transit survey. Simulations of the performance of NGTS in this thesis show that the survey can be expected to detect ~200 low mass planets. The simulations of NGTS also show that a sample of bright super-Earths and hot-Neptunes can be expected to be detected, which would be sensitive to the same techniques performed on hot Jupiters in this thesis. One day, these same techniques could be important tools for characterizing the atmospheres of Earth analogs.

We present SPIDERMAN, a fast code for calculating exoplanet phase curves and secondary eclipses with arbitrary surface brightness distributions in two dimensions. Using a geometrical algorithm, the code solves exactly the area of sections of the disc of the planet that are occulted by the star. The code is written in C with a user-friendly Python interface, and is optimised to run quickly, with no loss in numerical precision. Approximately 1000 models can be generated per second in typical use, making Markov Chain Monte Carlo analyses practicable. The modular nature of the code allows easy comparison of the effect of multiple different brightness distributions for the dataset. As a test case we apply the code to archival data on the phase curve of WASP-43b using a physically motivated analytical model for the two dimensional brightness map. The model provides a good fit to the data; however, it overpredicts the temperature of the nightside. We speculate that this could be due to the presence of clouds on the nightside of the planet, or additional reflected light from the dayside. When testing a simple cloud model we find that the best fitting model has a geometric albedo of \\(0.32 \\pm0.02\\) and does not require a hot nightside. We also test for variation of the map parameters as a function of wavelength and find no statistically significant correlations.

We measure wind velocities on opposite sides of the hot Jupiter HD\\(\\,\\)189733b by modeling sodium absorption in high-resolution HARPS transmission spectra. Our model implicitly accounts for the Rossiter-McLaughlin effect, which we show can explain the high wind velocities suggested by previous studies. Our results reveal a strong eastward motion of the atmosphere of HD\\(\\,\\)189733b, with a redshift of$2.3^{+1.3}_{-1.5}$ $\\,\\(km\\)\\,\\(s\\)^{-1}\\( on the leading limb of the planet and a blueshift of \\)5.3^{+1.0}_{-1.4}$ $\\,\\(km\\)\\,\\(s\\)^{-1}\\( on the trailing limb. These velocities can be understood as a combination of tidally locked planetary rotation and an eastward equatorial jet; closely matching the predictions of atmospheric circulation models. Our results show that the sodium absorption of HD\\)\\,$ 189733b is intrinsically velocity broadened and so previous studies of the average transmission spectrum are likely to have overestimated the role of pressure and thermal broadening.

The atmosphere of the exoplanet HD 209458b is undergoing sustained mass loss, believed to be caused by X-ray and extreme-ultraviolet (XUV) irradiation from its star. The majority of this flux is not directly observable due to interstellar absorption, but is required in order to correctly model the photo-evaporation of the planet and photo-ionisation of the outflow. We present a recovered high energy spectrum for HD\\,209458 using a Differential Emission Measure (DEM) retrieval technique. We construct a model of the stellar corona and transition region for temperatures between 10\\(^{4.1}\\) and 10\\(^{8}\\) K which is constrained jointly by ultraviolet line strengths measured with the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST) and X-ray flux measurements from XMM-Newton. The total hydrogen ionising luminosity (\\(\\lambda < 912\\) \\AA) is found to be 10\\(^{28.26}\\) erg s\\(^{-1}\\), which is similar to the value for the mean activity level of the Sun. This luminosity is incompatible with energy limited mass loss rates estimated from the same COS dataset, even the lower bound requires an uncomfortably high energetic efficiency of >40%. However, our luminosity is compatible with early estimates of the mass loss rate of HD 209458b based on results from the HST Space Telescope Imaging Spectrograph (STIS). Precisely reconstructed XUV irradiation is a key input to determining mass loss rates and efficiencies for exoplanet atmospheres.

We determine the observability in transmission of inhomogeneous cloud cover on the limbs of hot Jupiters through post processing a general circulation model to include cloud distributions computed using a cloud microphysics model. We find that both the east and west limb often form clouds, but that the different properties of these clouds enhances the limb to limb differences compared to the clear case. Using JWST it should be possible to detect the presence of cloud inhomogeneities by comparing the shape of the transit lightcurve at multiple wavelengths because inhomogeneous clouds impart a characteristic, wavelength dependent signature. This method is statistically robust even with limited wavelength coverage, uncertainty on limb darkening coefficients, and imprecise transit times. We predict that the short wavelength slope varies strongly with temperature. The hot limb of the hottest planets form higher altitude clouds composed of smaller particles leading to a strong rayleigh slope. The near infrared spectral features of clouds are almost always detectable, even when no spectral slope is visible in the optical. In some of our models a spectral window between 5 and 9 microns can be used to probe through the clouds and detect chemical spectral features. Our cloud particle size distributions are not log-normal and differ from species to species. Using the area or mass weighted particle size significantly alters the relative strength of the cloud spectral features compared to using the predicted size distribution. Finally, the cloud content of a given planet is sensitive to a species' desorption energy and contact angle, two parameters that could be constrained experimentally in the future.

We present analysis of XMM-Newton Optical Monitor observations in the near-ultraviolet of HD 189733, covering twenty primary transits of its hot Jupiter planet. The transit is clearly detected with both the UVW2 and UVM2 filters, and our fits to the data reveal transit depths in agreement with that observed optically. The measured depths correspond to radii of \\(1.059^{+0.046}_{-0.050}\\) and \\(0.94^{+0.15}_{-0.17}\\) times the optically-measured radius (1.187 R\\(_{\\rm J}\\) at 4950 Å) in the UVW2 and UVM2 bandpasses, respectively. We also find no statistically significant variation in the transit depth across the 8 year baseline of the observations. We rule out extended broadband absorption towards or beyond the Roche lobe at the wavelengths investigated, although observations with higher spectral resolution are required to determine if absorption out to those distances from the planet is present in individual near-UV lines.

We present a ground-based transmission spectrum and comprehensive retrieval analysis of the highly inflated Saturn-mass planet WASP-39b. We obtained low-resolution spectra (\\(R \\approx 400\\)) of a transit of WASP-39b using the ACAM instrument on the 4.2m William Herschel Telescope as part of the LRG-BEASTS survey. Our transmission spectrum is in good agreement with previous ground- and space-based observations of WASP-39b, and covers a wavelength range of 4000-9000A. Previous analyses of this exoplanet have retrieved water abundances that span more than four orders of magnitude, which in turn lead to conclusions of a subsolar or highly supersolar atmospheric metallicity. In order to determine the cause of the large discrepancies in the literature regarding WASP-39b's atmospheric metallicity, we performed retrieval analyses of all literature data sets. Our retrievals, which assume equilibrium chemistry, recovered highly supersolar metallicities for all data sets. When running our retrievals on a combined spectrum, spanning 0.3-5\\(\\mu\\)m, we recovered an atmospheric metallicity of \\(282^{+65}_{-58} \\times\\) solar. We find that stellar activity has a negligible effect on the derived abundances and instead conclude that different assumptions made during retrieval analyses lead to the reported water abundances that differ by orders of magnitude. This in turn has significant consequences for the conclusions we draw. This is the fourth planet to be observed as part of the LRG-BEASTS survey, which is demonstrating that 4m class telescopes can obtain low-resolution transmission spectra with precisions of around one atmospheric scale height.

We present a new ground-based optical transmission spectrum of the ultrahot Jupiter WASP-103b (\\(T_{eq} = 2484\\)K). Our transmission spectrum is the result of combining five new transits from the ACCESS survey and two new transits from the LRG-BEASTS survey with a reanalysis of three archival Gemini/GMOS transits and one VLT/FORS2 transit. Our combined 11-transit transmission spectrum covers a wavelength range of 3900--9450A with a median uncertainty in the transit depth of 148 parts-per-million, which is less than one atmospheric scale height of the planet. In our retrieval analysis of WASP-103b's combined optical and infrared transmission spectrum, we find strong evidence for unocculted bright regions (\\(4.3\\sigma\\)) and weak evidence for H\\(_2\\)O (\\(1.9\\sigma\\)), HCN (\\(1.7\\sigma\\)), and TiO (\\(2.1\\sigma\\)), which could be responsible for WASP-103b's observed temperature inversion. Our optical transmission spectrum shows significant structure that is in excellent agreement with the extensively studied ultrahot Jupiter WASP-121b, for which the presence of VO has been inferred. For WASP-103b, we find that VO can only provide a reasonable fit to the data if its abundance is implausibly high and we do not account for stellar activity. Our results highlight the precision that can be achieved by ground-based observations and the impacts that stellar activity from F-type stars can have on the interpretation of exoplanet transmission spectra.

We present an XMM-Newton X-ray observation of TRAPPIST-1, which is an ultracool dwarf star recently discovered to host three transiting and temperate Earth-sized planets. We find the star is a relatively strong and variable coronal X-ray source with an X-ray luminosity similar to that of the quiet Sun, despite its much lower bolometric luminosity. We find L_x/L_bol=2-4x10^-4, with the total XUV emission in the range L_xuv/L_bol=6-9x10^-4, and XUV irradiation of the planets that is many times stronger than experienced by the present-day Earth. Using a simple energy-limited model we show that the relatively close-in Earth-sized planets, which span the classical habitable zone of the star, are subject to sufficient X-ray and EUV irradiation to significantly alter their primary and any secondary atmospheres. Understanding whether this high-energy irradiation makes the planets more or less habitable is a complex question, but our measured fluxes will be an important input to the necessary models of atmospheric evolution.