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Many hot Jupiters have been discovered from the ground and Kepler discovered many exoplanets around fainter stars, but searches push for more smaller planets around bright stars. Transit photometry also relies on knowledge of host star properties, less certain for later spectral types. This thesis aimed to investigate detecting planets smaller than Jupiter around bright, later spectral types and to help better understand M dwarf properties. NGTS aims to discover Neptune-sized exoplanets around K and M dwarfs. In Chapter 3, I investigated the brightest stars, saturated in the standard pipeline. While relatively few, these targets are highly valuable. I investigated whether saturated star fluxes could be recovered by using custom apertures, finding tailored rectangular apertures captured them well. I show the difference in noise between bright unsaturated stars and saturated stars is reduced using rectangular apertures rather than the standard pipeline, and is the same for a number of cases. However, to perform saturated star photometry, NGTS needs to change its operational gain setting, to allow charge to be conserved. Earth-sized habitable zone exoplanets are easier to detect around later spectral types as smaller, cooler stars are more favourable for transit photometry. However, their exoplanets are difficult to find in wide-field transit surveys due to their inherent comparative faintness. For this reason, I conducted a targeted survey of mid-late M dwarfs, presented in Chapter 4. Six relatively bright M dwarfs with spectral types M5−M8 were intensely observed, aiming for 50% phase coverage into their habitable zones. Some late M dwarfs also exhibit high flaring frequencies, so for the highest flaring rate target, I investigated the habitability for any habitable zone planets, finding them likely uninhabitable. Stellar properties are a large source of potential uncertainty in determining exoplanet properties, especially for mid-to-late M dwarfs, where stellar evolutionary models show inconsistency with observed data. Testing these models requires independently determined parameters, which is possible using EBLMs from transit surveys. In Chapter 5, I fit two EBLMs from SuperWASP. I find both secondary stars sit close to the bottom of the main sequence, where there are few other measured stars. However, properties of the higher mass (primary) star can still have large uncertainties, affecting uncertainties for the secondary star, and values vary depending on how primary star mass and radius are determined. Overall, this thesis shows some ongoing areas of interest within transit surveys. I determined how saturated star fluxes in NGTS could be used to search for valuable transiting exoplanets around bright stars. I conducted a survey of relatively bright mid-to-late M dwarfs specifically searching for Earth-sized exoplanets within their habitable zone. While no exoplanets were found, transit injections showed that we were sensitive to these planet sizes around at least some of our targets. I also fitted two low mass eclipsing binaries with the aim of better informing low mass stellar evolutionary models. I find that both secondary stars have potential but increased precision on their primary star properties is still needed.

About 1 out of 200 Sun-like stars has a planet with an orbital period shorter than one day: an ultrashort-period planet 1 , 2 . All of the previously known ultrashort-period planets are either hot Jupiters, with sizes above 10 Earth radii ( R ⊕ ), or apparently rocky planets smaller than 2  R ⊕ . Such lack of planets of intermediate size (the ‘hot Neptune desert’) has been interpreted as the inability of low-mass planets to retain any hydrogen/helium (H/He) envelope in the face of strong stellar irradiation. Here we report the discovery of an ultrashort-period planet with a radius of 4.6  R ⊕ and a mass of 29  M ⊕ , firmly in the hot Neptune desert. Data from the Transiting Exoplanet Survey Satellite 3 revealed transits of the bright Sun-like star LTT 9779 every 0.79 days. The planet’s mean density is similar to that of Neptune, and according to thermal evolution models, it has a H/He-rich envelope constituting 9.0 − 2.9 + 2.7 % of the total mass. With an equilibrium temperature around 2,000 K, it is unclear how this ‘ultrahot Neptune’ managed to retain such an envelope. Follow-up observations of the planet’s atmosphere to better understand its origin and physical nature will be facilitated by the star’s brightness ( V mag  = 9.8). LTT 9779 b is Neptune-sized planet rotating around its star with a period of 0.79 days and an equilibrium temperature of 2,000 K. It is not clear how it retained its atmospheric envelope, which contains ~10% of H/He, as it should have been photoevaporated by now.

We report the discovery of a new ultra-short period transiting hot Jupiter from the Next Generation Transit Survey (NGTS). NGTS-10b has a mass and radius of \\(2.162\\,^{+0.092}_{-0.107}\\) M\\(_{\\rm J}\\) and \\(1.205\\,^{+0.117}_{-0.083}\\) R\\(_{\\rm J}\\) and orbits its host star with a period of \\(0.7668944\\pm0.0000003\\) days, making it the shortest period hot Jupiter yet discovered. The host is a \\(10.4\\pm2.5\\) Gyr old K5V star (\\(T_\\mathrm{eff}\\)=\\(4400\\pm100\\)\\,K) of Solar metallicity ([Fe/H] = \\(-0.02\\pm0.12\\)\\,dex) showing moderate signs of stellar activity. NGTS-10b joins a short list of ultra-short period Jupiters that are prime candidates for the study of star-planet tidal interactions. NGTS-10b orbits its host at just \\(1.46\\pm0.18\\) Roche radii, and we calculate a median remaining inspiral time of \\(38\\)\\,Myr and a potentially measurable transit time shift of \\(7\\)\\,seconds over the coming decade, assuming a stellar tidal quality factor \\(Q'_{\\rm s}=2\\times10^{7}\\).

About one out of 200 Sun-like stars has a planet with an orbital period shorter than one day: an ultra-short-period planet (Sanchis-ojeda et al. 2014; Winn et al. 2018). All of the previously known ultra-short-period planets are either hot Jupiters, with sizes above 10 Earth radii (Re), or apparently rocky planets smaller than 2 Re. Such lack of planets of intermediate size (the \"hot Neptune desert\") has been interpreted as the inability of low-mass planets to retain any hydrogen/helium (H/He) envelope in the face of strong stellar irradiation. Here, we report the discovery of an ultra-short-period planet with a radius of 4.6 Re and a mass of 29 Me, firmly in the hot Neptune desert. Data from the Transiting Exoplanet Survey Satellite (Ricker et al. 2015) revealed transits of the bright Sun-like star \\starname\\, every 0.79 days. The planet's mean density is similar to that of Neptune, and according to thermal evolution models, it has a H/He-rich envelope constituting 9.0^(+2.7)_(-2.9)% of the total mass. With an equilibrium temperature around 2000 K, it is unclear how this \"ultra-hot Neptune\" managed to retain such an envelope. Follow-up observations of the planet's atmosphere to better understand its origin and physical nature will be facilitated by the star's brightness (Vmag=9.8).

We report the discovery of a new ultra-short period hot Jupiter from the Next Generation Transit Survey. NGTS-6b orbits its star with a period of 21.17~h, and has a mass and radius of \\(1.330^{+0.024}_{-0.028}\\)\\mjup\\, and \\(1.271^{+0.197}_{-0.188}\\)\\rjup\\, respectively, returning a planetary bulk density of 0.711\\(^{+0.214}_{-0.136}\\)~g~cm\\(^{-3}\\). Conforming to the currently known small population of ultra-short period hot Jupiters, the planet appears to orbit a metal-rich star ([Fe/H]\\(=+0.11\\pm0.09\\)~dex). Photoevaporation models suggest the planet should have lost 5\\% of its gaseous atmosphere over the course of the 9.6~Gyrs of evolution of the system. NGTS-6b adds to the small, but growing list of ultra-short period gas giant planets, and will help us to understand the dominant formation and evolutionary mechanisms that govern this population.

We report the discovery of Qatar-6b, a new transiting planet identified by the Qatar Exoplanet Survey (QES). The planet orbits a relatively bright (V=11.44), early-K main-sequence star at an orbital period of P~3.506 days. An SED fit to available multi-band photometry, ranging from the near-UV to the mid-IR, yields a distance of d = 101 +/- 6 pc to the system. From a global fit to follow-up photometric and spectroscopic observations, we calculate the mass and radius of the planet to be Mp = 0.67 +/- 0.07 Mjup and Rp = 1.06 +/- 0.07 Rjup, respectively. We use multi-color photometric light curves to show that the transit is grazing, making Qatar-6b one of the few exoplanets known in a grazing transit configuration. It adds to the short list of targets that offer the best opportunity to look for additional bodies in the host planetary system through variations in the transit impact factor and duration.

We report the discovery of NGTS-4b, a sub-Neptune-sized planet transiting a 13th magnitude K-dwarf in a 1.34d orbit. NGTS-4b has a mass M=\\(20.6\\pm3.0\\)M_E and radius R=\\(3.18\\pm0.26\\)R_E, which places it well within the so-called \"Neptunian Desert\". The mean density of the planet (\\(3.45\\pm0.95\\)g/cm^3) is consistent with a composition of 100% H\\(_2\\)O or a rocky core with a volatile envelope. NGTS-4b is likely to suffer significant mass loss due to relatively strong EUV/X-ray irradiation. Its survival in the Neptunian desert may be due to an unusually high core mass, or it may have avoided the most intense X-ray irradiation by migrating after the initial activity of its host star had subsided. With a transit depth of \\(0.13\\pm0.02\\)%, NGTS-4b represents the shallowest transiting system ever discovered from the ground, and is the smallest planet discovered in a wide-field ground-based photometric survey.

We present the discovery of NGTS-1b, a hot-Jupiter transiting an early M-dwarf host (\\(T_{eff}=3916^{+71}_{-63}~K\\)) in a P=2.674d orbit discovered as part of the Next Generation Transit Survey (NGTS). The planet has a mass of \\(0.812^{+0.066}_{-0.075}~M_{J}\\), making it the most massive planet ever discovered transiting an M-dwarf. The radius of the planet is \\(1.33^{+0.61}_{-0.33}~R_{J}\\). Since the transit is grazing, we determine this radius by modelling the data and placing a prior on the density from the population of known gas giant planets. NGTS-1b is the third transiting giant planet found around an M-dwarf, reinforcing the notion that close-in gas giants can form and migrate similar to the known population of hot Jupiters around solar type stars. The host star shows no signs of activity, and the kinematics hint at the star being from the thick disk population. With a deep (2.5%) transit around a \\(K=11.9\\) host, NGTS-1b will be a strong candidate to probe giant planet composition around M-dwarfs via JWST transmission spectroscopy.

We describe the Next Generation Transit Survey (NGTS), which is a ground-based project searching for transiting exoplanets orbiting bright stars. NGTS builds on the legacy of previous surveys, most notably WASP, and is designed to achieve higher photometric precision and hence find smaller planets than have previously been detected from the ground. It also operates in red light, maximising sensitivity to late K and early M dwarf stars. The survey specifications call for photometric precision of 0.1 per cent in red light over an instantaneous field of view of 100 square degrees, enabling the detection of Neptune-sized exoplanets around Sun-like stars and super-Earths around M dwarfs. The survey is carried out with a purpose-built facility at Cerro Paranal, Chile, which is the premier site of the European Southern Observatory (ESO). An array of twelve 20cm f/2.8 telescopes fitted with back-illuminated deep-depletion CCD cameras are used to survey fields intensively at intermediate Galactic latitudes. The instrument is also ideally suited to ground-based photometric follow-up of exoplanet candidates from space telescopes such as TESS, Gaia and PLATO. We present observations that combine precise autoguiding and the superb observing conditions at Paranal to provide routine photometric precision of 0.1 per cent in 1 hour for stars with I-band magnitudes brighter than 13. We describe the instrument and data analysis methods as well as the status of the survey, which achieved first light in 2015 and began full survey operations in 2016. NGTS data will be made publicly available through the ESO archive.