We analyze archival data from Bailey and co-workers from the Magellan adaptive optics system and present the first 0.9 mu m detection (z' = 20.3 +/- 0.4 mag; Delta z' = 13.0 +/- 0.4 mag) of the 11 M-Jup circumbinary planet HD 106906AB b, as well as 1 and 3.8 mu m detections of the debris disk around the binary. The disk has an east-west asymmetry in length and surface brightness, especially at 3.8 mu m where the disk appears to be one-sided. The spectral energy distribution of b, when scaled to the K-S-band photometry, is consistent with 1800 K atmospheric models without significant dust reddening, unlike some young, very red, low-mass companions such as CT Cha B and 1RXS 1609 B. Therefore, the suggested circumplanetary disk of Kalas and co-workers might not contain much material, or might be closer to face-on. Finally, we suggest that the widest (a greater than or similar to 100 AU) low mass ratio (M-p/M-star = q less than or similar to 0.01) companions may have formed inside protoplanetary disks but were later scattered by binary/planet interactions. Such a scattering event may have occurred for HD 106906AB b with its central binary star, but definitive proof at this time is elusive.

We present spectrophotometry spanning 1-5 mu m of 51 Eridani b, a 2-10 M-Jup planet discovered by the Gemini Planet Imager Exoplanet Survey. In this study, we present new K1 (1.90-2.19 mu m) and K2 (2.10-2.40 mu m) spectra taken with the Gemini Planet Imager as well as an updated L-P (3.76 mu m) and new M-S (4.67 mu m) photometry from the NIRC2 Narrow camera. The new data were combined with J (1.13-1.35 mu m) and H (1.50-1.80 mu m) spectra from the discovery epoch with the goal of better characterizing the planet properties. The 51 Eri b photometry is redder than field brown dwarfs as well as known young T-dwarfs with similar spectral type (between T4 and T8), and we propose that 51 Eri b might be in the process of undergoing the transition from L-type to T-type. We used two complementary atmosphere model grids including either deep iron/silicate clouds or sulfide/salt clouds in the photosphere, spanning a range of cloud properties, including fully cloudy, cloud-free, and patchy/intermediate-opacity clouds. The model fits suggest that 51 Eri. b has an effective temperature ranging between 605 and 737 K, a solar metallicity, and a surface gravity of log(g) = 3.5-4.0 dex, and the atmosphere requires a patchy cloud atmosphere to model the spectral energy distribution (SED). From the model atmospheres, we infer a luminosity for the planet of -5.83 to -5.93 (logL/L circle dot),leaving 51 Eri b in the unique position of being one of the only directly imaged planets consistent with having formed via a cold-start scenario. Comparisons of the planet SED against warm-start models indicate that the planet luminosity is best reproduced by a planet formed via core accretion with a core mass between 15 and 127 M-circle plus.

One of the highest resolution astronomical images ever taken in the visible were obtained by combining the techniques of adaptive optics and lucky imaging. The Adaptive Optics Lucky Imager (AOLI), being developed at Cambridge as part of a European collaboration, combines these two techniques in a dedicated instrument for the first time. The instrument is designed initially for use on the 4.2m William Herschel Telescope (WHT) on the Canary Island of La Palma. This thesis describes the development of AOLI, in particular the adaptive optics system and a new type of wavefront sensor, the non-linear curvature wavefront sensor (nlCWFS), being used within the instrument. The development of the nlCWFS has been the focus of my work, bringing the technique from a theoretical concept to physical realisation at the WHT in September 2013. The non-linear curvature wavefront sensor is based on the technique employed in the conventional curvature wavefront sensor where two image planes are located equidistant either side of a pupil plane. Two pairs of images are employed in the nlCWFS providing increased sensitivity to both high- and low- order wavefront distortions. This sensitivity is the reason the nlCWFS was selected for use with AOLI as it will provide significant sky-coverage using natural guide stars alone, mitigating the need for laser guide stars. This thesis is structured into three main sections; the first introduces the non-linear curvature wavefront sensor, the relevant background and a discussion of simulations undertaken to investigate intrinsic effects. The iterative reconstruction algorithm required for wavefront reconstruction is also introduced. The second section discusses the practical implementation of the nlCWFS using two demonstration systems as the precursor to the optical design used at the WHT and includes details of subsequent design changes. The final section discusses data from both the WHT and a laboratory setup developed at Cambridge following the observing run. The long-term goal for AOLI is to undertake science observations on the 10.4m Gran Telescopio Canarias, the world's largest optical telescope. The combination of AO and lucky imaging, when used on this telescope, will provide resolutions a factor of two higher than ever before achieved at visible wavelengths. This offers the opportunity to probe the Cosmos in unprecedented detail and has the potential to significantly advance our understanding of the Universe.

La formation, l’évolution et la structure des planètes géantes font parties des grandesproblématiques de l’astrophysique moderne. Les planète géantes ont un rôle majeur carelles possèdent la plupart de la masse des systèmes planétaires et donc influencent leursévolutions dynamiques. Mon travail de thèse s’inscrit dans une démarche observationnellequi est essentielle pour apporter des contraintes sur la diversité des systèmes exoplanétaires.Mes premiers résultats de thèse sont issus d’une campagne d’observations sur trois ansréalisées avec l’instrument NaCo au VLT. Mes observations de HD142527 excluent laprésence d’une planète géante dans le disque et favoriseraient plutôt un système multiplede faible masse pour expliquer les structures de ce disque de transition. J’ai égalementdétecté une planète géante autour de HD95086. Cette planète possède des propriétés atmosphériquesparticulières. Sa présence fait de HD95086 un rare exemple de systèmesimagés possédant un disque de débris et une planète géante. Enfin, j’ai réalisé une étudestatistique sur l’ensemble du relevé et montré que les planètes géantes sur des orbiteséloignées sont rares (10 − 20 %) et ne peuvent pas s’être formées majoritairement pareffondrement direct du gaz dans un disque instable.La dernière partie de mon travail de thèse a été consacrée à l’étude du mode d’imageriedifférentielle simultanée spectrale. J’ai identifié les biais particuliers liés à la réductionde ce type de données et ai montré que leurs analyses nécessitent l’utilisation de modèlesd’évolution. Cette étude permettra d’exploiter les données de l’instrument IRDISde SPHERE installé au VLT. / How giant exoplanet form, evolve and are made of is one of the biggest challenge of modernastronomy. They play an important role as they carry most of the planetary systemmasses. Therefore, they strongly impact their dynamics and the fate of these systems tohost life. My PhD thesis falls within an observational approach that is mandatory to bringconstraints on the diversity of planetary systems.I got my first results from a three-year survey, with NaCo at VLT. My observations ofHD142527 excluded the presence of massive giants planets to explain the structures of thistransitional disk and might favor a light multiple system. I also detected a giant planetaround HD95086 and I showed that it has particular atmospheric properties. Finally, Icarried out a statistical analysis of the whole survey to show that giant planets on wideorbitsare rare (10 − 20 %) and could not be formed via direct collapse of unstable disks.I dedicated the last part of my work to investigate the spectral simultaneous differentialimaging mode. I pointed out the biases associated to the reduction of these data andshowed that evolutionary models have to be used to analyze them. This study might helpto exploit the full potential of SPHERE/IRDIS data.