The first transiting planetesimal orbiting a white dwarf was recently detected in K2 data of WD 1145+017 and has been followed up intensively. The multiple, long and variable transits suggest the transiting objects are dust clouds, probably produced by a disintegrating asteroid. In addition, the system contains circumstellar gas, evident by broad absorption lines, mostly in the u' band, and a dust disc, indicated by an infrared excess. Here we present the first detection of a change in colour of WD 1145+017 during transits, using simultaneous multiband fast-photometry ULTRACAM measurements over the u'g'r'i' bands. The observations reveal what appears to be 'bluing' during transits; transits are deeper in the redder bands, with a u' - r' colour difference of up to similar to-0.05 mag. We explore various possible explanations for the bluing, including limb darkening or peculiar dust properties. 'Spectral' photometry obtained by integrating over bandpasses in the spectroscopic data in and out of transit, compared to the photometric data, shows that the observed colour difference is most likely the result of reduced circumstellar absorption in the spectrum during transits. This indicates that the transiting objects and the gas share the same line of sight and that the gas covers the white dwarf only partially, as would be expected if the gas, the transiting debris and the dust emitting the infrared excess are part of the same general disc structure (although possibly at different radii). In addition, we present the results of a week-long monitoring campaign of the system using a global network of telescopes.

Avec la découverte d'anciens réseaux de rivières sur Mars et la détection de planètes telluriques tempérées autour d'étoiles voisines, nous disposons à présent d'un terrain de jeu formidable pour explorer si la vie est abondante ou rare dans l'Univers. Mon travail de thèse vise à mieux comprendre les conditions dans lesquelles une planète peut maintenir de l'eau liquide - substrat essentiel de la vie - à sa surface. À l'aide de modèles numériques de climat 3-D, et de calculs et mesures spectroscopiques, j'ai mené pendant ma thèse deux enquêtes. Premièrement, j'ai exploré les climats passés de Mars, pour comprendre comment se sont formées les énigmatiques rivières martiennes. À part la Terre, Mars est la seule planète qui a été habitable, mais nous ne savons toujours pas pourquoi. J'ai montré que les évènements extrêmes (formation des vallées de débâcle, impacts de météorites) qui ont pourtant profondément marqué la surface de Mars ne peuvent pas expliquer à eux seuls la formation de ces réseaux fluviaux. Mes travaux de thèse ont également permis d'établir que la présence de gaz à effet de serre réduits (hydrogène, méthane) offre une solution alternative prometteuse. Deuxièmement, j'ai étudié les atmosphères possibles des exoplanètes solides et tempérées, notamment celles orbitant autour de petites étoiles comme Proxima et TRAPPIST-1. J'ai montré que certaines de ces planètes ont des caractéristiques très favorables à la présence d'eau liquide en surface. Ce résultat est d'autant plus prometteur qu'il sera possible - comme démontré dans ma thèse pour le cas de la planète Proxima b - de caractériser l'atmosphère de ces planètes avec les futurs télescopes JWST et ELTs. / Ancient rivers and lakes discovered on Mars. Numerous temperate, Earth-sized exoplanets detected around nearby stars. Thanks to ground and space-based telescope observations and Solar System exploration missions, we now have a fantastic playground to explore how prevalent life is in the Universe. The main goal of my thesis work is to better understand the conditions necessary for a planet to maintain liquid water - a primary building block for life - on its surface. Using 3-D numerical climate models, as well as spectroscopic calculations and measurements, I conducted two major investigations during my thesis. First, I explored the environments of ancient Mars at multiple epochs in order to understand the conditions in which the enigmatic Martian rivers were carved. Apart from Earth, Mars is the only planet that has been habitable, but we don't know why. I showed that extreme events (outflow channel formation, meteoritic impacts) that scarred the surface of Mars cannot explain the formation of these valley networks. Nonetheless, I showed that the presence of reducing greenhouse gases such as hydrogen and methane offers a promising alternative solution. Secondly, I studied the possible atmospheres of solid, temperate exoplanets, with a particular focus on those orbiting small stars such as Proxima Centauri and TRAPPIST-1. I showed that some of these planets have characteristics that are highly favourable to the presence of liquid water on their surface. This result is really promising as it will be soon become possible - as demonstrated in my thesis for Proxima b - to characterize the atmosphere of these planets with the future JWST and ELTs astronomical observatories.

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This thesis presents algorithms for performing the next two reduction steps, namely orthogonalization and extraction. More specifically, this thesis addresses the following research question: What are proper methods of orthogonalizing spectral images in preparation for extraction?

We carried out a near-infrared radial velocity search for Jupiter-mass planets around 36 late M dwarfs. This survey was the first of its kind undertaken to monitor radial velocity variability of these faint dwarfs. For this unique survey we employed the 10-m Keck II on Mauna Kea in Hawaii. With a resolution of 20,000 on the near-infrared spectrograph, NIRSPEC, we monitored these stars over four epochs in 2007. In addition to the measurement of relative radial velocity we established physical properties of these stars. The physical properties of M dwarfs we determined included the identification of neutral atomic lines, the measurement of pseudo-equivalent widths, masses, surface gravity, effective temperature, absolute radial velocities, rotational velocities and rotation periods. The identification of neutral atomic lines was carried out using the Vienna Atomic line Database. We were able to confirm these lines that were previously identified. We also found that some of the lines observed in the K-type stars, such as Mg I though weak, still persist in late M dwarfs. Using the measurement of pseudo-equivalent widths (p-EW) of 13 neutral atomic lines, we have established relations between p-EW and spectral type. Such relations serve as a tool in determining the spectral type of an unknown dwarf star by means of measuring its p-EW. We employed the mass-luminosity relation to compute the masses of M dwarfs. Our calculations indicate these dwarfs to be in the range of 0.1 to 0.07 solar masses. This suggests that some of the late M dwarfs appear to be in the Brown dwarf regime. Assuming their radii of 0.1 solar radii, we calculated their surface gravity. The mean surface gravity is, log g = 5.38. Finally their effective temperature was determined by using the spectral-type iii temperature relationship. Our calculations show effective temperatures in the range of 3000 2300 K. Comparison of these values with models in literature show a good agreement. The absolute radial and rotational velocities of our targets were also calculated. Values of rotational velocities indicate that M dwarfs are, in general, slow rotators. Using our result and that from literature, we extended our study of rotational velocities to L dwarfs. Our observations show an increase in rotational velocities from late M to L dwarfs. We also find that the mean periods of M dwarfs are less than 10 hours. In order to improve our precision in measuring relative radial velocity (RV), we employed the use of deconvolution method. With this method we were able to ameliorate relative RV precision from 300 m/s to 200 m/s. This was a substantial improvement in our ability to detect gas-giant planets. However none of the 15 dwarfs we monitored indicate a presence of companions. This null result was then used to compute the upper limit to the binary frequency and close-in Jupiter mass planetary frequency. We find the binary frequency to be 11% while the planetary frequency was 1.20%.

Exploring the Planets (http://explanet.info) is a free online college textbook covering thebasic concepts of planetary science emphasizing the character and evolution of the planetarybodies in the Solar System. The latest edition (3rd edition) was published online in 2007 by EricH Christiansen. Since the release of the third edition, two important planetary missions havebeen completed: MESSENGER (to Mercury) and New Horizons (to Pluto). These missionsprovided new information and fundamental insights into these planetary bodies, which have notyet been included in Exploring the Planets. The modern results based on recent investigations ofMercury and Pluto are critical for our understanding of the nature and history of these bodies andthe Solar System and build upon the previous information on Mercury and Pluto gained fromMariner 10 (1974-1975) and the Hubble Space Telescope, respectively. These two planetarybodies are end members in a spectrum of objects in the Solar System. Mercury is small, hot,dense, and a silicate metal rich end member of the planets, helping scientists understand thethermal and accretionary evolution of the terrestrial planets. Pluto is cold, icy, distant from theSun, and a representative object of the vast Kuiper Belt, and is thus another end member amongplanetary bodies. These two bodies refine models of how different planets will evolve over time,and how our Solar System has evolved. For these reasons, it is important to update Exploring thePlanets to summarize the current understanding of the geology of Mercury and Pluto. This way,students can better understand their formation and evolution and the implications for theevolution of our Solar System.

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Le système TRAPPIST-1 offre une opportunité sans précédent de caractériser les premières planètes potentiellement habitables en dehors de notre système solaire. Dans ce mémoire est décrit le développement d’un pipeline de réduction de données personnalisé pour le mode WIRC+Spec de la caméra infrarouge à grand champ récemment mise à niveau à l’observatoire Palomar. Nous introduisons une nouvelle approche d’ajustement de la fonction d’étalement du point basée sur la transformation de Karhunen-Loève pour extraire des courbes de lumière photométrique et spectroscopique de sources de forme irrégulière, que nous appliquons aux observations de l’exoplanète TRAPPIST-1d pour mesurer ses spectres de transmission dans les bandes J (1.1 à 1.4 µm) et Ks (1.95 à 2.35 µm). Un guide détaillé est présenté pour l’implémentation d’un calcul de profils de température incluant l’équilibre radiatif et convectif pour une modélisation atmosphérique efficace et précise. En comparant une multitude de scénarios atmosphériques aux observations de TRAPPIST-1d, nous obtenons des contraintes sur la composition et la structure de son atmosphère, excluant les scénarios sans nuages avec des métallicités inférieures à 300 fois la valeur solaire à 3σ. / The TRAPPIST-1 system provides an unprecedented opportunity to characterize the first potentially habitable planets outside our solar system. In this work we describe the development of a custom data reduction pipeline for the WIRC+Spec mode of the recently upgraded Wide Field Infrared Camera instrument on Palomar Observatory. We introduce a novel, Karhunen-Loève transform based approach to extract photometric and spectroscopic light curves from irregularly shaped sources, which we apply to observations of the TRAPPIST-1d exoplanet to measure the J band (1.1 to 1.4 µm) and Ks band (1.95 to 2.35 µm) transmission spectra. We also present a detailed guide into the implementation of a self-consistent, radiative-convective temperature structure calculation for efficient and accurate atmospheric modelling. Comparing a host of atmospheric scenarios to the observations of TRAPPIST-1d to date, we constrain its atmosphere, ruling out cloud-free atmospheres with metallicities lower than 300 times the solar value at 3σ confidence.