This thesis provides a multi-pronged approach towards paving the way for future space and ground based exoplanet characterisation e↵orts as well as providing new analysis of the atmosphere of the exoplanet HD 179949 b. This is done, firstly, by outlining engineering trade studies conducted for the attitude and orbit control system (AOCS) and sun shield for the Exoplanet Characterisation Observatory (EChO) spacecraft (a proposed European Space Agency exoplanet space mission). These trade studies were conducted in collaboration with EADS Astrium. A cold gas system with the possibility of a hybrid system which would include the use of reaction wheels is recommend for the design of the AOCS. For the sun shield, a V-groove cone shield is concluded to provide the best thermal coverage while also providing stay light protection as well as being more mechanically symmetric than other options. Simulations are then conducted to determine the number of transiting planets future surveys should expect to find around stars within 50 parsecs of the sun. This is done by taking the known stars within 50 parsecs and adding a simulated planet population based on current models and observations to each star. Assumptions are made regarding observability of a planetary transit and a Monte Carlo simulation run to gain statistics on the number and type of planetary systems that can be expected to be found. The results of the simulation show a mean expected number of 27 detectable transiting planets within 50 parsecs. Next, using the Position and Proper Motion Extended-L (PPMXL) catalogue, optical and near-infrared colour cuts were used together with a reduced proper motion cut to find bright M dwarfs for future exoplanet transit studies. PPMXL’s low proper motion uncertainties allow this work to probe down to smaller proper motions than previous similar studies. Unique objects found with this method were combined with that of previous work to produce 8479 K < 9 M dwarfs. Low-resolution spectroscopy was obtained of a sample of the objects found using this selection method to gain statistics on their spectral type and physical properties. Results show a spectral-type range of K7-M4V. This catalogue is the most complete collection of K < 9 M dwarfs currently available and is made available here. High resolution spectroscopy and model spectra of planetary atmospheres is then used along with a spectral deconvolution technique to attempt to detect the Doppler shifted signal of the non-transiting planet HD 179949 b. The signal was not detected but new upper limits were set ruling out the presence of TiO down to a log10 ✏0 = -4.09 with 99.9 per cent confidence. Simulations conducted by this work imply a loss of sensitivity occurring possibly due to varying telluric interference or instrumental systematics.

The cloud and haze properties in the troposphere and stratosphere of Saturn are investigated in this thesis by analysing Cassini/VIMS spectra at 0.8-3.5 μm and 4.5-5.1 μm. The aerosol properties are derived from VIMS data by using the retrieval tool NEMESIS developed at the University of Oxford. Near-infared VIMS data between 4.5 and 5.1 μm are mainly sensitive to deeper tropospheric levels down to approximately 5 bar. At such pressure levels, thermochemical models predict the formation of condensed clouds made of ammonia and ammonium hydrosulphide ices, although none of these species has been spectrally detected so far. In addition, phosphine and ammonia are responsible for most of the gaseous absorption at these wavelengths. Therefore, the cloud properties and gas distributions can be retrieved from VIMS near-infared spectra. In this thesis, the analysis of limb-darkening data at 4.5-5.1 μm is performed in order to constrain the aerosol properties in Saturn’s atmosphere. The best-fitting model consists of a scattering cloud between 2 and 3 bar, and a scattering haze layer which can be placed anywhere between 10 and 500 mbar. The composition is still poorly constrained for both the deep cloud and haze layer. The haze physical and optical properties can however be independently retrieved from VIMS near-infrared spectra at shorter wavelengths in the 0.8-3.5 μm spectral range. Saturn’s hazes in the troposphere and stratosphere reflect the sunlight at 0.8-3.5 μm. The properties and vertical structure of tropospheric and stratospheric hazes are then investigated from VIMS reflection spectra in the near-infared. The latitudinal variation of haze properties is compared to a thermal feature known as "the temperature knee", corresponding to a local increase of temperature right below the tropopause. The north-south temperature variations in the troposphere are obtained from the analysis of thermal infrared data measured with the Composite InfraRed Spectromete (CIRS) instrument on board Cassini. Finally, VIMS near-infrared data at 0.8-3.5 and 4.5-5.1 μm are combined in order to obtain a cloud and haze model coherent with both wavelength ranges.

This thesis investigates how increased global mean temperatures on Earth, induced by the increase in the luminosity of the Sun as it ages, change the types of habitable environments on the planet at local scales over the next 3 Gyr. Rising temperatures enhance silicate weathering rates, reducing atmospheric CO₂ levels to below the threshold for photosynthesis, while simultaneously pushing environments past the temperature tolerances of plant and animal species. This leads to the end of all plant life and animal life (due to reduced food, O₂ and H₂O availability, as well as higher temperatures) within the next 1 Gyr. The reduction in the extent of the remaining microbial biosphere due to increasing temperatures and rapid ocean evaporation is then modelled, incorporating orbital parameter changes until all known types of life become extinct; a maximum of 2.8 Gyr from the present. The biosignatures associated with these changes are determined and the analysis extended to Earth-like extrasolar planets nearing the end of their habitable lifetimes. In particular, the stages in the main sequence evolutions of Sun-like stars within 10 pc are evaluated and used to extrapolate the stage that an Earth-analogue planet would be at in its habitable evolution, to determine the best candidate systems for a far-future Earth-analogue biosphere, highlighting the Beta Canum Venaticorum system as a good target. One of the most promising biosignatures for a microbial biosphere on the far-future Earth (and similar planets) may be CH₄, which could reach levels in the atmosphere that make it more readily detectable than it is for a present-day Earth-like atmosphere. Determining these biosignatures will help expand the search for life to the wider range of environments that will be found as the habitable exoplanet inventory grows and planets are found at different stages in their habitable evolution.

We present a primary transit observation for the ultra-hot (T-eq similar to 2400 K) gas giant expolanet WASP-121b, made using the Hubble Space Telescope Wide Field Camera 3 in spectroscopic mode across the 1.12-1.64 mu m wavelength range. The 1.4 mu m water absorption band is detected at high confidence (5.4 sigma) in the planetary atmosphere. We also reanalyze ground-based photometric light curves taken in the B, r', and z' filters. Significantly deeper transits are measured in these optical bandpasses relative to the near-infrared wavelengths. We conclude that scattering by high-altitude haze alone is unlikely to account for this difference and instead interpret it as evidence for titanium oxide and vanadium oxide absorption. Enhanced opacity is also inferred across the 1.12-1.3 mu m wavelength range, possibly due to iron hydride absorption. If confirmed, WASP-121b will be the first exoplanet with titanium oxide, vanadium oxide, and iron hydride detected in transmission. The latter are important species in M/L dwarfs and their presence is likely to have a significant effect on the overall physics and chemistry of the atmosphere, including the production of a strong thermal inversion.

The solid-metal/silicate-melt partition coefficient for W has been determined experimentally for the temperature and oxygen fugacity conditions at which eucritic basalts formed. The partition coefficient for W is 25 ± 5 at 1190°C and an oxygen fugacity of 10⁻¹³∙⁴. The solid-metal/silicate-melt partition coefficient for P, D(P), has been determined experimentally at 1190°C and 1300°C. The dependence of the partition coefficient on oxygen fugacity is consistent with a valence state of 5 for P in the silicate melt. The experimental partition coefficients are given by: (1) log D(P) = -1.21 log fO₂ -15.95 at 1190°C (2) log D(P) = -1.53 log fO₂ -17.73 at 1300°C The partition coefficients may be used to interpret the depletion of W/La and P/La ratios in the Earth, Moon, and eucrites relative to Cl chondrites. The depletion of the W/La ratios in the eucrites may be explained by partitioning of W into 2% to 10% solid metal assuming equilibration and separation of the metal from the silicates at low degrees of partial melting of the silicates. The depletion of P/La ratios requires an additional 5% to 25% sulfur-bearing metallic liquid. The depletion of both P/La and W/La ratios in the Moon can be explained by partitioning of P and W into liquid metal during formation of a small lunar core by metal-silicate separation at low degrees of partial melting of the silicates. The W/La ratios in the Earth and Moon are virtually indistinguishable, while P/La ratios differ by a factor of two. The concentrations of FeO also appear to be different. These observations are difficult to reconcile with the hypothesis of a terrestrial origin of the Moon following formation of the Earth's core, but are consistent with an independent formation of the Earth and Moon. In contrast to the Moon and eucrites, the depletion of P/La and W/La ratios in the Earth cannot be explained by an internally consistent model involving equilibrium between metal and silicate at low pressures.

We present a new velocity-resolved survey of 2.9 mu m spectra of hot H2O and OH gas emission from protoplanetary disks, obtained with the Cryogenic Infrared Echelle Spectrometer at the VLT (R similar to 96,000). With the addition of archival Spitzer-IRS spectra, this is the most comprehensive spectral data set of water vapor emission from disks ever assembled. We provide line fluxes at 2.9-33 mu m that probe from the dust sublimation radius at similar to 0.05 au out to the region of the water snow line. With a combined data set for 55 disks, we find a new correlation between H2O line fluxes and the radius of CO gas emission, as measured in velocity-resolved 4.7 mu m spectra (R-co), which probes molecular gaps in inner disks. We find that H2O emission disappears from 2.9 mu m (hotter water) to 33 mu m (colder water) as R-co increases and expands out to the snow line radius. These results suggest that the infrared water spectrum is a tracer of inside-out water depletion within the snow line. It also helps clarify an unsolved discrepancy between water observations and models by finding that disks around stars of M-star > 1.5M(circle dot) generally have inner gaps with depleted molecular gas content. We measure radial trends in H2O, OH, and CO line fluxes that can be used as benchmarks for models to study the chemical composition and evolution of planet-forming disk regions at 0.05-20 au. We propose that JWST spectroscopy of molecular-gas may be used as a probe of inner disk gas depletion, complementary to the larger gaps and holes detected by direct imaging and by ALMA.

The Planet Formation Imager (PFI) project aims to provide a strong scientific vision for ground-based optical astronomy beyond the upcoming generation of Extremely Large Telescopes. We make the case that a breakthrough in angular resolution imaging capabilities is required in order to unravel the processes involved in planet formation. PFI will be optimised to provide a complete census of the protoplanet population at all stellocentric radii and over the age range from 0.1 to similar to 100 Myr. Within this age period, planetary systems undergo dramatic changes and the final architecture of planetary systems is determined. Our goal is to study the planetary birth on the natural spatial scale where the material is assembled, which is the "Hill Sphere" of the forming planet, and to characterise the protoplanetary cores by measuring their masses and physical properties. Our science working group has investigated the observational characteristics of these young protoplanets as well as the migration mechanisms that might alter the system architecture. We simulated the imprints that the planets leave in the disk and study how PFI could revolutionise areas ranging from exoplanet to extragalactic science. In this contribution we outline the key science drivers of PFI and discuss the requirements that will guide the technology choices, the site selection, and potential science/technology tradeoffs.

Context. HR4796A is surrounded by a debris disc, observed in scattered light as an inclined ring with a high surface brightness. Past observations have raised several questions. First, a strong brightness asymmetry detected in polarised reflected light has recently challenged our understanding of scattering by the dust particles in this system. Secondly, the morphology of the ring strongly suggests the presence of planets, although no planets have been detected to date. Aims. We aim here at measuring with high accuracy the morphology and photometry of the ring in scattered light, in order to derive the phase function of the dust and constrain its near-infrared spectral properties. We also want to constrain the presence of planets and set improved constraints on the origin of the observed ring morphology. Methods. We obtained high-angular resolution coronagraphic images of the circumstellar environment around HR4796A with VLT/SPHERE during the commissioning of the instrument in May 2014 and during guaranteed-time observations in February 2015. The observations reveal for the first time the entire ring of dust, including the semi-minor axis that was previously hidden either behind the coronagraphic spot or in the speckle noise. Results. We determine empirically the scattering phase function of the dust in the H band from 13.6 degrees to 166.6 degrees. It shows a prominent peak of forward scattering, never detected before, for scattering angles below 30 degrees. We analyse the reflectance spectra of the disc from the 0.95 mu m to 1.6 mu m, confirming the red colour of the dust, and derive detection limits on the presence of planetary mass objects. Conclusions. We confirm which side of the disc is inclined towards the Earth. The analysis of the phase function, especially below 45 degrees, suggests that the dust population is dominated by particles much larger than the observation wavelength, of about 20 mu m. Compact Mie grains of this size are incompatible with the spectral energy distribution of the disc, however the observed rise in scattering efficiency beyond 50 degrees points towards aggregates which could reconcile both observables. We do not detect companions orbiting the star, but our high-contrast observations provide the most stringent constraints yet on the presence of planets responsible for the morphology of the dust.

We have obtained new images of the protoplanetary disk orbiting TW Hya in visible, total intensity light with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST), using the newly commissioned BAR5 occulter. These HST/STIS observations achieved an inner working angle of similar to 0."2, or 11.7 au, probing the system at angular radii coincident with recent images of the disk obtained by ALMA and in polarized intensity near-infrared light. By comparing our new STIS images to those taken with STIS in 2000 and with NICMOS in 1998, 2004, and 2005, we demonstrate that TW Hya's azimuthal surface brightness asymmetry moves coherently in position angle. Between 50 au and 141 au we measure a constant angular velocity in the azimuthal brightness asymmetry of 22 degrees.7. 7 yr(-1) in a counterclockwise direction, equivalent to a period of 15.9. yr assuming circular motion. Both the (short) inferred period and lack of radial dependence of the moving shadow pattern are inconsistent with Keplerian rotation at these disk radii. We hypothesize that the asymmetry arises from the fact that the disk interior to 1 au is inclined and precessing owing to a planetary companion, thus partially shadowing the outer disk. Further monitoring of this and other shadows on protoplanetary disks potentially opens a new avenue for indirectly observing the sites of planet formation.

To investigate the potential connection between the intense X-ray emission from young low-mass stars and the lifetimes of their circumstellar planet-forming disks, we have compiled the X-ray luminosities (L-X) of M stars in the similar to 8 Myr old TW Hya Association (TWA) for which X-ray data are presently available. Our investigation includes analysis of archival Chandra data for the TWA binary systems TWA 8, 9, and 13. Although our study suffers from poor statistics for stars later than M3, we find a trend of decreasing L-X/L-bol with decreasing T-eff for TWA M stars, wherein the earliest-type (M0-M2) stars cluster near log(L-X/L-bol) approximate to -3.0 and then log(L-X/L-bol) decreases, and its distribution broadens, for types M4 and later. The fraction of TWA stars that display evidence for residual primordial disk material also sharply increases in this same (mid-M) spectral type regime. This apparent anticorrelation between the relative X-ray luminosities of low-mass TWA stars and the longevities of their circumstellar disks suggests that primordial disks orbiting early-type M stars in the TWA have dispersed rapidly as a consequence of their persistent large X-ray fluxes. Conversely, the disks orbiting the very lowest-mass pre-MS stars and pre-MS brown dwarfs in the Association may have survived because their X-ray luminosities and, hence, disk photoevaporation rates are very low to begin with, and then further decline relatively early in their pre-MS evolution.