Trans-neptunian objects (TNOs) are believed to be pristine remnants of planets formation, providing constrains on the early stages of the solar system evolution. The knowledge of this population composition and dynamics provides constrains on the formation processes of the early solar nebula, as well as formation processes of other planetary systems around young stars. Nonetheless, because of their great heliocentric distance, and their resulting faintness, all studies are very challenging. 
More than a thousand objects have been detected though, and orbits have been determined for most of them. The resulting dynamical structure is complex and not fully understood: we divide the trans-neptunian region into the Kuiper Belt –itself divided into resonant objects and the classical belt–, the scattered disk and detached objects. 
TNOs physical properties remain poorly known, but we can get constrains on their size, shape, mass, albedo, density or color using different observational methods. Composition is the most difficult property to access though. Only few spectra are available, and they show nevertheless features due to the presence of diverses ices. All those properties result from the competition of several processes that will be discussed. 
The more we learn about TNOs, the more the picture seems complicated. Ultimately, extremely large telescopes, new satellites such as Herschel and space missions like New Horizons will be of great help, since a better understanding of their properties and evolution is critical to improve solar and extra-solar systems formation models.

The magnetorotational instability (MRI) is the most likely source of MHD turbulence in accretion disks. Recently, it has been realized that microscopic diffusion coefficients (viscosity and resistivity) are important in determining the saturated state of the turbulence and thereby the rate of angular momentum transport. In this paper, we use a set of numerical simulations performed with a variety of numerical methods to investigate the dependance of α, the rate of angular momentum transport, on these coefficients. We show that α is an increasing function of the magnetic Prandtl number Pm, the ratio of viscosity over resistivity. In the absence of a mean field, we also find that MRI–induced MHD turbulence decays at low Pm.

Long hypothesized to be the origin of planetary systems, disks around newly formed stars have been studied in detail in the last twenty years. Most, if not all stars form surrounded by a disk, with typical masses of 0.001–0.3 M๏ and up to several hundred AU. Molecular emission lines show the gas to be in Keplerian rotation, with most species (but not H2) frozen out onto dust grains in the cold and dense disk interior. The fraction of stars with disks decreases from > 80% at < 1 Myr to < 10% at 10 Myr. The disk “half-life" is 2–3 Myr, with the inner and outer disks disappearing nearly simultaneously. There is a small but distinct populations of disks with cleared-out inner regions, so-called cold or transitional disks, explained by photoevaporation, planet formation, or binarity. Inside the disks, planet formation begins, with clear observational evidence for the growth of dust grains to sizes of a few cm at least.

We review models of protoplanetary disks. In the earlier stages of evolution, disks are subject to gravitational instabilities that redistribute mass and angular momentum on short timescales. Later on, when the mass of the disk is below ten percent or so of that of the central star, accretion occurs through the magnetorotational instability. The parts of the disks that are not ionized enough to couple to the magnetic field may not accrete or accrete inefficiently.
We also review theories of planet migration. Tidal interaction between a disk and an embedded planet leads to angular momentum exchange between the planetary orbital motion and the disk rotation. This results in low mass planets migrating with respect to the gas in the disk, while massive planets open up a gap in the vicinity of their orbit and migrate in as the disk is accreted.

Photolysis of methane in Titan's stratosphere is the starting point of gas phase carbon chemistry. Quantitative studies of methane photolytic products are of utmost importance for Titan atmosphere models. With this aim, two experimental strategies are presented in this article. Preliminary results demonstrate the possibility of using CRDS absorption coupled with pulsed photolysis on the example of a halogenated derivative of methane: Trichloromethane (CHCl3).

Due to their high vsini, early-type main-sequence stars have been considered as unsuitable targets for radial-velocity surveys. Knowing about the presence of planets around those stars is however a key issue to understand the impact of the mass of the central star on the way planets form and evolve. We developped in recent years a method that allows to search for planets around those stars and started radial-velocity surveys focused on these targets. We report here some results obtained so far in terms of performances and detection.

The formation of resonant planets pairs in exoplanetary systems involves planetary migration inside the protoplanetary disc: an inwards migrating outer planet captures in Mean Motion Resonance an inner planet. During the migration of the resonant pair of planets, the eccentricities are expected to rise excessively, if no damping mechanism is applied on the inner planet. We express the required damping action to match the observations, and we show that the inner disk can play this role. This result applies for instance to the system GJ 876: we reproduce the observed orbital elements through a fully hydrodynamical simulation of the evolution of the resonant planets.

Over 300 extrasolar planets have been found since 1992, showing that planetary systems are common and exhibit an outstanding variety of characteristics. As the number of detections grows and as models of planet formation progress to account for the existence of these new worlds, statistical studies and confrontations of observation with theory allow to progressively unravel the key processes underlying planet formation. In this chapter we review the dominant contribution of Doppler spectroscopy to the present discoveries and to our general understanding of planetary systems. We also emphasize the synergy of Doppler spectroscopy and transit photometry in characterizing the physical properties of transiting extrasolar planets. As we will see, Doppler spectroscopy has not reached its limits yet and it will undoubtly play a leading role in the detection and characterization of the first Earth-mass planets.

Tight binaries discovered in young nearby associations are ideal targets to provide dynamical mass measurments through orbital monitoring. Coupled with estimated temperatures, surface gravities and luminosities, direct mass measurments provide benchmarks for evolutionary models of low-mass stars (M ≤ 0.5 M๏) and brown dwarfs (M ≤ 0.078 M๏) at young ages (Age ≤ 100 Myrs). TWA22 AB is likely to be a member of the nearby TW Hydrae association (Age ~ 8 Myr). It was resolved in a tigh binary with a projected separation of a few AU. In this paper we present preliminary results on the companion orbital monitoring and on the spectral characterisation of the system.

Atomic hydrogen escaping from the extrasolar giant planet HD 209458b provides the largest observational signature ever detected for an extrasolar planet atmosphere. In fact, the upper atmosphere of this planet is evaporating. Observational evidences and interpretations coming from various models are reviewed. Implications for exoplanetology are discussed.

The available results of deep imaging searches for planetary companions around nearby stars provide us useful constraints on the frequencies of giant planets in very wide orbits. Here we present some preliminary results of the Monte Carlo simulation which compare the published detection limits with the generated planetary masses and orbital parameters. This allow us to consider the impications of the null detection, which comes from the direct imaging techniques, on the distributions of mass and semimajor axis derived from the results of the other search techniques and also to check the agreement of the observations with the available planetary formation models.

Dawn space mission will provide the first, detailed data of two of the major bodies in the Main Belt, Vesta and Ceres, and will allow us to study in depth the crater record on the surface of these asteroids and get an insight on their place in the context of Solar System history. In the framework of our studies on the origin of Solar System, we modelled the accretion of Jupiter and, through an N-Body code developed on purpose, we evaluated the flux of impactors on Vesta and Ceres keeping track of their formation zones. We also studied the effects of the possible inward migration of Jupiter on the rate and the characteristics of the impacts. We here describe the different scenarios and their implications for the evolution of Solar System to provide a reference frame for future studies of Vesta's and Ceres' cratering histories.

We examine the implications for the distribution of extrasolar planets based on the null results from two of the largest direct imaging surveys published to date. Combining the measured contrast curves from Masciadri et al.  (2005) and Biller et al.  (2007), we consider what distributions of planet masses and semi-major axes can be ruled out by these data, based on Monte Carlo simulations of planet populations. We can set the following upper limit with 95% confidence: the fraction of stars with planets with semi-major axis between 20 and 100 AU, and mass above 4 MJup, is 20% or less. Also, with a distribution of planet mass of $\frac{{\rm d}N}{{\rm d}M} \propto M^{-1.16}$ in the range of 0.5–13 MJup, we can rule out a power-law distribution for semi-major axis ($\frac{{\rm d}N}{{\rm d}a} \propto a^{\alpha}$) with index 0 and upper cut-off of 18 AU, and index –0.5 with an upper cut-off of 48 AU. For the distribution suggested by Cumming et al.  (2008), a power-law of index –0.61, we can place an upper limit of 75 AU on the semi-major axis distribution.


Planet–star plasma interactions can be described as the interaction of a plasma flow with an obstacle, each being possibly magnetized. Examples of the 4 possible situations are found in our solar system, with intense radio emissions produced in 3 cases out of 4, when either the flow or the obstacle is strongly magnetized. Scaling laws are derived that relate the emitted radio power to the power dissipated in the various corresponding flow–obstacle interactions. They are generalized as a “radio–magnetic” scaling law between the output radio power and the magnetic energy flux convected onto the obstacle. Extrapolating it to the case of exoplanets, we find that hot Jupiters may produce very intense radio emissions due to planetary magnetospheric interaction with a strong stellar wind, reconnection between planetary and stellar magnetic fields, or unipolar interaction between the planet and a magnetic star (or strongly magnetized regions of the stellar surface). Emitted radio power is expected in the hecto–decameter range with intensity 103 to 106 times that of Jupiter (unless some “saturation” mechanism occurs). Corresponding flux densities should be detectable at tens of parsecs range with modern radio arrays. We briefly discuss ongoing observational searches as well as the interests of direct radio detection.

Debris disks are dusty and/or gasous disk that are viewed in scattered light and thermal emission around stars around 107–108 yr. It is well known that the dust in these system is not primodial. It is short lived and must be continuously replenished by colliding planetesimals. Most of them appear distorted by the gravitational pertubations by inner planets or stellar companions. This is why these systems are viewed today as young planetary systems. 
Debris disks are collisional systems. Thanks to collisional cascade towards smaller size, the dust particles are transported outwards by radiation or stellar wind pressure. Below a given blow-off size they escape the system. This model explains the radial density profiles observed.
The various asymmetries, clumps and other dynamical structures such as spiral arms are though to originate in gravitational perturbations by planets and/or companions. Planets usually create gaps in disks, but they also sculpt disks via their mean-motion resonances. Clumpy structures are often invoked as resulting from such an interaction. Stellar companions usually truncate the disk, sometimes confining them to thin annular structures. They also help creating spiral patterns, either tidally or by secular interaction. In this context, the situation is different whether the perturbing companions are bound or just passing stars. In any case, dynamical studies (often specific to each system) can greatly help constraining the configuration and the past history of these systems.

Current design concepts for an optical terrestrial planet-finding mission (TPF) are limited to the spectral range, 0.4–1.05 µm. However, there are good reasons to extend the spectral range of observation to shorter wavelengths such as higher angular resolution. But the most important reason is that the sensitivity to the presence of life on habitable exoplanets is increased over a hundred-fold by access to the ozone biomarker in the mid-ultraviolet.

Earthshine is sunlight that has been reflected from the dayside Earth onto the dark side of the Moon and back again to Earth. In recent times, there has been a renewed interest in ground-based visible and near infrared measurements of the earthshine, as a proxy for exoplanet observations. Observations of the earthshine allow us to explore and characterize the globally-integrated photometric, spectral and polarimetric features of the Earth, and to extract precise information on what are the distinctive characteristics of our planet, and life in particular. They also allow us to quantify how this features changes with time and orbital configurations.

Here, gravitational scattering of planetesimals (viscous stirring and dynamical friction) and their growth through pairwise collisions (runaway/oligarchic growth) are explained in detail. The masses of accreted protoplanets depend on initial disk surface density and semimajor axis. When the mass of a protoplanet becomes larger than a critical value that is a few to ten earth masses, runaway gas accretion from the disk onto the planet starts. The dependence of the protoplanet masses is responsible for the diversity of extrasolar planetary systems, although orbital migrations of planets are also important drivers to produce the diversity.

CoRoT is the first instrument aiming at detecting exoplanets from space, using the transit method. It has been built in order to explore the domain of planets with orbital period less than three months, and size down to about two Earth radii. Successfully launched on the 27th of December 2006, the instrument started its science observations by the beginning of February 2007. The first detections show the capability of the instrument to enlarge the parameter space and to explore the transition regime between gaseous giant and terrestrial planets. Five new transiting giant planets discovered by CoRoT are fully characterized at the time of writing.

The emerging field of extrasolar planet search has shown an extraordinary ability to combine research by astrophysics, chemistry, biology and geophysics into a new and exciting interdisciplinary approach to understand our place in the universe. Are there other worlds like ours? How can we characterize those planets and assess if they are habitable? After a decade rich in giant exoplanet detections, observation techniques have now reached the ability to find planets of less than $10\,M_{\rm Earth}$ (so called Super-Earths) that may potentially be habitable. 
The detection and characterization of Earth-like planet is approaching rapidly with dedicated space observatories already in operation (Corot) or in development phase (Kepler, James Webb Space Telescope, Extremely Large Telescope (ELT), Darwin/TPF). Space missions like CoRoT (CNES, Rouan et al. 1998) and Kepler (NASA, Borucki et al. 1997) will give us statistics on the number, size, period and orbital distance of planets, extending to terrestrial planets on the lower mass range end as a first step, while missions like Darwin/TPF are designed to characterize their atmospheres. 
In this chapter we discuss how we can read a planet's spectral fingerprint and characterize if it is potentially habitable. We discuss the first steps to detect a habitable planet and set biomarker detection in context in Section 1. In Section 2 we focus on biomarkers, their signatures at different wavelengths, abiotic sources and cryptic photosynthesis – using Earth as our primary example – the only habitable planet we know of so far. Section 3 concentrates on planets around different stars, and Section 4 summarizes the chapter.