This Les Houches School offers students a wide ranging view of the field of exoplanets and the search for life beyond the solar system. Observational and theoretical opportunities abound in a new field of astronomy that will be growing for decades to come. I give a brief introduction and overview to the many detailed talks that will be presented in this volume.

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.

Studying the internal structure of exoplanet-host stars compared to that of similar stars without detected planets is particularly important for the understanding of planetary formation. The observed average overmetallicity of stars with planets is an interesting point in that respect. In this framework, asteroseismic studies represent an excellent tool to determine the structural differences between stars with and without detected planets. It also leads to more precise values of the stellar parameters like mass, gravity, effective temperature, than those obtained from spectroscopy alone. Interestingly enough, the detection of stellar oscillations is obtained with the same instruments as used for the discovery of exoplanets, both from the ground and from space. The time scales however are very different, as the oscillations of solar type stars have periods around five to ten minutes, while the exoplanets orbits may go from a few days up to many years. Here I discuss the asteroseismology of exoplanet-host stars, with a few examples.

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.

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.

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.

We present the results of our study of astrometric stability of 200-in Hale (Mt. Palomar) and 10-m Keck II (Mauna Kea) telescopes, both with Adaptive Optics (AO) facilities. A group of nearby visual binaries and multiples was observed in near infrared, relative separations and position angles measured. We have also checked the influence of some systematic effects (e.g. atmospherical refraction, varying plate scale factor) on result and precision of astrometric measurements. We conclude that in visual binaries astrometrical observations it is possible to achieve much better precision than 1 miliarcsecond [mas], which in many cases allows detection of the astrometrical signal produced by planetary-mass object.

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.


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.

For stars with ages ≥ 10 Myr, circumstellar disks are dominated by a population of optically thin dust grains most likely associated with the erosion of a planetesimal population in a system that may have also formed planets. The key goal of this chapter is to provide a core introduction to circumstellar debris disks that will allow the reader to more easily grasp the vast literature. We review key concepts involving the thermal and scattered light detection of debris disks, stellar age determinations, dust destruction mechanisms, and the connection between debris disks and exosolar planets. The material is presented at a level suitable for the non-astronomy graduate students attending the Les Houches winter school, but also includes tables and previously unpublished figures that will interest advanced astronomers.

Circumstellar disks surrounding young forming stars, are likely the location where planets form. While the gaseous phase represents up to ~99% of the disk mass and control the dynamics, most of disk properties relies on dust analyses. The main constituent of the gaseous component, molecular hydrogen (H2), remains nearly out of reach and the gas disk is probed through emission lines of minor tracers, such as CO. In this lecture, we will first recall how H2 symmetric molecular structure makes its detection difficult. We will then review the most significant results achieved so far, thanks to new generation of ground and space-based telescopes, with a special emphasize given to Herbig Ae/be, which are pre-main sequence stars of intermediate mass. Though the first direct estimates of circumstellar disk mass have been reported, observation of H2 is still challenging detection.

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.

We present a model for the dispersal of protoplanetary disks by winds from either the central star or the inner disk. These winds obliquely strike the flaring disk surface and strip away disk material by entraining it in an outward radial-moving flow at the disk-wind interface. This interface lies several disk scale heights above the mid plane. The disk dispersal time depends on the velocity at which disk material flows into the mixing layer. If this velocity is ~10% of the sound speed, the disk dispersal time at ~1–10 AU is ~5 Myr for a 0.01 M๏ disk around a solar mass star, with a spherical wind launched from the inner disk or central star with a typical mass loss rate of 10-8M๏yr-1 and terminal velocity of vw = 100 kms-1. We conclude that wind stripping is not a dominant disk dispersal mechanism compared with viscous accretion and photoevaporation. Nevertheless, wind stripping may affect the evolution of the intermediate disk regions.

We have studied the effect of the gas accretion flow on the distribution of molecules in hot inner regions of young circumstellar disks. The gas-phase reactions initiated by evaporation of icy mantle on dust grains are calculated along the accretion flow, and the molecular line emission is simulated using the obtained abundance profiles. Our results have shown that some evaporated molecules keep high abundances and emit strong transition lines only when the accretion velocity is high enough.


We have modelled a detailed physical structure of protoplanetary disks, taking into account X-ray and UV irradiation from a central star, as well as dust size growth and settling towards the disk midplane. In addition, we have calculated the level populations and line emission of molecular hydrogen in the disks. As a result, we reproduce the observed strong H2 line flux if the disks are influenced by strong UV and X-ray irradiation. Also, the dust evolution changes the physical properties of the disk, and thus the H2 line ratios.


This pedagogical review covers an unsolved problem in the theory of protoplanetary disks: the growth of dust grains into planetesimals, solids at least a kilometer in size. I summarize timescale constraints imposed on planetesimal formation by circumstellar disk observations, analysis of meteorites, and aerodynamic radial migration. The infall of ≲ meter-sized solids in a hundred years is the most stringent constraint. I review proposed mechanisms for planetesimal formation. Collisional coagulation models are informed by laboratory studies of microgravity collisions. The gravitational collapse (or Safronov-Goldreich-Ward) hypothesis involves detailed study of the interaction between solid particles and turbulent gas. I cover the basics of aerodynamic drag in protoplanetary disks, including radial drift and vertical sedimentation. I describe various mechanisms for particle concentration in gas disks – including turbulent pressure maxima, drag instabilities and long-lived anticylonic vortices. I derive a general result for the minimum size for a vortex to trap particles in a sub-Keplerian disk. Recent numerical simulations demonstrate that particle clumping in turbulent protoplanetary disks can trigger gravitational collapse. I discuss several outstanding issues in the field.


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.

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.

The discovery that the short-lived radionucleide 60Fe was present in the oldest meteorites suggests that the formation of the Earth closely followed the death of a massive star. I discuss three astrophysical origins: winds from an AGB star, injection of supernova ejecta into circumstellar disks, and induced star formation on the boundaries of HII regions. I show that the first two fail to match the solar system 60Fe abundance in the vast majority of star forming systems. The cores and pillars on the edges of HII regions are spectacular but rare sites of star formation and larger clumps with masses 103-4 M๏ at tens of parsec from a supernova are a more likely birth environment for our Sun. I also examine γ-ray observations of 60Fe decay and show that the Galactic background could account for the low end of the range of meteoritic measurements if the massive star formation rate was at least a factor of 2 higher 4.6 Gyr ago.