Significant progress has been made over the last few decades in probing the gaseous component of planet-forming disks. I discuss how an understanding of the evolution of the gas in disks can help us to understand the processes of giant and terrestrial planet formation. I also discuss the observational tools that are currently available to study the gaseous component. These include in situ probes of the gas, as well as more indirect probes, such as stellar accretion rates. These tools can be used to probe the evolutionary status of various classes of young stars and thereby provide additional insights into the physical processes that govern planet formation.

Kepler is a Discovery-class mission designed to determine the frequency of Earth-size and smaller planets in and near the habitable zone (HZ) of spectral type F through M dwarf stars. The instrument consists of a 0.95 m aperture photometer to do high-precision photometry of 100,000 solar-like stars to search for patterns of transits. The depth and repetition time of transits provide the size of the planet relative to the star and its orbital period. Multi-band ground-based observation of these stars is currently underway to estimate the stellar parameters and to choose appropriate targets. With these parameters, the true planet radius and orbit scale—hence the relation to the HZ—can be determined. These spectra are also used to discover the relationships between the characteristics of planets and the stars they orbit. In particular, the association of planet size and occurrence frequency with stellar mass and metallicity will be investigated. At the end of the four-year mission, several hundred terrestrial planets should be discovered with periods between 1–400 days, if such planets are common. A null result would imply that terrestrial planets are rare. Based on the results of the recent Doppler-velocity discoveries, over a thousand giant planets will also be found. Information on the albedos and densities of those giants showing transits will be obtained. The mission is now in Phase C/D development and is scheduled for launch in 2008 into a 372-day heliocentric orbit.

The Doppler technique has continuously improved its precision during the past two decades, attaining the level of 1 ms−1. The increasing precision opened the way to the discovery of the first extrasolar planet, and later, to the exploration of a large range of orbital parameters of extrasolar planets. This ability to detect and characterize in great detail companions down to Neptune-mass planets has provided many new and unique inputs for the understanding of planet formation and evolution. In addition, the success of the Doppler technique introduced a great dynamic in the whole domain, allowing the exploration of new possibilities.

Nowadays, the Doppler technique is no longer the only means to discover extrasolar planets. The performance of new instruments, like the High Accuracy Radial-velocity Planet Searcher (HARPS), has shown that the potential of the Doppler technique has not been exhausted; Earth-mass planets are now within reach. In the future, radial velocities will also play a fundamental role in the follow-up and characterization of planets discovered by means of other techniques—for transit candidates, in particular. We think, therefore, that the follow-up of candidates provided by, e.g., the COnvection, ROtation and planetary Transits (COROT) and Kepler space telescopes, will be of primary importance.

Human beings have long thought that planetary systems similar to our own should exist around stars other than the Sun. However, the astronomical search for planets outside our Solar System has had a dismal history of decades of discoveries that were announced, but could not be confirmed. All that changed in 1995, when we entered the era of the discovery of extrasolar planetary systems orbiting main-sequence stars. To date, well over 130 planets have been found outside our Solar System, ranging from the fairly familiar to the weirdly unexpected. Nearly all of the new planets discovered to date appear to be gas giant planets similar to our Jupiter and Saturn, though with very different orbits about their host stars. In the last year, three planets with much lower masses have been found, similar to those of Uranus and Neptune, but it is not yet clear if they are also ice giant planets, or perhaps rock giant planets, i.e., super-Earths. The long-term goal is to discover and characterize nearby Earth-like, habitable planets. A visionary array of space-based telescopes has been planned that will carry out this incredible search over the next several decades.

The host stars of extrasolar planets (ESPs) tend to be metal rich. We have examined other properties of these stars in search of systematic trends that might distinguish exoplanet hosts from the hoi polloi of the Galactic disk; we find no evidence for such trends among the present sample. The α-element abundance ratios show that several ESP hosts are likely to be members of the thick disk population, indicating that planet formation has occurred throughout the full lifetime of the Galactic disk. We briefly consider the radial metallicity gradient and age-metallicity relation of the Galactic disk, and complete a back-of-the-envelope estimate of the likely number of solar-type stars with planetary companions with 6 < R < 10 kpc.

In a protoplanetary disk that is sufficiently cold and massive, gravitational instabilities (GIs) will lead to the development of dense spiral waves on a dynamic time scale. For sufficiently short cooling times, comparable to about half a rotation period, an unstable disk will fragment into dense clumps that could be the precursors of gas giant protoplanets. At moderate cooling rates, the strong spiral waves which permeate the disk do not fragment, but nevertheless generate significant mass and angular momentum transport. I will review recent research on GIs with an emphasis on several critical questions: Do GIs cause planets to form? How fast do they transport mass? When do they occur? How do they affect the solids in the disk? The physical processes that are central to answering these questions are radiative and possibly convective cooling, irradiation of the disk, and gas-solid interactions. I conclude that, while it is unlikely that gas giant planets are formed directly by disk instability, GIs may substantially accelerate both planetesimal formation and core accretion.

The extrasolar planets known to date have masses and orbital periods spanning a large range. Those for which we have definite knowledge about physical composition have much more restricted properties: they are either transiting planets with near-Jovian masses and orbital periods of a few days, or (as in a couple of recent discoveries) they are distant low-mass companions to objects that are themselves low-mass and young. Here we will concentrate on the former group of objects, and try to summarize what is known and conjectured concerning their atmospheres based on observations of their transits. By way of motivation and illustration of the ultimate possibilities available to transit observations, we begin by discussing recent observations of the transit of Venus in June 2004.

The core accretion—gas capture model is generally accepted as the standard formation model for gas giant planets. It proposes that a solid core grows via the accretion of planetesimals, and then captures a massive envelope from the solar nebula gas. Simulations have been successful in explaining many features of giant planets. This chapter will present an overview of the historical and scientific developments of the model, a description of the computer code based on the core accretion hypothesis with a summary of results of recent computer simulations, and the effect the observational achievement of finding extrasolar planets has had on the core accretion—gas capture model.

Models of planetary growth are based upon data from our own Solar System, as well as observations of extrasolar planets and the circumstellar environments of young stars. Collapse of molecular cloud cores leads to central condensations (protostars) surrounded by higher specific angular momentum circumstellar disks. Planets form within such disks, and play a major role in disk evolution. Terrestrial planets are formed within disks around young stars via the accumulation of small dust grains into larger and larger bodies—until the planetary orbits become separated enough that the configuration is stable for the age of the system. Giant planets begin their growth as do terrestrial planets, but they become massive enough to accumulate substantial amounts of gas before the protoplanetary disk dissipates. A potential hazard to planetary systems is radial decay of planetary orbits, resulting from interactions between the planets and the natal disk. Massive planets can sweep up disk material in their vicinity, eject planetesimals and small planets into interstellar space or into their star, and confine disks in radius and azimuth. Small planetary bodies (asteroids and comets) can sequester solid grains for long periods of time and subsequently release them.

Gravitational torques between a planet and gas in the protoplanetary disk result in orbital migration of the planet, and modification of the disk surface density. Migration via this mechanism is likely to play an important role in the formation and early evolution of planetary systems. For masses comparable to those of observed giant extrasolar planets, the interaction with the disk is strong enough to form a gap, leading to coupled evolution of the planet and disk on a viscous time scale (Type II migration). Both the existence of hot Jupiters and the statistical distribution of observed orbital radii are consistent with an important role for Type II migration in the history of currently observed systems. We discuss the possibility of improving constraints on migration by including information on the host stars' metallicity, and note that migration could also form a population of massive planets at large orbital radii that may be indirectly detected via their influence on debris disks. For lower mass planets with Mp ~ M⊕, surface density perturbations created by the planet are small, and migration in a laminar disk is driven by an intrinsic and apparently robust asymmetry between interior and exterior torques. Analytic and numerical calculations of this Type I migration are in reasonable accord, and predict rapid orbital decay during the final stages of the formation of giant planet cores.