Planet formation

Abstract. One particular fact that is helping us to understand the mechanisms of planetary formation has to do with the planet host stars themselves. In fact, these were found to have, on average, a metal content higher than the one found in stars without detected planetary companions. In this contribution we will mainly focus on the most recent results on the chemical abundances of planet-host stars, and what kind of constraints they are bringing to the theories of planet formation. 

With the number of known exoplanets increasing very fast, current results are giving us the chance to undertake the first statistical studies of the properties of the exoplanets, as well of their host stars [24,21,8]. This is bringing new interesting constraints for the models of planet formation and evolution. 

Yin Q, Jacobsen SB, Yamashita K, Blichert-Toft J, Telouk P, Albarede F (2002) A short time scale for terrestrial planet formation from the Hf-W chronometry of meteorites. Nature 418 949-951 Young ED, Russell SS (1998) Oxygen reservoirs in the early solar nebula inferred from an Allende CAL Science 282 452-455... 

Yin, Q. Z., Jacobsen, S. B., Yamashita, K. etal. (2002) A short timescale for terrestrial planet formation from Hf-W chronometry of meteorites. Nature, 418, 949-952. 

Boss, A. P. (2004) Convective cooling of protoplanetary disks and rapid giant planet formation. Astrophysical Journal, 610, 456-463. 

Figure 8.1 Whirlpool galaxy M51 (Courtesy Hubble Heritage Team, ESA, NASA) and a fossil shell with the same spiral structure as the proposed distribution of matter in the solar system at the time of planet formation...

Abstract Planet formation is a very complex process through which initially submicron-sized dust grains evolve into rocky, icy, and giant planets. The physical growth is accompanied by chemical, isotopic, and thermal evolution of the disk material, processes important to understanding how the initial conditions determine the properties of the forming planetary systems. Here we review the principal stages of planet formation and briefly introduce key concepts and evidence types available to constrain these. 

Tiny solid cosmic particles - often referred to as dust - are the ultimate source of solids from which rocky planets, planetesimals, moons, and everything on them form. The study of the dust particles genesis and their evolution from interstellar space through protoplanetary disks into forming planetesimals provides us with a bottom-up picture on planet formation. These studies are essential to understand what determines the bulk composition of rocky planets and, ultimately, to decipher the formation history of the Solar System. Dust in many astrophysical settings is readily observable and recent ground- and space-based observations have transformed our understanding on the physics and chemistry of these tiny particles. 

However, planet formation is a uniquely fortunate problem, as our extensive meteorite collections abound with primitive materials left over from the young Solar System, almost as providing a perfect sample-return mission from a protoplanetary disk. A remarkable achievement of geochronology is that many of these samples can be dated and the story of the Solar System s formation reconstructed. 

In this chapter we first introduce the types of evidence, basic concepts, and planet-formation timeline that are used throughout the book and briefly review the constraints available for different epochs of planet formation within the first 10 Myr. Table 1.1 provides a summary of the types of constraints on the different stages of planet formation and the chapters in which they are discussed in the book. 

In the remainder of the chapter we review the major stages and key open questions of planet formation, drawing on the detailed discussions presented in the subsequent chapters. 

Figure 1.3 Chronology of the planet formation in the Solar System and astronomical analogs. The isotopes given identify the radioisotope systems that served as a basis for the dating. For the astronomical ages, Li refers to ages derived from stellar atmospheric Li abundances, dyn refers to dynamically derived ages, iso refers to ages derived through stellar isochrone fitting. Note that the zero points of the two systems were assumed here to coincide.

The extent to which the dust from the ISM survives planet formation intimately depends on the details of the core collapse and the formation of the accretion disk. 

The lifetime of protoplanetary disks determines the time available for planet formation with the loss of the dusty gas disks no raw material is left to form planetesimals or giant planets. Thus, disk mass as a function of time is perhaps the single most important constraint on the formation of both the rocky and the giant planets. The most readily observable, albeit imperfect, indicator of disks is the presence of excess emission above the stellar photosphere, emerging from small, warm dust grains. 

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