Planetesimals, protoplanetary

The paramount importance of carbon in the cosmos is shown by the fact that more than 75% of the approximately 120 interstellar and circumstellar molecules so far identified are carbon containing (Henning and Salama, 1998). Molecules apparently travel from the ISM via protoplanetary discs to the planetesimals and from there, via accretion, to the heavenly bodies formed. The molecules so far identified in ISM come from quite different types of compounds ... 

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. 

While the amount of dust and small particles that underwent thermal processing remains difficult to constrain both in the entire proto-solar nebula and in protoplanetary disks around other stars, in the Asteroid Belt over 80% of the pre-chondritic components have been melted. These heating events may play a crucial role in defining the bulk composition of planetesimals and planets by reprocessing much or all... 

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. 

Observations of protoplanetary disks indicate that these objects remain optically thick for timescales of millions of years, meaning that a population of dust is sustained for that period of time (see Chapter 9 for a detailed discussion of disk lifetimes and dispersal mechanisms). As will be discussed in Chapter 10, the timescale for dust growth and incorporation into planetesimals is less than this time period. Additionally, the timescale for dust settling is much less than the age of these disks. However, the apparent contradictions between these timescales and the observations can be explained within the context of the processes described thus far. 

In this chapter, we focus on the insights that chondrites offer into the earliest stages in the formation of asteroids and planetesimals in the solar nebula— the protoplanetary disk of dust and gas that evolved into the planetary system. Other chapters review many other aspects of chondrite studies. 

Sanders 1. S. (1996) A chondmle-forming scenario involving molten planetesimals. In Chondrules and the Protoplanetary Disk (eds. R. H. Hewins, R. H. Jones, and E. R. D. Scott). Cambridge University Press, Cambridge, pp. 327—334. 

Meteorites have great scientific value because most of them are fragments of asteroids which are themselves remnants of larger bodies that formed in the solar system at the time the planets accreted from planetesimals in the protoplanetary disk that formed around the Sun at the time of its formation about 4.6 x 10 years ago. Therefore, meteorites are our principal source of information about the origin of Earth and of the other planets of the solar system. 

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