Published online by Cambridge University Press: 24 February 2010
Abstract Circumstellar disks are a natural outcome of the star-formation process and the sites where planets form. Gas, mainly hydrogen and helium, accounts for about 99% of the disk's initial mass while dust, in the form of submicron-sized grains, only for about 1%. In the process of forming planets circumstellar disks disperse: submicron dust grains collide and stick together to form larger aggregates; gas accretes onto the star, onto the cores of giant and icy planets, and evaporates from the disk surface. A key question in planet formation is the timescale and physical mechanism for the clearing of protoplanetary disks. How rapidly gas and dust disperse determines what type of planets can form.
In this chapter we compare the evolution of protoplanetary disks to that of the proto-solar nebula. We start by summarizing the observational constraints on the lifetime of protoplanetary disks and discuss four major disk-dispersal mechanisms. Then, we seek constraints on the clearing of gas and dust in the proto-solar nebula from the properties of meteorites, asteroids, and planets. Finally, we try to anchor the evolution of protoplanetary disks to the Solar System chronology and discuss what observations and experiments are needed to understand how common is the history of the Solar System.
The observed lifetime of protoplanetary disks
Observations at different wavelengths trace different disk regions (see e.g. Chapter 3). Therefore, determining when disks disperse requires multi-wavelength observations of disks around stars of different ages. The ages of young stars (younger than ∼100 Myr) are typically estimated by comparing their positions in the Hertzsprung–Russell diagram to predictions from pre-main-sequence evolutionary tracks.