The Formation Of Planetary Systems

Abstract

The orbital characteristics of the planets imply their formation in the rotating disk of gas and dust that earlier produced the sun. Observations of star-forming regions suggest that such protoplanetary disks are common around young stars, and hence that planetary systems are abundant in the galaxy. Our understanding of the star formation process is fairly robust, in large part because of our ability to study pre-collapse clouds, protostars, young stellar objects, and pre-mainsequence stars in nearby star-forming regions, and to revise and extend the theory of star formation accordingly. While we are achieving a similar understanding of the large scale structure of protoplanetary disks, the absence of examples of other newly-formed or mature planetary systems (prior to the discoveries of 1995–96) has greatly hampered our ability to test and improve upon theories of planet formation. The recent discovery of several giant planets and brown dwarf stars orbiting nearby stars marks the advent of a new era, the era of the discovery of extrasolar planetary systems. Current theories of the formation of terrestrial and giant planets and of brown dwarf stars are discussed in the light of these discoveries. The extremely small separation (0.05 AU) between 51 Peg and its ∼ 1/2 Jupiter-mass companion implies that if 51 Peg B formed as a giant planet, then it must have migrated inward to its present location following its formation. Gravitational interaction with an exceptionally long-lived disk is the likely cause of this migration.