Formation of Sunlike Stars and Planetary Systems

We discuss the main events in the birth and evolution of a sunlike star from the collapse of a cloud of molecules, to the accretion of a magnetized disk of material from which planets form, to the appearance of jets and bipolar outflows from young stellar objects. We relate how this sequence of events corresponds to the classification scheme from infrared and radio astronomy of class 0, I, II, and III objects. Apart from the reprocessing of electromagnetic radiation, recurring themes are the outward transport of angular momentum and the inward transport of mass mediated by the long-range forces of self-gravitation and magnetic fields that naturally produce configurations where most of the mass of the system ends up as a ball at the center, and most of the angular momentum ends up in a flattened protoplanetary disk that revolves about the central ball that is the host star. From the disk, the dust separates from the gas, by processes whose details remain obscure, to aggregate into planetesimals that grow to become protolanets and planets. Radio astronomical observations in spectral lines and the continuum will be crucial to deciphering how this separation of gas and solids occurs, and the likely crucial role played by water ice in forming icy giant planets relative to rocky terrestrial planets. A major advance in our theoretical understanding of the process of planet formation came with the realization that protoplanets can interact strongly with the surrounding disk in the launching of spiral density waves. The dissipation of the spiral density waves can not only can open up gaps in the disk, but can also, in back reaction, cause a protoplanet to migrate far from its original place of birth. This migration helps to explain the diversity of formed planetary systems around sunlike stars revealed by the explosion of studies of exoplanets. It also creates new puzzles concerning how inwardly spiraling planets can stop their motion before being swallowed by the central star. In any case, the developments of the past few decades have revealed that star and planet formation is a far more dynamic and violent event than contemplated by classical traditions that started with quasi-static Kelvin-Helmholtz contraction.