From disks to planets

Theories for the formation and evolution of planets depended primarily on geophysical data from the solar system (see Brush�1990, and references therein). Starting in the 1940s, astrophysical data began to provide new insights. Discoveries of pre-main-sequence stars in Taurus- Auriga, Orion, and other regions led to the concept that stars form in giant clouds of gas and dust (see�Kenyon et�al.�2008b, and references therein). Because nearly every young star has a circumstellar disk with enough mass to make a planetary system, theorists began to connect the birth of stars to the birth of planets. Still, the solar system remained unique until the 1990s, when the first discoveries of exoplanets began to test the notion that planetary systems are common. With thousands of (candidate) planetary systems known today, we are starting to have enough examples to develop a complete theory for the origin of the Earth and other planets.