BIRTH OF THE SOLAR SYSTEM (˜4.7—4.6 GA)

Many theories have been proposed for the origin of the solar system, but the one advanced by German philosopher ImmanuelKant in 1755 in Universal Natural History and Theories of the Heavens is acknowledged as essentially correct. Kant proposed that the planets originated from a rapidly rotating disk of matter orbiting the sun, which coalesced to form larger and larger objects, eventually creating planets. A French mathematician, Pierre-Simon de Laplace, refinedKant's model. In the Kant—Laplace proto-planetary or accretion disk hypothesis, the solar system began as a dense cloud of interstellar gas and dust. About 4.7 billion years ago (4.7 Ga), a shock wave from an exploding supernova triggered the gravitational collapse of the interstellar cloud into a rapidly rotating disk (Figure 17.1). Dust grains attracted by electrostatic forces clumped together, settling in the mid-plane of the disk, where they eventually grew to the size of pebbles. By processes that are poorly understood, the pebbles aggregated to form kilometer-sized planetesimals with sufficient mass to rapidly accrete moonsize bodies. Accretion of these bodies to form the inner planets produced the most intense phase of impacts in the history of the solar system.