encounter theoriesTheories that propose that the planets formed from material ejected from the Sun or a companion star during an encounter with another object. Although the first such theory, by G.L.L. de Buffon in 1745, postulated that a comet collided with the Sun, most later theories have invoked an approach or collision involving another star, protostar, or a giant molecular cloud.
The planetesimal theory (1901–05) of T.C. Chamberlin and F.R. Moulton suggested that a passing star caused the Sun to eject filaments of material. These condensed into planetesimals from which the planets formed by accretion.
Tidal theories (1916–18) of Harold Jeffreys and James Jeans suggested that a star grazed the Sun, drawing out into solar orbit a cigar-shaped filament of material that fragmented to form the planets: the larger planets – Jupiter and Saturn – condensed from the thicker central regions of the filament.
Variations on these ideas postulate that the encounter took place between a passing star and a binary companion star to the Sun. This overcomes one objection to most encounter theories: that material ejected from the Sun could not enter near-circular orbits at the observed planetary distances. However a remaining major problem with all these ideas is that most of the material drawn out from the Sun or its postulated companion would have stayed very close to the parent object. Also any material ejected at stellar temperatures would not condense but would expand violently and dissipate into space before it could form planets.
A modification of the Jeans–Jeffreys theory, the capture theory of M.M. Woolfson, attempts to overcome these problems. It proposes that the filament of matter was drawn off a relatively cool tenuous protostar moving past and tidally distorted by the condensed and more massive Sun. Condensations of protoplanets formed from the fragmented filament, of which six were captured by the Sun. A gradual rounding-off of the initial elliptical orbits and a major collision between the two innermost bodies led to the present distribution of solar-system bodies.