The criteria to be satisfied by a successful theory of the origin of
the solar system are discussed before highlighting the difficulties
of early two-body interaction theories. It is shown that these do
not apply to Woolfson's capture theory which proposes that a light'
diffuse protostar was tidally disrupted during a close encounter with
the Sun and that the captured material might condense to form massive
The smoothed particle hydrodynamic technique of Gingold & Monaghan is
improved to provide a model for the computer simulation of stellar
encounters. The protostar is replaced by a number of randomly
distributed fluid elements, from which a density distribution may be
recovered by a statistical technique. This distribution leads to a
gravitational field and with a suitable thermodynamic model the
pressure is determined at any point, enabling the dynamical evolution
of the protostar to be simulated. The model is tested on a number
of hydrostatic and hydrodynamic problems.
A collapsing protostar is seen to be tidally disrupted during a close
stellar encounter and the captured material moves quickly away from
the Sun. This provides the optimal conditions for protoplanetary
condensation. The effect of an initial protostellar rotation is also
considered. It is shown that the quantity of material captured is
critically dependent on the alignment of the protostellar spin axis
to the orbital plane and, in an extreme case, might prevent the
disruption of the protoster.
The behaviour of a contracting protoplanet in the solar tidal field
is investigated. It is shown that, despite severe distortion, it
may well survive a perihelion passage without disruption and after
further contraction may regain its spheroidal shape.
Finally, a number of extensions to the capture theory are suggested.