The Nebular Hypothesis is a model of solar system Planet Formation hypothesizing a disk (the Solar Nebula) surrounding the young Sun providing the material by which planets were formed. The most modern and complete version is the Solar Nebular Disk Model (SNDM). The model is consistent with the common observation of disks around young stars.
The nebular hypothesis is generally accepted as the source of the material making up the planets. Supplanted theories include:
Both suffer from the seeming improbability given the distance between stars, and in the case of the solar system, the passing cloud would seem to have to be related to the Sun to explain the similar chemical makeup of the Sun and its planets.
Core Accretion Model
Gravitational Instability Model
Protoplanetary Nebula (PPN)