The nebular hypothesis is a model of solar system planet formation hypothesizing a slowly-rotating nebula that formed the young Sun (the solar nebula) and then evolved into a surrounding protoplanetary disk 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. Planets could be formed according to the gravitational instability model and/or core accretion model.
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 consistency of the chemical makeup of the Sun and its planets.
The term nebular hypothesis was coined regarding the formation of the solar system planets, but the same basic idea is accepted as the usual method of planet formation around other stars.