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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. This was hypothesized in the 18th century; it is still thought of as valid, though modern theory differs from the 18th century in details and some of the presumed mechanisms. The most modern and complete version is termed the solar nebular disk model (SNDM). The model is consistent with the occurrence of disks observed around young stars. The gravitational instability model and/or core accretion model could describe formation of some of the planets.
The nebular hypothesis is generally accepted regarding 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.