The solar nebula is the original  protoplanetary disk from which the Solar System formed. The solar nebula itself formed from a pre-stellar cloud during the collapse phase of the Sun’s formation. The solar nebula had an initial composition similar to that of the outer layers of the present Sun and had a minimum mass sufficient to form the current planets. The latter model nebula is known as the “minimum-mass solar nebula” (MMSN), which is estimated by adding enough light elements to the masses of all the planets to bring them to solar composition.