For the longest time the nebular theory of planet formation reigned supreme. The main reason was that classical mechanics supported the chances of stars and planets forming from nebulas. In fact the theory is one of the oldest originating in the late 1700s. However the nebula theory had holes that still troubled scientists. One major problem was angular momentum. According to physics momentum just like energy and matter has to be conserved it doesn’t disappear. Angular momentum has to be transferred to another object.
The angular momentum issue was important because according to the nebula theory a nebula due to the accumulation of mass at the center exerted gravity becoming a spinning flat disk. This motion would speed up the accumulation of gas and dust until a proto sun was formed. The planets were then from the remainder of the nebula cloud once the sun “turned on.” However if classical mechanics was accurate the original angular momentum of the nebula would be completely transferred to the Sun at its center. However we know that is not the case.
The other problems presented were that if according to the original theory rings of matter split off then Solar winds would have dispelled them. An independent nebula theory was developed to answer these many questions by Carl Von Wiezsacker and Gerard Kuiper. This new theory was called the protoplanet hypothesis. The new theory had the same basic premises as the original theory with adjustments.
First it decentralized coagulation of gases and dusts in a nebula. The sun was still formed but instabilities inherent in the disk caused gases and dust to clump together alongside the sun. The small clumps were the origins of the planets and were called planetessimals or protoplanets. Over time the planetessimals eventually collided and combined to make larger bodies until the current planets and the asteroid belt were formed. Solar winds then blew away the rest of gases. This theory more accurately explained planet formation without the unanswered questions.
This theory better explained the formation of our universe. However, there is still a lot we don’t know. We don’t know much about the period of collisions that formed the planets. We don’t know how this time produced moons as well as the asteroid belt and more importantly what made life succeed in developing on Earth rather than our Neighbors. Even with all these unanswered questions the protoplanet hypothesis gives us a starting point.
We’ve also recorded a series of episodes of Astronomy Cast about every planet in the Solar System. Start here, Episode 49: Mercury.