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The solar nebula theory, also known as the nebular hypothesis, is the most widely accepted model explaining the formation and evolution of our solar system. It was originally applied to our solar system only but, this method of planetary system formation is now thought to hold true throughout the universe. The modern variant that is most accepted is the Solar Nebular Disk Model (SNDM).
According to SNDM stars form in massive, dense clouds of molecular hydrogen called giant molecular clouds(GMC). These clouds are gravitationally unstable, and matter coalesces into smaller, denser clumps inside. These clouds collapse and form stars. This can give birth to planets under the right circumstances, which are not fully understood. So, the formation of planetary systems is thought to be a natural result of star formation. The process is thought to take at least 100 million years.
According to SNDM, rocky planets form in the inner part of the protoplanetary disk where the temperature is high enough to prevent condensation of water ice and other substances into grains. This results in the coagulation of purely rocky grains and later into the formation of rocky planetesimals. After small planetesimals have formed, runaway accretion begins. Growth accelerates as mass accumulates. This leads to the growth of larger bodies by the destruction of smaller bodies. This lasts between 10,000 and 100,000 years and ends when the largest bodies exceed approximately 1,000 km in diameter. Next, oligarch accretion begins. Several hundred of the largest bodies(oligarchs), continue to accrete planetesimals. Only the oligarchs grow. Occasionally, oligarchs impact each other and form a larger body. The final result of the oligarchic stage is the formation of about 100 bodies between the size of the Moon and Mars. Last is the merger phase. This begins when the oligarchs become massive enough to perturb each other causing their orbits to become chaotic. This lasts for 10 to 100 million years and forms a limited number of Earth sized bodies. Some of the oligarchs are thought to have brought water to Earth. The resulting rocky planets eventually settle into nearly stable orbits.
Scientists are still trying work out how to apply the solar nebula theory to the formation of giant planets. It is great to see that there are still challenges for our scientists. Unanswered questions mean unexplored frontiers.
We have written many articles about the solar nebula theory for Universe Today. Here’s an article about how the solar system was formed, and here’s an article about the new theory proposed for solar system formation.
We’ve also recorded a series of episodes of Astronomy Cast about the formation of the Solar System. Start here, Episode 174: Mysteries of the Solar System, Part 1.