Nebular hypothesis

The nebular hypothesis is the leading scientific model used to explain how solar systems form and evolve. It suggests that stars and planets emerge from massive clouds of gas and dust. This process is not unique to our own Solar System. Astronomers believe this mechanism is at work throughout the entire universe. The modern version is called the solar nebular disk model, or SNDM. This model explains why planets follow nearly circular, flat orbits. It also explains why they rotate in the same direction as the Sun.

The process begins inside giant molecular clouds, known as GMCs. These clouds are made of molecular hydrogen and are incredibly massive. A single cloud can be 300,000 times the mass of our Sun. Gravity causes these clouds to become unstable and fragment into dense cores. These cores then collapse and rotate to form new stars. Every young star is surrounded by a gaseous protoplanetary disk, or proplyd. This disk is an accretion disk that feeds the growing central star.

As the system evolves, the disk undergoes several distinct stages. Initially, the disk is extremely hot. It later cools during the T Tauri star stage. During this cooling phase, tiny grains of rock and ice begin to form. These grains eventually coagulate into kilometer-sized objects called planetesimals. If the disk is massive enough, runaway accretion occurs. This process rapidly forms planetary embryos the size of the Moon or Mars. This rapid growth can take only 100,000 to 300,000 years.

Different types of planets form in different parts of the disk. Near the star, embryos undergo violent mergers to create terrestrial planets. Farther out, past the frost line, the environment changes. In this colder region, embryos are made mostly of various types of ice. These icy embryos become much more massive than those in the inner disk. Some grow to 5 to 10 Earth masses, which is a critical threshold. Once they reach this mass, they can begin accreting hydrogen and helium gas.

The formation of giant planets is a complex and fast process. Once a core reaches about 30 Earth masses, gas accumulation accelerates. This is known as runaway accretion. Jupiter-like planets can accumulate most of their mass in just 10,000 years. This happens before the gas in the disk is completely exhausted. However, some planets like Uranus and Neptune may be "failed cores." These ice giants likely formed too late when the disk was almost gone.

Our understanding of this model has a long history of discovery. Immanuel Kant first proposed parts of the theory in 1755. Pierre Laplace later modified it in 1796 with a more detailed model. For a time, the Laplacian model was the standard in the 19th century. However, it struggled to explain why planets hold 99% of the system's angular momentum. Many other theories were proposed in the 20th century to fix these gaps. The modern SNDM model was largely shaped by Victor Safronov in 1969.

Even with modern advances, some mysteries remain for scientists to solve. One major problem is how tiny 1 cm particles grow into 1 km planetesimals. Another issue is how material loses angular momentum to reach the star. Scientists suggest processes like magnetic braking or solar winds might help. We also study how planets migrate long distances within the disk. Understanding these details helps us grasp how the universe builds its many worlds.