Summarize four steps in the formation of a planetary system
1) Formation of a protoplanetary disk: A protoplanetary disk forms from the gas and dust left over after the birth of a star. This disk consists of a rotating, flattened structure around the star, where particles within the disk begin to clump together.
2) Accretion of planetesimals: Within the protoplanetary disk, small particles called planetesimals start to collide and stick together due to gravity. These collisions gradually build up larger bodies, called protoplanets, as they continue to accrete more material from the surrounding disk.
3) Protoplanet formation: Protoplanets continue to grow through more collisions and accretion of material, eventually becoming massive enough to dominate their immediate surroundings. As protoplanets grow, their gravitational pull influences nearby planetesimals, while also trapping and accumulating gas from the surrounding disk, further increasing their mass.
4) Clearing out of the disk and planet migration: Over time, protoplanets begin to clear out the remaining gas and dust in their vicinity, either through their own gravitational pull or through interactions with the surrounding disk. The process of clearing out the disk can also cause some protoplanets to migrate towards the star or be scattered to other regions, leading to the final configuration of the planetary system.
Step 1: Nebula Collapse - A nebula, which is a vast cloud of gas and dust in space, starts to collapse under its own gravity. This collapse could be triggered by a nearby supernova explosion or a collision with another nebula.
Step 2: Protostar Formation - As the nebula collapses, it forms a dense core called a protostar. The core continues to gather more material from the surrounding cloud, growing in mass and temperature.
Step 3: Accretion Disc Formation - As the protostar continues to grow, a rotating disk of gas and dust forms around it, called an accretion disc. The particles in the disc start to collide and stick together, forming planetesimals, which are small cosmic bodies.
Step 4: Planetary Formation - Planetesimals continue to collide and merge, gradually growing and forming planets. Over time, the interior planets form from metals and rocky materials, while the outer planets form from ice and gas. The planets clear out their orbits by sweeping up smaller debris or scattering them away, leading to the formation of a stable planetary system.