Processes in the protoplanetary disk
Planets are thought to form out of the dust that collide and stick to form larger and larger bodies. The growth of small bodies (less than 1 km) relies on random, turbulent motions in the gas to cause collisions. Bodies larger than 1 km, called planetesimals, can attract each other by gravity, which is a force where two bodies of mass are directly drawn to eachother. Even larger bodies (100 to 1000 km), called protoplanets have enough mass, not only to keep together by gravity, but to also change the path of approaching rocks and are able to grow much faster.
The time early in our Solar System’s history when planetesimals and protoplanets formed was active and energetic.
The energy came in multiple forms:
1. Initial energy of the Solar Nebula: After the supernova explosion, the solar nebula was very hot. Innermost portions of protoplanetary disks are thought to have reached temperatures as high as 1000 degrees Kelvin. The protoplanetary disk cooled slowly with time.2. Decay of radioactive isotopes: Early in the Solar System’s history there was a higher abundance of short-lived radioactive isotopes. These isotopes are atoms with large masses and short half-lives (seconds to 1000s years). Radioactive decay of short-lived isotopes to more stable isotopes (lower masses, longer or infinite half-lives) releases particles, energy (heat) and radiation. The type of energy and radiation that is released by radioactive decay is the same type that is released by a nuclear bomb explosion. The difference is that natural radioactive decay is more dispersed and cumulative, rather than a sudden, violent explosion.
3. Collision of bodies, or impacts: Impacts are one of the major processes leading to the formation of planets. It is also a very energetic process that can cause wholesale melting of a body, bringing about planetary differentiation (separation into core, mantle, and crust). The next section will discuss this process in greater detail.
