Solar System/Planet Formation and Structure
Core Accretion and Disk Instability
The 2 main/most common theories for planet formation are disk instability and core accretion.
Core Accretion
Core Accretion is the theory that planets form from small particles gathering together and forming lumps that accumulate more and more mass. This process is slow, taking millions of years.
Disk Instability
Disk Instability is the theory that planets form from spinning disks of material that accumulates so much mass, it violently collapses on itself and forms a planet. This is a quick process (speaking relatively), taking just thousands of years.
Toomre-Q Parameter
The Toomre Q-parameter is a dimensionless quantity used in astrophysics to measure the gravitational stability of a rotating disk of gas or stars (such as a galaxy or protoplanetary disk). It determines whether a disk will remain stable or fragment into smaller clumps due to its own gravity.
Stability Thresholds-
Q > 1 : Disk is stable
Q < 1 : Disk is unstable
Q ≈ 1 : Disk is marginally stable