Physics – 6.1.2 The Solar System | e-Consult

The accretion model proposes that our Solar System formed from a giant, rotating cloud of gas and dust called a solar nebula. Gravity was the primary force driving this process. As the nebula collapsed under its own gravity, it began to spin faster, flattening into a rotating disk – the accretion disc.

The interstellar cloud contained a variety of elements, including hydrogen, helium, and heavier elements produced in the cores of dying stars. The composition of the material varied with distance from the protosun (the early Sun). Closer to the protosun, temperatures were much higher. This meant that only materials with high melting points, like metals and silicates, could condense into solid particles. These particles then collided and stuck together through electrostatic forces and gravity, gradually growing into planetesimals.

Further from the protosun, temperatures were lower. Volatile substances like water, ammonia, and methane could also freeze and condense. This abundance of icy material allowed the planetesimals to grow much larger and faster. The larger planetesimals had stronger gravitational pulls, attracting even more material. This led to the formation of the gas giants (Jupiter and Saturn) and ice giants (Uranus and Neptune), which could accrete vast amounts of gas from the nebula. The rocky planets, being closer to the Sun, lacked the icy materials needed for significant growth, resulting in their smaller size and rocky composition. The accretion disc provided a continuous supply of material for planet formation, with the composition of that material changing with distance from the protosun.