The Formation of the Solar System: Nuclear Disc Model (Neo-Laplacian model)

Earlier Theories

German philosopher Immanuel Kant’s Nebular Hypothesis (Theory) of Laplace (1796) tried to explain the formation of the solar system. But it had many drawbacks as the theory was based on scientifically erroneous assumptions. But one assumption it got right was that the solar system was born from a giant interstellar cloud called nebula (a vast, swirling cloud of gas and dust).
In 1900, Chamberlain and Moulton considered that a wandering star approached the sun. As a result, a cigar-shaped extension of material was separated from the solar surface. As the passing star moved away, the material separated from the solar surface continued to revolve around the sun and slowly condensed into planets. Sir James Jeans and later Sir Harold Jeffrey supported this argument.
In 1950, Otto Schmidt in Russia and Carl Weizascar in Germany somewhat revised the Nebular Hypothesis. They considered that the sun was surrounded by solar nebula containing mostly hydrogen and helium along with what may be termed as dust. The friction and collision of particles led to the formation of a disk-shaped cloud and the planets were formed through the process of accretion.
The most popular argument is the Big Bang Theory (expanding universe hypothesis (Edwin Hubble, in 1920, provided evidence that the universe is expanding)). It was followed up by the Nuclear Disc Model (Neo-Laplacian model) which dealt primarily with the formation of the solar system.

According to the Nuclear Disc Model (Neo-Laplacian model), the nebula started its collapse and core formation some 5-5.6 billion years ago, and the Sun and the planets were formed about 4.6 billion years ago (the age of the earth is 4.543 billion years).

The Formation of the Sun

The nebula began to collapse (gravitational collapse) in on itself after becoming gravitationally unstable. This was possibly because of a nearby supernova sending shock waves rippling through space.
Gravity then caused dust and gas to coalesce to the centre of the nebular cloud. As more matter got pulled in, the centre got denser and hotter, increasing the gravity and pulling even more dust inwards causing a snowball effect.
About 99.9% of the material fell into the centre and became the protosun (no sunlight yet). Once the centre of the cloud became hot enough it triggered nuclear fusion, and the Sun was born. The 0.1% of matter that remained orbited around the Sun, causing the randomly shaped gas cloud to form a flat disc shape. This flat disc, called the protoplanetary disc, was where the planets formed.

The Formation of the Planets

Within the solar nebula, the dust particles in the gas occasionally collided and clumped together. Through this accretion process, the microscopic particles formed larger bodies that eventually became planetesimals (infant stage of a planet) with sizes up to a few kilometres across.
As the disc continued to cool, the planetesimals grew through accretion to form protoplanets. Gradually they got larger and larger, sweeping up all the leftover dust, other protoplanets, and planetesimals until they grew into the planets.

In the inner, hotter part of the solar nebula, planetesimals were composed mostly of silicates and metals. This hot, rocky material near the centre of the solar system gave rise to terrestrial planets with metal cores (mostly composed of iron and nickel): Mercury, Venus, Earth, and Mars.
In the outer, cooler portion of the nebula, water ice was the dominant component. This gave rise to the gas and ice giants: Saturn, Jupiter, Neptune, and Uranus.
Rocks that escaped the pull of planets were left as asteroids, scattered through the solar system. Many of these rocks orbit the Sun in an area between Mars and Jupiter known as the asteroid belt.

Iron Catastrophe and Planetary Differentiation

When Earth formed about 4.5 billion years ago, it was a uniform ball of hot rock. Radioactive decay and leftover heat (primordial heat) from planetary formation caused the ball to get even hotter. Eventually, after about 500 million years, our young planet’s temperature heated to the melting point of iron — about 1,538° Celsius. This pivotal moment in Earth’s history is called the iron catastrophe.
The iron catastrophe allowed the greater, more rapid movement of Earth’s molten, rocky material. Relatively buoyant materials, such as silicates, water, and even air, stayed close to the planet’s exterior. Droplets of iron, nickel and other heavy metals gravitated to Earth’s centre, becoming the early core. This important process is called planetary differentiation.