The formation of the solar system can be summarised using the accretion model. This divides the formation of the solar system into two parts:
- The formation of a star at the centre of the nebula:
  - A star is formed from a nebula, which is considered the building blocks of the solar system and contains the required dust and gases for planet and star formation.
  - If massive enough, the nebula collapses under its own gravity and the centre pulls the rest of the mass inwards.
  - As more atoms occupy the same space, its density increases, which also increases the pressure and temperature.
  - When the pressure and temperature are high enough to trigger nuclear fusion of hydrogen into helium, which releases a lot of energy, the star is born. This is how the Sun is formed in the centre of the solar system.
- The formation of planets around the star:
  - As the nebula continuously shrinks and swirls, its motion causes it to flatten and form a disk around the Sun. This is the accretion/protoplanetary disk.
  - Atoms and molecules in the accretion disk clump together to form larger particles. These larger clumps randomly collide with each other, forming larger masses or shattering each other, all whilst orbiting the protostar.
  - When the clumps grow to the size of over 160 km, they can effectively exert their gravitational fields and attract smaller objects into it, forming planetesimals.
  - Over time, these planetesimals grew in size, absorbing other planetesimals and forming the 8 main planets we know today.
  - Those that did not form planets later formed asteroids, natural satellites, comets and other smaller bodies.
- The planets formed nearer to the Sun are small, rocky, and dense.
  - Due to high temperatures near the Sun during the formation, only rocks and metals can withstand the heat and form planets.
  - The lack of materials results in the formation of small, rocky, and dense planets.
- The planets formed further from the Sun are large, gaseous, and less dense.
  - Lower temperatures further from the Sun allow ice to form planet-building materials, resulting in much larger planetesimals.
  - These larger planetesimals begin to accrete their own gases around themselves, swelling into much larger sizes and forming the gas giants we know today.
The Sun makes up over 99% of the materials in the solar system.
- Due to the Sun having so much mass, the gravitational pull exerted by the Sun is much stronger than the rest of the planets.
- Since lighter objects orbit heavier objects, all the other objects in the solar system orbit the Sun.

The Sun’s gravitational pull keeps an object in orbit with the Sun.
- If no gravitational force is present, the object will continue its path in a straight line, just like letting go of a spinning ball tied to a string as seen here:

The further away the object is from the Sun, the lower the strength of its gravitational field.
- As a consequence, the object would slow down when it moves further away from the Sun.
In an elliptical orbit, the orbital speed of a planet is not constant. It moves slower when it moves further away from the Sun, but faster as it moves closer. This can be explained using the law of the conservation of energy: the planet has constant energy, but its kinetic energy and gravitational potential energy fluctuate according to the position of the planet.
- As the planet moves further away from the Sun, the kinetic energy is converted into the gravitational potential energy, and thus the planet slows down.
- As the planet moves closer to the Sun, the gravitational potential energy is converted into the kinetic energy, and thus the planet increases its speed.

6.04 - Solar System (III)

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