23.4 - Solar System Formation

The accretion model of Solar System formation

• The accretion model explains how the Solar System formed from a giant cloud of gas and dust.
• Gravity is the driving force throughout: it pulls the cloud together, heats the centre enough to form the Sun, and assembles the planets.

How the Solar System formed

1. The Solar System began as a large, rotating cloud of interstellar gas and dust containing heavy elements from earlier supernovae.
2. Gravity pulled the material closer together.
3. As the matter collapsed inwards, the cloud became denser, especially near the centre.
4. The denser regions had a stronger gravitational pull, attracting more matter and becoming denser still.
5. Compression raised the temperature and pressure in the densest regions.
6. The centre became hot and dense enough to form a star: the Sun.
7. The remaining gas and dust settled into an accretion disc orbiting the new Sun.
8. The disc was hottest near the Sun and cooler further out. High temperatures vaporised lighter materials close in, leaving heavier substances near the Sun and lighter substances further out.
9. Within the disc, material clumped together under gravity to form the planets.
10. Heavier rocky substances formed the rocky planets near the Sun; lighter substances formed the gas planets further out.

Gravitational field strength of a planet

• Every planet has a gravitational field around it that pulls objects towards its centre.
• Two factors determine its strength:
  1. Mass - A more massive planet has a stronger gravitational field at its surface.
  2. Distance - The field weakens as distance from the planet increases.
• Surface gravity does not depend on size alone; for planets of similar size, the one with the greater mass has the stronger surface field.

The Sun's field and orbital speed with distance

• The Sun's gravitational field holds the planets in orbit.
• The field weakens with distance, so planets further from the Sun experience a weaker gravitational pull and travel at lower orbital speeds.

Interpreting trends in planetary data

• Planetary data shows patterns linked to a planet's average orbital radius (distance from the Sun).
• As average orbital radius increases:
  • Orbital period increases - A more distant planet travels a longer path at a lower orbital speed, so it takes longer to complete an orbit.
  • Density decreases - The inner planets are dense and rocky, while the outer gas planets have much lower densities.
  • Surface temperature decreases - A more distant planet receives less concentrated radiation from the Sun.

Surface gravity

• Surface gravity does not follow a trend with distance from the Sun.
• The gravitational field strength at a planet's surface depends on the planet's own mass and radius, so it shows no pattern with orbital radius.