solar system

• The Solar System formed about 4.6 billion years ago from a solar nebula, which was a large cloud of gas (mainly hydrogen and helium) and dust.
  • The cloud collapsed under gravity, causing it to:
    • Spin faster (conservation of angular momentum)
    • Flatten into a disc shape
  • Most of the mass concentrated at the centre → formed the Sun
  • Remaining material formed planets, moons, asteroids, and comets
• The Sun’s gravity is the dominant force in the Solar System:
  • It holds all objects in orbit
  • It accounts for ~99.8% of the total mass
  • This is why all planets orbit the Sun rather than each other

Sun:

• The Sun is a main sequence star made of plasma (ionised gas where electrons are not bound to atoms).
• It produces energy through nuclear fusion:
  • Hydrogen nuclei fuse to form helium:
    • 4H → He + energy
  • Some mass is converted into energy (E = mc²)
• Conditions required for fusion:
  • Very high temperature (~15 million °C) → gives particles high kinetic energy
  • High pressure → forces nuclei close enough to overcome electrostatic repulsion

Planets:

• The 8 planets orbit the Sun in elliptical orbits
• They are divided into:
  • Inner (Terrestrial) Planets:
    • Mercury, Venus, Earth, Mars
    • Characteristics:
      • Rocky, solid surfaces
      • High density (made of metals + silicates)
      • Few or no moons
      • No ring systems
    • Formed closer to the Sun:
      • High temperatures caused lighter gases to escape
  • Outer (Giant) Planets:
    • Jupiter, Saturn, Uranus, Neptune
    • Characteristics:
      • Large size, low density
      • Mostly gases (H, He) or ices (water, methane, ammonia)
      • Many moons and ring systems
    • Formed further away:
      • Cooler → gases could condense and remain
  • Gas giants: Jupiter, Saturn
  • Ice giants: Uranus, Neptune (contain more frozen compounds)

Other Objects:

• Satellites (moons):

  • Natural objects that orbit planets
  • Held in orbit by gravitational attraction
  • Example: Earth’s Moon
    • Causes tides due to gravitational pull on oceans
    • Helps stabilise Earth’s axial tilt, keeping climate stable

• Asteroids:

  • Small rocky bodies, mostly in the asteroid belt (Mars–Jupiter)
  • Leftover material from Solar System formation

  Types:

  • C-type → carbon-rich, dark, most common
  • S-type → silicate rock
  • M-type → metallic

• Comets:

  • Made of ice, dust, and frozen gases
  • Originate from:
    • Kuiper Belt (near Neptune)
    • Oort Cloud (far outer Solar System)
  • When near the Sun:
    • Heat causes sublimation → forms coma
    • Solar wind pushes material → forms tail
    • Tail always points away from the Sun

Relative Scale: