Core Idea

• The solar system formed from a rotating interstellar nebula (gas + dust).
• Explains Sun and planets formation together, unlike earlier encounter theories.

Timeline

• Nebula collapse began: ~5–5.6 billion years ago
• Formation of Sun & planets: ~4.6 billion years ago
• Age of Earth: ~4.54 billion years

Formation of the Sun

• Nebula became gravitationally unstable (possibly due to a nearby supernova shock).
• Gravitational collapse led to a dense, hot central core.
• ~99.9% of matter accumulated at centre → Protosun.
• Nuclear fusion started → Sun formed.
• Remaining ~0.1% matter formed a rotating protoplanetary disc.

Protoplanetary Disc

• Flat, rotating disc of gas and dust around the Sun.
• Site for planet formation.

Formation of the Planets

• Dust particles collided → accretion.
• Formation sequence:
  1. Dust grains
  2. Planetesimals (a few km-sized bodies)
  3. Protoplanets
  4. Planets

Planet Types (Temperature-based differentiation)

Inner Solar System (Hot Region)

• Composition: Silicates & metals
• Planets: Mercury, Venus, Earth, Mars
• Type: Terrestrial (rocky) planets
• Metal-rich cores (Iron, Nickel)

Outer Solar System (Cool Region)

• Composition: Ice, hydrogen, helium
• Planets: Jupiter, Saturn, Uranus, Neptune
• Type: Gas & ice giants

Asteroids

• Leftover rocky material not accreted.
• Concentrated mainly in the Asteroid Belt (Mars–Jupiter).

Iron Catastrophe

• Occurred ~4.5 billion years ago.
• Earth heated beyond the iron melting point (~1538°C).
• Triggered a large-scale internal rearrangement.

Planetary Differentiation

• Heavy metals (iron, nickel) sank to form the core.
• Lighter materials (silicates, water, gases) rose upward.
• Led to the formation of:
  • Core
  • Mantle
  • Crust