Origin of Solar System : Hypotheses

Big-Bang Theory : (most accepted)

• Concept: Universe originated ~13.8 billion years ago from an extremely dense, hot singularity.
• Process:
  • Sudden expansion → matter, energy, space, and time created.
  • Formation of hydrogen, helium (primordial nucleosynthesis).
  • Galaxies, stars, and later → solar systems formed.
• Evidence:
  • Redshift of galaxies (Hubble’s law).
  • Cosmic Microwave Background Radiation (CMB).
  • Abundance of light elements (H, He, Li).
• Relevance for Solar System: Provides the cosmic setting → solar system formed later (~4.6 Ga) from stellar nebulae produced after supernova explosions.

———- Other Theories ———-

Nebular Hypothesis (Kant-Laplace Hypothesis)

• Proposed by: Immanuel Kant (1755), refined by Pierre-Simon Laplace (1796).
• Idea: A hot, rotating nebula (gas + dust) contracted under gravity.
• Process:
  • Nebula flattened into a rotating disk.
  • Sun formed at the center.
  • Planets formed by condensation of matter in rings shed off from the disk.
• Limitations: Could not explain angular momentum distribution (Sun has most mass but little angular momentum).

Planetesimal Hypothesis

• Proposed by: Chamberlin & Moulton (1904).
• Idea:
  • A passing star’s gravitational pull drew filaments of matter from the Sun.
  • These filaments condensed into planetesimals (small solid bodies).
  • Planetesimals collided & accreted → planets.
• Limitations: Close stellar encounter is extremely improbable

Tidal Hypothesis

• Proposed by: James Jeans & Harold Jeffreys (1917–1925).
• Idea: A massive star passed close to the Sun → gravitational tides pulled matter → planets formed from it.
• Limitations: Same improbability of close stellar encounter.