Universe began as a rotating gaseous nebula.
Gradual cooling → contraction → faster rotation → disc formation.
Central mass → Sun; remaining matter → planets.
Criticism: Could not explain angular momentum distribution.
A passing star pulled material from the Sun → elongated “cigar-shaped” mass → condensed into planets.
Criticism: Improbable stellar pass; inconsistent with observed solar system features.
Sun had a companion star; gravitational interactions produced solar debris → planets.
Sun surrounded by solar nebula of hydrogen, helium, dust.
Collisions → disc formation → accretion → planets.
Most accepted explanation of universe formation.
Proposed as Expanding Universe Hypothesis (Evidence by Edwin Hubble, 1920s).
Singularity – all matter in an infinitely dense, hot point.
Explosion (13.7 billion years ago) – rapid expansion; energy → matter.
Formation of first atoms (within 3 minutes).
Universe became transparent (300,000 years) – temperature ↓ to ~4500 K → formation of neutral atoms → radiation escaped.
Hubble’s Redshift → galaxies moving away.
Cosmic Microwave Background Radiation (CMBR).
Abundance of light elements.
Universe is eternal with continuous matter creation.
Rejected due to strong observational support for Big Bang.
From nebulae: large clouds of hydrogen gas + dust.
Gravity → clumping → protostar → nuclear fusion → star.
Our Sun formed 5–6 billion years ago.
Stages
Rotating disc around newborn star.
Condensation of dust → small solid particles.
Planetesimals formation by cohesion.
Repeated collision & gravity → protoplanets → planets (~4.6 billion years ago).
Small, rocky, metal-rich; high density.
Solar winds removed gases; low gravity couldn’t retain atmosphere.
Formed far from Sun → gases condense → massive, low density.
Thick atmospheres: H₂, He.
E.g., Pluto, Eris (2003 UB313).
IAU (2006): reclassified Pluto as dwarf planet.
~4.44 billion years ago, a Mars-sized body (Theia) collided with Earth.
Ejected material → Earth’s orbit → accreted to form Moon.
Earlier rejected ideas:
Fission Theory (George Darwin): Moon split from Earth (Pacific Ocean basin).
Capture Theory: Moon formed elsewhere & captured by Earth’s gravity.
Hot, molten, volcanic, barren.
Thin atmosphere of H & He (lost due to solar wind).
Cooling → density stratification.
Differentiation:
Heavy materials (Fe, Ni) → core.
Lighter silicates → mantle & crust.
Resulted in layered Earth:
Crust → Mantle → Outer Core → Inner Core.
Three Stages:
Loss of Primordial Atmosphere (H & He blown away).
Secondary Atmosphere via Degassing
volcanic eruptions released H₂O vapor, CO₂, N₂, NH₃, CH₄.
Oxygenation Stage
Photosynthesis by cyanobacteria (3.0–2.5 billion years ago).
2 billion years ago → O₂ filled oceans → atmosphere.
Cooling → atmospheric water vapour condensed → rains.
CO₂ dissolved → further cooling → continuous rainfall.
Water accumulated in depressions → oceans formed ~4 billion years ago.
Earth initially hostile; high temperature, no O₂.
Life began ~3.8 billion years ago.
Likely started as complex organic molecules formed via chemical reactions (“Primordial Soup”).
Early life: unicellular, anaerobic, marine (blue-green algae / cyanobacteria).
Geological Time Scale shows progression from bacteria → plants → animals → mammals → humans.
Hadean (4.6 – 3.8 BYA) – Earth formation, oceans form.
Archean (3.8 – 2.5 BYA) – First life (bacteria).
Proterozoic (2.5 BYA – 570 MYA) – Oxygenation, soft-bodied organisms.
Phanerozoic (570 MYA – Present) – Complex life.
Palaeozoic – first vertebrates, fishes, amphibians.
Mesozoic – age of dinosaurs; extinction at 65 MYA.
Cenozoic – mammals dominate; humans evolve.
Differentiation: separation of Earth materials by density.
Degassing: release of gases from Earth’s interior via volcanism.
Light Year: distance light travels in 1 year = 9.461 × 10¹² km.
Terrestrial Planets: small, dense, rocky.
Jovian Planets: large, gaseous, low density.