D.4] Origin of Solar System

1. Formation Theories: Nebular Hypothesis

1.1 Nebular Hypothesis

• Proposed by: Immanuel Kant (1755), Pierre-Simon Laplace (1796)
• Key Idea: The solar system formed from a rotating cloud of gas and dust (nebula)
• Stages of Formation:
  • Nebula Collapse: Gravitational collapse of a molecular cloud due to disturbance (e.g., supernova shockwave)
  • Formation of Protostar: Central mass forms the Sun
  • Accretion Disk: Remaining material forms a rotating disk around the Sun
  • Planet Formation: Dust and gas clump together to form planetesimals, which further accrete into planets

1.2 Other Theories (Less Accepted)

• Capture Theory: Planets captured from other stars (not widely accepted)
• Binary Star Theory: Solar system formed from a binary star system (not widely accepted)

2. Facts on Sun and 8 Planets

2.1 Sun

• Composition: 73% hydrogen, 25% helium, 2% other elements
• Mass: ~1.989 × 10³⁰ kg (99.86% of total solar system mass)
• Temperature: Surface ~5,500°C, Core ~15 million °C
• Luminosity: ~3.828 × 10²⁶ Watts
• Age: ~4.6 billion years

2.2 Planets (Inner & Outer)

3. Meteors and Meteorites

3.1 Meteors

• Definition: A meteor is a streak of light in the sky caused by a meteoroid entering Earth’s atmosphere
• Also Known As: Shooting star
• Composition: Mostly rock and metal
• Speed: ~110,000 km/h (varies depending on entry angle)
• Fate: Most burn up in the atmosphere (typically at ~80–120 km altitude)

3.2 Meteorites

• Definition: A meteorite is a meteor that survives atmospheric entry and lands on Earth
• Types:
  • Stony Meteorites: Most common, composed mainly of silicates
  • Iron Meteorites: Composed mainly of iron and nickel
  • Stony-Iron Meteorites: Rare, mix of silicates and metal
• Significance: Provide insights into the early solar system and planetary formation

3.3 Famous Meteorite Events

• Tunguska Event (1908): Massive explosion over Siberia, believed caused by a meteoroid or comet
• Chelyabinsk Meteor (2013): Large meteor exploded over Russia, causing widespread damage

4. Asteroids and Comets

4.1 Asteroids

• Definition: Small rocky bodies orbiting the Sun, primarily found in the asteroid belt between Mars and Jupiter
• Composition: Mostly rock and metal
• Size Range: ~1 meter to ~1,000 km in diameter
• Notable Asteroids:
  • Ceres: Largest asteroid, classified as a dwarf planet
  • Vesta: Second-largest asteroid, studied by NASA’s Dawn mission
• Potential Threat: Some asteroids may have orbits that intersect with Earth’s

4.2 Comets

• Definition: Icy bodies composed of dust, rock, and frozen gases (volatiles)
• Composition: 50% ice, 50% rock and dust
• Orbit: Highly elliptical, often from the outer solar system (Oort Cloud or Kuiper Belt)
• Tail Formation: Ion tail (blue, ionized gas) and dust tail (yellow, particles)
• Notable Comets:
  • Halley’s Comet: Periodic, visible every ~76 years
  • Comet Hale-Bopp: One of the most visible comets in modern times
  • Comet 67P/Churyumov–Gerasimenko: Target of Rosetta mission

4.3 Differences Between Asteroids and Comets

5. Key Facts for Competitive Exams (SSC, RRB)

• Nebular Hypothesis is the most accepted theory for solar system formation
• Sun is the most massive object in the solar system
• Jupiter is the largest planet, with the Great Red Spot
• Meteorites are valuable for studying early solar system materials
• Comets have tails formed by solar radiation and solar wind
• Asteroids are mostly found in the asteroid belt
• Halley’s Comet has a period of ~76 years
• Ceres is the only dwarf planet in the asteroid belt
• Tunguska Event is a famous meteoroid impact event
• Rosetta Mission studied comet 67P/Churyumov–Gerasimenko