What is a Planet?

A planet is a celestial body that:

• Revolves around a star (Sun)
• Moves in an elliptical orbit
• Does not emit its own light

All 8 planets revolve around the Sun counter-clockwise (as seen from above the Sun’s north pole).

Classification of Planets

Inner Planets (Terrestrial Planets)

Members

• Mercury
• Venus
• Earth
• Mars

Key Features

• Made of silicates + iron & nickel core
• Smaller but denser
• Solid rocky surface
• Impact craters, volcanoes, and rift valleys are common
• Venus, Earth & Mars have atmospheres

 Mercury

• Closest planet to the Sun
• Almost no atmosphere
• Extreme temperature variation
  • Day: +427°C
  • Night: −173°C
• Heavily cratered → geologically inactive
• Smaller than moons Ganymede & Titan, but more massive
• MESSENGER mission found:
  • Evidence of volcanism
  • Water ice at the poles

Seen only near the horizon during sunrise/sunset

Venus

• Brightest planet (highest albedo)
• Called Morning Star / Evening Star
• Thick atmosphere: 96% CO₂
• The hottest planet due to the extreme greenhouse effect
• Atmospheric pressure = 92 times Earth’s
• Retrograde rotation (clockwise)
• One day (243 Earth days) > one year (224 days)

Known as Earth’s twin (size & mass similar, conditions opposite)

 Earth

• Densest planet in the Solar System
• Only known planet supporting life
• One moon
• Axial tilt: 5° → seasons
• Balanced atmosphere & liquid water

Mars

• Known as the Red Planet (iron oxide)
• Thin atmosphere (≈96% CO₂)
• Lost magnetosphere → atmosphere stripped by solar wind
• Evidence of past liquid water
• Water is now locked in polar ice caps
• Largest volcano: Olympus Mons
• Largest canyon: Valles Marineris
• Two moons: Phobos & Deimos
• Seasons similar to Earth

Methane presence suggests an active geological or biological source

The Moon (Earth’s Satellite)

Key Facts

• Diameter = ¼ of Earth
• Distance from Earth ≈ 3,84,400 km
• Tidally locked → only one side visible
• Stabilises Earth’s axial tilt

 Formation

• Giant Impact Theory (“Big Splat”)
• A Mars-sized body collided with early Earth

 Important Discoveries

• Luna-2 (1959) → first object to reach the Moon
• Apollo-11 (1969) → first humans on Moon
• Chandrayaan-1 (2009) → water at lunar poles

Outer Planets

Members

• Jupiter
• Saturn
• Uranus
• Neptune
• (Pluto → dwarf planet)

 General Features

• Composed mainly of hydrogen & helium
• No solid surface
• Thick atmospheres
• Strong magnetospheres
• Rings & numerous moons
• Account for 99% of planetary mass

Gas Giants vs Ice Giants

Why are Inner Planets Rocky & Outer Planets are Gaseous?

• Inner planets formed close to the Sun
• High temperature → gases could not condense
• Strong solar wind blew away gases
• Smaller gravity → could not retain gases
• Outer planets formed farther → cooler → gases condensed & retained

Jupiter

• Largest planet
• Rapid rotation → equatorial bulge
• No solid surface
• Four Galilean moons:
  • Io, Europa, Ganymede, Callisto
• Ganymede → largest moon in the Solar System
• Latest mission: Juno

Saturn

• Least dense planet → can float on water
• Prominent ring system
• Moon Titan:
  • Nitrogen-rich atmosphere
  • Only the moon with a dense atmosphere

Uranus

• Rotates on its side
• Axis almost parallel to orbital plane
• Retrograde rotation
• Extreme seasons

Neptune

• Strongest winds in the Solar System (~2100 km/h)
• Ice giant
• Twin of Uranus