Solar System — Introduction

The Solar System consists of the Sun and all celestial bodies that orbit it — planets, moons, asteroids, comets, and dwarf planets.

• ›The Sun contains 99.86% of the Solar System's total mass.
• ›The Solar System is located in the Milky Way galaxy, in the Orion Arm.
• ›Age of the Solar System: approximately 4.6 billion years.
• ›The force keeping planets in orbit: Gravity (Newton's Law of Gravitation).

This chapter frequently appears in BPSC TRE-4 General Knowledge / Science sections.

The Sun

The Sun is the star at the centre of our Solar System.

• ›It is a medium-sized yellow star (G-type main-sequence star).
• ›Surface temperature: ~5,500°C; core temperature: ~15 million°C.
• ›Energy source: Nuclear Fusion — hydrogen fuses into helium.
• ›Light from Sun reaches Earth in: 8 minutes 20 seconds.
• ›Distance from Earth: ~150 million km (1 AU — Astronomical Unit).
• ›Sunspots: Dark, cooler areas on the Sun's surface; cycle of ~11 years.
• ›The outermost layer of the Sun: Corona (visible during solar eclipse).

The Eight Planets (Order from Sun)

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(Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune)

• ›Mercury — smallest; closest to Sun; no atmosphere; extreme temperatures.
• ›Venus — hottest planet (~465°C); thick CO₂ atmosphere; Earth's twin in size; brightest planet in sky.
• ›Earth — only planet with life; has one natural satellite (Moon).
• ›Mars — Red Planet; highest volcano: Olympus Mons; two moons: Phobos & Deimos.
• ›Jupiter — largest planet; Great Red Spot (giant storm); 95 known moons.
• ›Saturn — famous for its rings; least dense planet (floats in water).
• ›Uranus — rotates on its side (axial tilt ~98°); ice giant.
• ›Neptune — farthest planet; strongest winds in Solar System.

Moon, Asteroids & Comets

Moon:

• ›Earth's only natural satellite.
• ›Distance from Earth: ~3.84 lakh km.
• ›Moon's gravity = 1/6 of Earth's.
• ›No atmosphere; extreme temperature range.
• ›Lunar eclipse: Earth's shadow falls on Moon.
• ›Solar eclipse: Moon's shadow falls on Earth.

Asteroids: Rocky bodies orbiting the Sun, mostly in the Asteroid Belt between Mars and Jupiter.

Comets: Icy bodies with bright tails when near the Sun. Famous: Halley's Comet — visible every ~76 years (last seen 1986; next ~2061).

Dwarf Planets: Pluto (reclassified in 2006), Eris, Ceres.

Points to Remember

• ›Order of planets — Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
• ›Largest planet — Jupiter; smallest — Mercury
• ›Hottest planet — Venus (not Mercury despite being closest to Sun)
• ›Red Planet — Mars; highest volcano Olympus Mons
• ›Saturn — least dense; famous for rings
• ›Uranus — rotates on its side
• ›Pluto — reclassified as dwarf planet in 2006
• ›Sun's light reaches Earth — 8 min 20 sec; distance = 1 AU = 150 million km
• ›Halley's Comet — every 76 years; last 1986
• ›Moon's gravity — 1/6 of Earth
• ›Exam tip: Venus is the hottest, not Mercury — because Venus has a thick CO₂ atmosphere that traps heat (greenhouse effect); Mercury has no atmosphere.
• ›Exam tip: Neptune is the farthest planet, not Pluto — Pluto is now a dwarf planet.