The Earth in the Solar System
Introduction
Have you ever looked up at the night sky and wondered what those twinkling lights are? Some are stars, some are planets, and some are other celestial bodies. Our Earth is just one small part of an enormous system called the Solar System. In this chapter, we will learn about the Sun, the planets, and other members of our cosmic neighbourhood.
The Solar System
The Solar System consists of the Sun and all the celestial bodies that revolve around it:
- 8 planets
- 5 known dwarf planets
- Asteroids
- Comets
- Meteors
- Moons (natural satellites)
- Dust and gas
The Sun is at the centre of the Solar System. All other bodies orbit around it due to its tremendous gravitational pull.
The Sun — Our Nearest Star
The Sun is a star — a huge ball of hot, glowing gases (mostly hydrogen and helium).
Key Facts About the Sun
| Property | Value |
|---|---|
| Diameter | About 1,392,000 km (109 times Earth's diameter) |
| Temperature (surface) | About 5,500°C |
| Temperature (core) | About 15,000,000°C |
| Distance from Earth | About 150 million km (1 AU — Astronomical Unit) |
| Age | About 4.6 billion years |
| Composition | Hydrogen (73%), Helium (25%), Other elements (2%) |
| Gravity | 28 times Earth's gravity |
The Sun is about 109 times wider than Earth — you could fit about 1.3 million Earths inside the Sun!
The Sun provides:
- Light and heat — Essential for life on Earth
- Energy — Drives weather, ocean currents, and plant growth (photosynthesis)
- Gravity — Keeps all planets in their orbits
The Planets
A planet is a celestial body that:
- Orbits the Sun
- Has sufficient mass to be round (due to its own gravity)
- Has cleared its orbit of other objects
The Eight Planets (In Order from the Sun)
| No. | Planet | Diameter (km) | Distance from Sun (million km) | Orbital Period | Rotation Period | Number of Moons |
|---|---|---|---|---|---|---|
| 1 | Mercury | 4,879 | 57.9 | 88 days | 58.6 days | 0 |
| 2 | Venus | 12,104 | 108.2 | 225 days | 243 days (retrograde) | 0 |
| 3 | Earth | 12,742 | 149.6 | 365.25 days | 24 hours | 1 |
| 4 | Mars | 6,779 | 227.9 | 687 days | 24.6 hours | 2 |
| 5 | Jupiter | 1,39,820 | 778.5 | 11.86 years | 9.93 hours | 95 |
| 6 | Saturn | 1,16,460 | 1,432 | 29.46 years | 10.7 hours | 146 |
| 7 | Uranus | 50,724 | 2,867 | 84 years | 17.2 hours (retrograde) | 27 |
| 8 | Neptune | 49,244 | 4,515 | 164.8 years | 16.1 hours | 16 |
Inner Planets vs Outer Planets
| Inner Planets (Mercury, Venus, Earth, Mars) | Outer Planets (Jupiter, Saturn, Uranus, Neptune) |
|---|---|
| Closer to the Sun | Far from the Sun |
| Small size | Large size |
| Rocky surface | Gaseous (no solid surface) |
| Few or no moons | Many moons |
| No rings | Have rings |
| High density | Low density |
Special Features of Each Planet
Mercury: Smallest planet; closest to the Sun; no atmosphere; extreme temperatures (-180°C to 430°C); looks like our Moon
Venus: Hottest planet (465°C); similar size to Earth; thick atmosphere of CO₂; rotates backwards (sunrise in west); called 'Morning Star' or 'Evening Star'
Earth: Only planet with known life; liquid water on surface; atmosphere with oxygen; one large moon
Mars: Called the 'Red Planet' (due to iron oxide/rust); has the largest volcano (Olympus Mons — 21.9 km high) and the largest canyon (Valles Marineris — 4,000 km long); ice at poles
Jupiter: Largest planet; Great Red Spot (giant storm); famous for its many moons (Io, Europa, Ganymede, Callisto)
Saturn: Famous for its beautiful ring system (made of ice and rock particles); least dense planet (would float in water!)
Uranus: Rotates on its side (tilted 98°); blue-green colour due to methane
Neptune: Farthest planet; strongest winds in the solar system (2,000 km/h); blue colour
A simple mnemonics to remember planets in order: My Very Eager Mother Just Served Us Noodles (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune).
Earth — The Unique Planet
Earth is special because:
- Life — It is the only planet known to have life.
- Water — About 71% of Earth's surface is covered with water. It is called the 'Blue Planet'.
- Atmosphere — Contains oxygen (21%), nitrogen (78%), and other gases. Protects from harmful solar radiation.
- Temperature — Not too hot, not too cold. The average temperature is 15°C.
- Distance from Sun — Earth is at the perfect distance (Goldilocks zone) for liquid water to exist.
- Magnetic field — Protects from solar wind.
- Ozone layer — Filters UV radiation.
Earth is the only planet not named after a Roman or Greek god. The name 'Earth' comes from Old English and Germanic words meaning 'ground' or 'soil'.
Other Members of the Solar System
Asteroids
- Small rocky bodies that orbit the Sun
- Mostly found between Mars and Jupiter (the Asteroid Belt)
- Largest asteroid: Ceres (also a dwarf planet) — 940 km in diameter
- Some asteroids are as small as pebbles
- An asteroid impact is believed to have caused the extinction of dinosaurs
Comets
- Made of ice, dust, and rock ('dirty snowballs')
- Have a glowing head and a long tail (which always points away from the Sun)
- Orbit the Sun in long, elliptical paths
- Famous comets: Halley's Comet (visible every 75-76 years, next seen in 2061)
- Oort Cloud is a huge cloud of comets at the edge of the Solar System
Meteors and Meteorites
- Meteoroid: A small piece of rock in space
- Meteor: When a meteoroid burns up entering Earth's atmosphere ('shooting star')
- Meteorite: When a meteoroid survives and lands on Earth's surface
The Moon — Earth's Satellite
A satellite is a body that revolves around a planet. The Moon is Earth's only natural satellite.
| Property | Value |
|---|---|
| Diameter | 3,474 km (about 1/4 of Earth) |
| Distance from Earth | 384,400 km |
| Orbital period | 27.3 days |
| Surface temperature | -233°C to 123°C |
| Gravity | 1/6 of Earth's gravity |
| Atmosphere | Almost none |
Features of the Moon
- No atmosphere, water, or life
- Surface covered with craters (from meteor impacts)
- We always see the same side of the Moon (tidal locking)
- The Moon causes tides on Earth
- Light of the Moon is actually reflected sunlight
The Moon is the only celestial body where humans have walked. Neil Armstrong and Buzz Aldrin landed on the Moon in 1969 (Apollo 11 mission).
Constellations
A constellation is a group of stars forming a recognizable pattern:
| Constellation | Meaning | Best Seen |
|---|---|---|
| Ursa Major | Great Bear (contains the Big Dipper) | All year (northern sky) |
| Ursa Minor | Little Bear (contains the North Star/Pole Star) | All year |
| Orion | The Hunter | Winter |
| Cassiopeia | Queen (W-shaped) | All year |
| Saptarishi | Seven Sages (Indian name for Ursa Major) | All year |
Saptarishi (Big Dipper) is important in Indian astronomy. It has seven stars that form the shape of a ladle. The Pole Star (Dhruva Tara) is located along a line drawn from two stars of the Saptarishi.
AP Context: Sriharikota Space Centre (SHAR)
Andhra Pradesh is home to India's spaceport:
- Location: Sriharikota Island, Tirupati district, Andhra Pradesh
- Full name: Satish Dhawan Space Centre (SDSC) SHAR
- Established: 1971
- Importance: India's primary launch centre for satellites and space missions
- Launches from here: PSLV, GSLV, and LVM3 rockets
- Famous missions: Chandrayaan (Moon), Mangalyaan (Mars), Aditya-L1 (Sun)
Sriharikota is on a barrier island off the Bay of Bengal coast. Its location near the equator gives rockets an extra boost from the Earth's rotation.
Common Mistakes Students Make
- Thinking Pluto is still a planet — it was reclassified as a dwarf planet in 2006.
- Confusing rotation (spinning on axis) with revolution (orbiting around the Sun).
- Believing stars and planets are the same — planets shine by reflected light; stars produce their own light.
- Thinking the Sun is a planet — it is a star.
- Forgetting the correct order of planets from the Sun.
Quick Self-Test
- What is the Solar System?
- Why is Earth called a unique planet?
- Name the planets in order from the Sun.
- What is the difference between a meteoroid, meteor, and meteorite?
- What are asteroids and where are they found?
- What is a constellation? Name two.
- What is special about the Moon?
- Why does Venus look like a star in the morning and evening?
- Where is Sriharikota located and why is it important?
- What is the difference between inner and outer planets?
Exam Focus (5 Marks)
- Describe the Solar System and its members.
- Explain why Earth is suitable for life.
- Describe the difference between inner and outer planets.
- What is the importance of Sriharikota for India's space programme?
