Cosmic Family Portrait
Sun at centre with orbits of eight planets
One Snapshot of Our Solar System
The Sun anchors everything with its strong gravity and enormous mass.
Eight major planets loop around it on nearly circular paths called orbits.
Key Points:
- Sun stays fixed at the centre; all motion happens around it.
- Planetary orbits are flat rings spread out like tracks on a disk.
- The farther a planet is, the bigger and slower its orbit becomes.
Planets & Moons
Planet & Moon
Planet: A large, nearly round body that orbits the Sun and has swept other objects from its path.
Moon: A natural satellite—a smaller body that orbits a planet and travels with it around the Sun.
Knowing these roles explains why Earth qualifies as a planet, while its companion is a moon.
Multiple Choice Question
Question
Which of these is NOT classified as a major planet?
Hint:
The International Astronomical Union lists only eight major planets.
Correct!
Great! Pluto is a dwarf planet, so it is not part of the eight major planets.
Incorrect
Remember: the major planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
Dwarf Planets
Pluto, Eris and Ceres are all classified as dwarf planets.
What Makes a Dwarf Planet?
A dwarf planet orbits the Sun and is round, but it is not heavy enough to sweep other objects from its path.
Think of it as an almost-planet sharing its orbit with nearby rocks and ice.
Key Points:
- Pluto – icy dwarf beyond Neptune.
- Eris – similar size to Pluto but even farther out.
- Ceres – rocky dwarf in the asteroid belt.
- Unlike major planets, none has cleared its orbit.
Asteroids
Most asteroids orbit in a belt between Mars and Jupiter.
Space Rocks in a Belt
Asteroids are irregular chunks of rock and metal that orbit the Sun.
About two million crowd a zone called the Asteroid Belt between Mars and Jupiter.
Key Points:
- Main home: Asteroid Belt (Mars–Jupiter gap).
- Composition: solid silicate rock with metals like iron and nickel.
- Too small for gravity to shape them into planets.
The Sun—Our Star
What Makes the Sun Special?
The Sun is an average-sized star made of about 75 % hydrogen and 24 % helium.
Its radius is roughly 7 × 105 km and its mass 2 × 1030 kg—holding 99.8 % of Solar System mass.
At a 15 million-degree core, hydrogen nuclei fuse into helium, producing the light and heat that reach Earth.
Key Points:
- 40 % of energy arrives as visible light, 50 % as infrared heat, 10 % as ultraviolet rays.
- Solar gravity keeps planets, moons, asteroids, and comets in their orbits.
- The Sun’s light and warmth make life and liquid water on Earth possible.
Held by Gravity
The Sun’s mass warps space-time; planets move around the dip.
Sun’s Gravitational Pull Keeps Planets in Orbit
Imagine space as a stretched rubber sheet.
The Sun’s huge mass presses the sheet down, forming a deep well.
Planets move sideways across the slope; gravity curves their paths into steady orbits.
Key Points:
- Gravitational pull is strongest at the Sun’s centre.
- It provides the inward force that holds planets in orbital paths.
- Sideways motion and gravity balance to create stable, nearly circular orbits.
- Without this pull, planets would drift away into space.