The Universe and the Solar System
NCERT Class 6 - The Earth Our Habitat • NCERT Class 11 - Fundamentals of Physical Geography, Chapter 2
The Universe and the Solar System
The universe is believed to have begun with the Big Bang, an expansion from an extremely dense and hot state about 13.7 billion years ago. Our solar system formed within the Milky Way galaxy and consists of the Sun, eight planets, their moons, asteroids, comets and meteoroids.
Universe, galaxy and stars
- The Big Bang theory holds that all matter was concentrated in a tiny volume that expanded, and that the universe is still expanding.
- A galaxy is a vast system of stars, gas and dust held together by gravity. Ours is the Milky Way, called Akash Ganga, a spiral galaxy; Andromeda is the nearest large galaxy to it.
- Stars form from clouds of gas and dust called nebulae; gravity pulls the matter together until nuclear fusion begins.
- Distance in space is measured in light years - the distance light travels in one year, about 9.46 trillion kilometres.
- Constellations are patterns of stars; the Saptarishi or Great Bear and the Pole Star are used for direction.
The Sun and the planets
- The Sun is a medium sized star of mostly hydrogen and helium, about 150 million kilometres from the earth. Its light takes about 8 minutes and 20 seconds to reach us.
- Inner or terrestrial planets - Mercury, Venus, Earth and Mars - are close to the Sun, made of rock and metal, small, dense and with few or no moons.
- Outer or Jovian planets - Jupiter, Saturn, Uranus and Neptune - are far from the Sun, made largely of gas, very large, less dense and with many moons and rings.
- The asteroid belt between Mars and Jupiter separates the two groups.
| Planet | Remembered for |
|---|---|
| Mercury | Smallest planet and closest to the Sun |
| Venus | Hottest planet, earth's twin in size, rotates east to west |
| Earth | Only planet known to support life, called the blue planet |
| Mars | The red planet, has the largest volcano in the solar system |
| Jupiter | Largest planet, has the Great Red Spot and the largest moon, Ganymede |
| Saturn | Bright rings, the least dense planet, would float in water |
| Uranus | Tilted nearly on its side, so it appears to roll along its orbit |
| Neptune | Farthest planet from the Sun, with the strongest winds |
Other members and the moon
- Pluto was reclassified as a dwarf planet in 2006, along with Ceres and Eris.
- Asteroids are rocky bodies mainly between Mars and Jupiter; comets are icy bodies with long orbits, of which Halley's comet, returning about every 76 years, is best known.
- Meteoroids entering the atmosphere burn up as meteors or shooting stars; those that reach the ground are meteorites.
- The moon is the earth's only natural satellite, about 384,400 kilometres away.
- It takes about 27 days both to rotate on its axis and to revolve around the earth, so the same side always faces us.
- The moon has no atmosphere and no water, and its gravity is about one-sixth of the earth's.
हिंदी संकेत: शुक्र (Venus) सबसे गर्म ग्रह है, सबसे निकट वाला बुध (Mercury) नहीं - क्योंकि शुक्र के घने वायुमंडल में ऊष्मा फंस जाती है। यह सबसे आम जाल है।
Exam me kaise aata hai
Match-the-following on planet and its distinguishing feature is almost certain. Factual questions ask the time sunlight takes to reach earth, the nearest galaxy, and when Pluto was reclassified. Statement-based questions test which planet is hottest and which is least dense.
UPSC / State PSC ke liye
Note why the two groups of planets differ - the young Sun's heat drove light gases away from the inner region, leaving rock and metal there, while the cold outer region retained hydrogen and helium. This explanation converts a list of facts into a reasoned answer.
Yahan confuse hote hain
✗ Mercury is the hottest planet because it is closest | ✓ Venus is hottest, because its dense atmosphere traps heat
✗ Pluto is the ninth planet | ✓ It was reclassified as a dwarf planet in 2006
✗ A light year measures time | ✓ It measures distance - how far light travels in one year
Ek nazar me
- The universe began with the Big Bang about 13.7 billion years ago and is still expanding.
- Our galaxy is the Milky Way or Akash Ganga; Andromeda is the nearest large galaxy.
- A light year is a unit of distance, about 9.46 trillion kilometres.
- Sunlight takes about 8 minutes 20 seconds to reach the earth.
- Inner planets are rocky and small; outer planets are gaseous and large.
- Venus is hottest, Jupiter largest, Saturn least dense, Uranus tilted on its side.
- Pluto became a dwarf planet in 2006; the moon keeps the same face towards earth.