Chapter 1 The Earth in the Solar System

IAS Exam preparation

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The Earth in the Solar System

The Earth is the fifth-largest planet in the solar system, and it is the third planet from the Sun. More than 70% of the Earth is covered in Water. So when we see it from outer space, the Earth appears to be blue and people call it the Blue Planet.

The Sun is the star at the center of our Solar System. It is a nearly perfect sphere of hot plasma, with internal convective motion that generates a magnetic field via a dynamo process.

Distance From Earth – 149.6 million km
Surface Temperature – 5,778 K
Mass – 1.989 × 10^30 kg
Radius – 696,340 km
Age – 4.603 Bn yrs /13.8 Bn yrs ago happened the Big Bang

What is a Planet?

A planet is a celestial body that is in orbit around the Sun, and has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and has cleared the neighbourhood around its orbit.

Under this definition, Pluto is NOT a planet, but has been deemed a dwarf planet because it has not yet cleared its orbit.

This definition is under discussion, particularly by members of the planetary science community, and it may yet be further refined.


  • There are 8 planets in our solar system, they are Mercury, Venus, Earth, Mars, jupitor, Saturn, Uranus and Neptune.

  • Planets in our solar system can be divided into two main groups, Terrestrial Planets  and Gas Giants.

The Kuiper belt, occasionally called the Edgeworth–Kuiper belt, is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune(30.1 AU) to approximately 50 AU(Astronomical Unit) from the Sun. It is similar to the asteroid belt, but is made up of ice and far larger—20 times as wide and 20 to 200 times as massive.

  • The terrestrial planets in our solar system orbit relatively close to the Sun, this gives them their other name; the “Inner Planets”

  • Earth is the most hospitable to life. Mars may have supported life in the past, but there is no evidence that conditions have ever been life-friendly on Mercury or Venus.

  • Each of the terrestrial planets has a central core made mostly of iron. The layer above the core is called the “mantle” and is usually made of silicate rocks. These are rocks rich in silicon and oxygen. The terrestrial planets are also sometimes referred to as the “rocky” planets.
  • The surfaces of terrestrial planets have mountains, craters, canyons, and volcanoes. About 75% of Earth’s surface is covered in water. Both Mars and Earth have permanent polar ice caps.

  • Terrestrial planets exist around other stars. Data from the Kepler mission suggest Earth-sized and so-called “super-Earth” worlds exist throughout the galaxy. There could be up to 40 billion such exoplanets in the Milky Way.

  • Planets that orbit other stars are referred  as Exoplanets.

  • The largest planet in the solar system is Jupiter, followed by Saturn, Uranus, Neptune, Earth, Venus, Mars with the smallest being Mercury.

  • Earth’s atmosphere protects us from meteoroids and radiation from the Sun.

  • Venus is the hottest planet in the solar system.

  • Jupiter has more than double the mass of all the other planets combined.

  • Saturn has more moons than any other planet in the Solar System.

  • Uranus has only been visited by a single spacecraft, Voyager 2.
  • It takes like more than 4 hours for light to reach Neptune from the Sun.(On Earth 490 to 507 seconds)

  • Together the planets make up 0.14% of the solar systems mass, 99% of which is the gas giants (Jupiter, Saturn, Uranus and Neptune).

  • Except for the Earth, the planets are named after gods from Roman and Greek mythology.
  • Celestial bodies

A celestial body can be defined as any natural body outside of the Earth’s atmosphere. Celestial bodies or Heavenly bodies are objects in space such as the Sun, moon, planets and stars, natural satellite, asteroid, meteoroids comet etc…

  • From largest to smallest they are: 


  • Constellations

Ursa Major or Big Bear is one such constellation. One of the most easily recognisable constellation is the Saptarishi (Sapta seven, rishi-sages). It is a group of seven stars (Figure 1.1) that forms a part of Ursa Major Constellation. Ask someone elder in your family or neighbourhood to show you more stars, planets and constellations in the sky.

  • Ancient time/Direction with constellations.

  • The North star indicates the north direction. It is also called the Pole Star.
  • Full moon
  • New moon

  • Jupiter, Saturn and Uranus have rings around them. These are belts of small debris. These rings may be seen from the earth with the help of powerful telescopes.

The Great Red Spot

is a persistent high-

pressure region in the

atmosphere of Jupiter,

producing an anti-

cyclonic storm, the

largest in the Solar System,

22 degrees south of Jupiter‘s equator. It has been continuously observed since 1830.

  • The Moon

Our earth has only one satellite, that is, the moon. Its diametre is only one-quarter that of the earth. It appears so big because it is nearer to our planet than other celestial bodies. It is about 3,84,400 km away from us. Now you can compare the distance of the earth from the sun and that from the moon.

  • The moon moves around the earth in about 27 days. It takes exactly the same time to complete one spin. As a result, only one side of the moon is visible to us on the earth. The moon does not have conditions favourable for life.

Currently there are over 2218 artificial satellites orbiting the Earth.

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Exercise For You

Upload answers of following:

1.How does a planet differ from a star?

2.What is meant by the ‘Solar System’?

3.Name all the planets according to their distance from the sun.

4.Why is the Earth called a unique planet?

5.Why do we see only one side of the moon always?

6.What is the Universe ?