GA Study Notes On Solar System : SSC CGL Tier-1 2017

Solar System

Our solar system consists of the sun (the star), 8 planets, 63 moons, millions of smaller bodies like asteroids and comets and a huge quantity of dust-grains and gases.Some celestial bodies do not have their own heat and light. They are lit by the light of the stars. Such bodies are called planets.

In order of their distance from the sun, the eight planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

Out of the eight planets, Mercury, Venus, Earth, and Mars are called as the inner planets as they lie between the sun and the belt of asteroids. They are called Terrestrial, meaning Earth-like as they are made up of rock and metals, and have relatively high densities.

The other four planets are called the outer planets. They are called Jovian or Gas Giant planets. Jovian means Jupiter-like. Most of them are much larger than the terrestrial planets and have a thick atmosphere, mostly of helium and hydrogen.

The Moon

The moon is the only natural satellite of the earth. It is now generally believed that the formation of the moon, as a satellite of the earth, is an outcome of ‘giant impact’ or what is described as “the big splat”. A body of the size of one to three times that of Mars collided into the earth sometime shortly

after the earth was formed. It blasted a large part of the earth into space. This portion of blasted material then continued to orbit the earth and eventually formed into the present moon about 4.44 billion years ago.

Evolution of Earth

The earth was mostly in a volatile state during its primordial stage. Due to the gradual increase in density the temperature inside has increased.As a result, the material inside started getting separated depending on their densities. This allowed heavier materials (like iron) to sink towards the centre of the earth and the lighter ones to move towards the surface. With the passage of time, it cooled further and solidified and condensed into a smaller size.This later led to the development of the outer

surface in the form of a crust.

The Sun

The sun is about 150 million km away from the earth.It is huge and made up of extremely hot gases.

Planets

Mercury (Budh) - It is nearest to the sun.It is the smallest planet of our solar system.It can be observed just before sunrise or just after sunset, near the horizon It takes only about 88 days to complete one round along its orbit. Mercury has no satellite of its own.
Venus (Shukra) - It is the brightest planet in the night sky. it is often called a morning or an evening star although it is not a star.Venus is considered as‘Earth’s-twin’ because its size and shape are very much similar to that of the earth.Venus has no moon or satellite of its own. It rotates from east to west while the Earth rotates from west to east.
The Earth (Prithvi) - The earth is the third nearest planet to the sun, it is the fifth largest planet.It is slightly flattened at the poles. That is why its shape is described as a Geoid.From space, the Earth appears blue-green due to the reflection of light from water and landmass on its surface. The Earth has only one moon.
Mars (Mangal) - It appears slightly reddish and, therefore, it is also called the red planet. Mars has two small natural satellites.
Jupiter (Brihaspati) - Jupiter is the largest planet of the solar system. It is so large that about 1300 earths can be placed inside this giant planet. However, the mass of Jupiter is about 318 times that of our Earth. It rotates very rapidly on its axis.Jupiter has a large number of satellites. It also has faint rings around it.
Saturn (Shani) - Saturn appears yellowish in colour.It looks beautiful because of rings. Saturn also has a large number of satellites. it is the least dense among all the planets. Its density is less than that of water.
Uranus and Neptune - Like Venus, Uranus also rotates from east to west. it has highly tilted rotational axis in its orbital motion and thus it appears to roll on its side.

Jupiter, Saturn, and Uranus have rings around them. These are belts of small debris.

Pluto - Pluto was also considered a planet. However, in a meeting of the International Astronomical Union, a decision was taken that Pluto-like other celestial objects (Ceres, 2003 UB₃₁₃) discovered in recent past may be called ‘dwarf planets.”