The Solar System:

Our sun is a star and hence has its star system known as the solar system. Everything within the sun’s gravitational control constitutes what is called our solar system. This sun-centered system comprises of the sun, nine planets (our Earth included), sixty one natural satellites called the Moons, about thousands of asteroids ( referred to as planetoids), scores of comets and uncounted millions meteors, all of which revolve round the sun.

The Sun:

The sun is just a member among the 136 billion stars of Milky Way galaxy. Our sun is a relatively small star. Epsilon Aurigie, largest star known so far in our galaxy is about 27 x 107 times as big as Sun.

Our sun is a huge sphere of hot gaseous mass. The elements present are form of burning gases and vapours.

The major elements which make up bulk of the Sun are hydrogen and helium. It emits radiant energy in form of heat and light and this energy is generated within it by conversi of hydrogen to helium through nuclear reaction. The temperatures at surface and at the centre of the sun are 6000° C and 20,000,000° C (million degree Celsius) respectively. Its diameter is about 190 tis greater than the earth’s diameter.

The Planets:

The name planet evolved from the Greek word “Planets” meaning wandering. Planets are, thus, heavenly bodies which appear to wander or move about the sky whereas the stars are seemingly fixed in their relative positions in the sky. The sun has nine planets which revolve round it in elliptical orbits and also rotate about their own axes.


The nine planets as arranged in the order of their increasing distances fro the Sun are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto

The nine planets fall into the following two groups which vary so much in size, density and composition.

(1) The inner group of planets which include Mercury, Venus, Earth and Mars.


(2) The outer group of planets which include Jupiter, Saturn, Uranus, Neptune and Pluto.

The inner planets are small in size, fairly dense (higher average density), hotter in comparison with the outer planets which are larger in size (except Pluto which is a tiny planet), lighter with low average density and cooler. The outer group contains much of light gases of hydrogen and helium; where as the planets of the inner groups are almost devoid of light gases. The inner planets, being hotter and of weak gravitational field, lack in lighter gases.

In general, the planetary motions show systematic uniformity. The orbits of all planets, except Pluto and Mercury lie in one plane. All planets revolve round the sun in elliptical orbits and in the same direction from west to the east except Uranus which rotates east to west.

Mercury is the tiniest planet and Jupiter is the largest. In respect of size, the Earth ranks fifth. The order of the planets based on their increasing sizes would be Pluto, Mercury, Mars, Venus, Earth, Uranus, Neptune, Saturn and Jupiter


The Earth is the densest planet of the solar system and Saturn is the lightest planet having the lowest density.

Three of the inner planets i.e. Earth, Venus and Mars are known to have definite atmosphere in some form. The astrophysicists are of the opinion that life may possibly exist on Mars that shows markings of dried river channels etc.

Among the outer planets, Jupiter and Saturn are known to have cold and dense atmosphere which is rich in methane and ammonia gases. The two planets commonly seen in the sky in naked eye are Venus and Mars. Venus is the brightest of all planets and clearly visible during sunrise and sunset. Mars looks reddish in colour. The numerical data of the planets of solar system are given in Table 1.1.


Also known as planetoids, these are a group of smaller bo”‘ of the system found in between the inner and the outer planets (betw Mars and Jupiter). These are thought to be remnants of a planet which broke into pieces by collision with another bigger heavenly body, asteroids very in size (diameter) from 2 km to over 800 km. There about 1500 asteroid bodies.


Comets are wandering members of the solar-system. They move around the sun in highly eccentric orbits. Comet derives its name from t’ Greek word ‘Kometes’ meaning long-haired. Comets are objects that g1 around the sun with regular periodicity. Some comets are known t reappear to our vision at predictable intervals. The famous “Halley’s comet” appears after every 76 years. It last appeared in 1986 and shall reappear next again in 2062.


Meteors are heavenly objects which move about in space within the solar system in definite courses. They greatly vary in size, shape and composition. During their flight in space, the meteors may come within the earth’s gravitational field. As they fall through the earth’s atmosphere, they get burnt up by frictional heat produced and emit flash of light.

Many of them may burn away to yield ashes and dusts, while some of them may survive total burning and fall on the earth’s surface in the form of solid bodies called “Meteorites”. The world’s largest meteorite that fell on South-West Africa weighs 70 tonnes. Meteorites vary in composition.

It is surmised that the meteorites represent either broken parts of comets or pieces of asteroids. Meteorites provide us with information about the composition of the earth’s interior.


Satellites (otherwise called the moons) are heavenly bodies which revolve round the natural planets. Like the planets, the satellites do not themselves produce light but get lighted by the Sun’s rays reflecting from their surface. There are in total sixty one (61) satellites belonging to the planetary domains of our solar system. Two planets e.g., Mercury and Venus do not possess moon. Saturn has the highest number of 18 satellites, followed by Jupiter (16).


The earth has one satellite (i.e. our moon) only. Some satellites of the outer planets are quite large in size and somewhat larger than Mercury. Titan, the largest satellite of Saturn is the only satellite known to have an atmosphere composed mainly of nitrogen.