The sun is the center of our solar system. The sun is a star, a self luminous mass of gases that emits radiant energy. Besides the sun, other members of the solar system are the nine planets, 32 satellites, asteroids, comets, meteors, and gaseous particles which reflect light known as zodiacal light.

The sun which occupies the central position in the solar system is the parent of all the other members. The tenth planet 2003 UB313 has also been recently sighted.

Mercury, Venus, Earth and Mars are called as the inner planets as they lie between the sun and the asteroids, while the other five planets namely Jupiter, Saturn, Uranus, Neptune and Pluto are called the outer planets.

Alternatively, the first four planets are known as terrestrial meaning earthlike as they are made up of rocks and metals, while the rest five are called Jovian or gas giant planets. Jovian means Jupiter like.


Most of the Jovian planets are larger than the terrestrial planets and have thick atmosphere, mostly of helium and hydrogen. All the planets were formed in the same period, sometime about 4.6 billion years ago.

All the planets revolve round the sun in the same counterclockwise direction. Their orbits are almost elliptical. They rotate on their own axis in anticlockwise direction (from west to east) except Venus and Uranus which rotate on their axis from east to west.

However, all the planets without any exception revolve round the sun anticlockwise. Their orbits follow an elliptical path. All have an atmospheric layer of gases with the possible exception of Pluto and Mercury which may not be dense enough to hold gases.

Although the planets are similar in many ways, but they differ in certain ways because of two factors, the position of a planet with respect to the sun and a planet’s mass. Mercury has a diameter less than half that of the earth and its mass. Mercury does not have the mass and sufficient gravitational pull to hold on to an atmosphere.


Since mercury rotates on its axis once every 59 days, the same side of the planet faces the sun for long periods with the result that the opposite side receives none of the solar energy. The side exposed to the sun is extremely hot and the opposite side is too cold to support life.

Uranus, Neptune and Pluto, the three planets are too distant to receive enough solar energy to have surface temperatures suitable to support life. In the absence of free oxygen, there is no development of life whatsoever, on these planets.

Pluto is the smallest planet and the farthest from the Sun. It makes a complete revolution around the Sun in 248 years. Its orbit is highly inclined.

Jupiter and Saturn are the two larges planets, and both have very low densities. These planets have small solid central cores. The atmosphere of these planets is composed mostly of hydrogen and helium.


Methane and ammonia are the minor constituents of their atmosphere. There is complete absence of free oxygen.

Despite the fact that these two largest planets are closer to the Sun than are Uranus, Neptune and Pluto, they still do not receive adequate solar energy to produce livable surface temperatures; their surface temperatures are as low as nearly -95°C and -150°C.

However, there are warmer areas that are nearly closer to the earth’s surface temperatures; it is true that no life exists there. Their opaque cloud cover does not allow a clear view of the surface.

The Jupiter has 13 known satellites which travel in nearly circular orbits around it. The Jupiter’s largest satellite is named as ‘Ganymede’, which is a little larger than the planet mercury, while the others are almost similar to the earth’s moon in size.


The most characteristic feature of the planet Saturn is its spectacular system of rings which were discovered by Galileo. The Saturn is also called the ‘Ringed Planet’. These rings are gaseous.

The Saturn has 14 satellites which lie beyond the outer edge of the last ring. The largest satellite of the Saturn is called ‘Titan’ which is believed to have some form of life.

Of all the planets accept our own planet, the earth, mars is better known to us. Thousands of pictures of its surface were taken by Mariner 9 which was launched in 1971. This planet has a solid surface.

Its surface is characterized by immense craters of volcanic origin. There are large flows of lava clearly visible on its surface. Besides, very deep and wide canyons are also seen there.


The surface temperatures on mars have been measured between – 88°C and + 26°C, but the average temperature is less than that of the earth. The polar regions of mars are covered by white ice caps which probably consist of frozen carbon dioxide.


An asteroid is a very small planet with a diameter of less than 800 km. There is a belt which has about 1500 heavenly bodies which revolve in the solar system between Jupiter and Mars. They are formed from the disintegration of a former planet. Majority of asteroids are about a kilometer in diameter. Many asteroids have irregular shapes.



A Comet has a head which is a collection of solid fragments, and a tail sometimes millions of kilometers long made up of gases. A comet usually follows an orbit around the Sun.

Comets are made of frozen gases which hold together small pieces of rocky and metallic materials. Some comets travel along very elongated orbits that carry them beyond the Pluto.

The comets are visible only after they have reached within the orbit of the Saturn. There is a great variation in the size of different comets. Despite the enormous size of their head and tail, the comets’ mass is very insignificant.

When a comet approaches the sun, the frozen gases forming it begin to vaporize and produce a glowing head called the ‘coma’. As a comet moves farther away from the sun, the gases begin to condense and the comet’s tails become non existent.

Halley’s Comet is a well-known comet. There is no denying the fact that comets are among the most spectacular and unpredictable heavenly bodies in the solar system.


Meteor is also called a ‘shooting star’. It is a heavenly body which travels at great velocity through space. After entering the atmosphere it is heated by friction and so becomes luminous. Because of friction, usually a meteor is reduced into meteoric dust.

It is to be noted that each day millions of meteoroids enter the terrestrial atmosphere. However, some very large meteors which survive the atmospheric friction fall on the earth’s surface creating very deep and extensive hollows. Such a solid body of the meteor which falls on the earth’s surface is called a meteorite.


Of all the planets the earth is unique in the sense that it is the only planet which is inhabited by man and other living creatures. Besides, it is the only planet which has an atmosphere, a hydrosphere, and a biosphere.

It is the presence of water that characterizes this planet as unique. About 71% of the total surface area of the earth is covered with water and the remaining 29% is the land. The earth is the fifth largest planet in the family of the solar system.

The earth’s crust and a portion of the upper mantle just below the crust forms what is called the lithosphere? This planet has a layered structure. From the outermost end of the atmosphere to the core of the earth, the material that exists is not uniform.

The atmospheric matter has the least density. From the surface to the interior of the earth there are different zones and each zone has materials with different characteristics. From the crust to the core, the density of the materials increases.