In the vastness of the Universe, the Earth, the Sun and planets are tiny dots. The Sun is a single star in a Galaxy comprising 100,000 million stars.

The Solar System is centred on the Sun. It consists of a star called the Sun and all the objects that travel around it. The Solar System includes : 9 planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto), along with the numerous satellites that travel around most of them; planet-like objects called asteroids (hundreds of asteroids); chunks of iron and stone called meteoroids; bodies of dust and foreign gases called comets (thousands of comets); and drifting particles called interplanetary dust and electrically charged gas called plasma that together make up the interplanetary medium.

The whole solar system by volume appears to be an empty void. This vacuum of ‘space’ comprises the interplanetary medium. The speed of the solar wind is about 400 kilometer per second in the vicinity of Earths' orbit.

The Solar System originated in a primitive solar nebula–a rotating disc of gas and dust. It is from this rotating disc that the planets and the rest of the Solar System evolved. The Solar System is also tucked away in a corner of the Milky Way at a distance of about 30,000 to 33,000 light years from the centre of the galaxy.

The Sun contains 99.85% of all the matter in the Solar System. The planets which condensed out of the same disk of material that formed the Sun, contains only 0.135% of the mass of the Solar System.

Jupiter contains more them twice the matter of all the other planets combined. Satellites of the planets, comets, asteroids, meteoroids, and the interplanetary medium constitute the remaining 0.015%.



The Planets
The bodies revolving around the sun (at the same time rotating on their imaginary axis) are called planets. They have no light of their own but shine by radiating the fight they receive from the sun. They all revolve around the sun in elliptical orbits. Until about 200 years ago only six planets were known. Three more planets were discovered later, the latest being Pluto (discovered in 1930). Nine planets can now be identified.

Mercury : Mercury is the planet nearest to the sun. It rotates on its own axis in 56.65 earth days. It takes 88 days to complete one revolution round the sun. Thus it is the fastest planet in our solar system.

Venus : Also known as the evening star and morning star, is the brightest object in the sky after the sun and the moon. It is slightly smaller than the earth and is the planet closest to the earth. It is also the hottest planet in our solar system and has a weak magnetic belt.

Mars : Mars is the fourth planet from the sun and is the next planet after the earth. Being favorably situated, it is brighter than most of the stars and, is therefore, known as the Red Planet. It has two small satellites called Phobos (Fear) and Deimos (Terror).

Jupiter : Jupiter is the largest planet in our solar system. It is about eleven times larger than the earth. Its volume is one and half times the volume of all the planets combined together. The most conspicuous aspect about Jupiter is its Great Red Spot. It is also known as the giant planet because of its huge size.

Saturn : Saturn is an outer planet visible to the naked eye. Second in size to Jupiter, it is the least dense of all the planets. The most spectacular feature of Saturn is its system of rings. The ring system is made up of a variety of separate particles which move independently in circular orbits. It has 46 satellites. Titan is its biggest.

Uranus : Uranus is the seventh planet from the sun and is not visible to the naked eye. It was identified as a planet in 1781 by William Herchel. It has completed only two revolutions round the sun since its discovery, and takes about 84 terrestrial years to circle round the sun. It has 27 satellites.

Neptune : Neptune is not visible to the naked eye but can be seen through a small telescope as a greenish star. It is eighth in position from the sun. This planet was discovered by J.G. Galle of Berlin in 1846. Till 1930, it was believed to be the farthest planet from the sun and the outermost in our solar system. It has eight satellites, and Triton and Nereid are the most con­spicuous of them.

Pluto : Pluto is the youngest planet to be discovered in our solar system. It was discovered photographically by C.W. Tombaugh (USA) in 1930. It is the smallest planet in our solar system; slightly smaller than Mercury and visible only through a tele­scope. The duration of its revolution round the sun is the longest and it is, therefore, the slowest planet in our solar system.