THE SOLAR SYSTEM

A star may be defined as a huge, massive body of luminescent matter radiating enormous amount of energy second. Each star holds around itself, under the influence of force of gravity, a variable number of relatively small sized, non-radiant bodies of matter, called planets. The basic difference between the star and planet is in their masses : a star is thousand times bigger than a planet. Because of such an enormous mass very high temperatures are reached inside the star that make it radiate energy. Our sun is also a typical star. It has nine planets held and revolving around it under gravitation. The sun and the planets taken together make a solar system. Each galaxy is believed to contain billions of solar systems. Our solar system_ the sun in the center and the nine planets revolving around it is just an insignificant part of our galaxy, named by us for our convenience, as MILKY WAY.

The nine planets revolving around the sun in order of their increasing distance from the sun have been named as Mercury, Venus, the Earth, mars, Jupiter, Saturn, Uranus, Neptune. The mean distance from the Earth to sun is computed as 149,642,000 km. this distance is commonly referred as One Astronomical Unit. it is very inconvenient to use this unit in describing the distance of other planets from the sun as follows:

S.no	Name	Distance(A.U)	S.no	Name	Distance(A.U)
1.	Mercury	0.39	5.	Jupiter	5.20
2.	Venus	0.72	6.	Saturn	9.54
3.	The Earth	1.00	7.	Uranus	19.20
4.	Mars	1.52	8.	Neptune	30.10

*A.U. =Astronomical unit = 14,96,42,000 km, taken approximately as 150 million kilometres

**Astronomers classify Pluto(38.0 A.U) as a ‘dwarf planet’.

All the nine planets including the earth revolve around the Sun along regular, stable and nearly circular paths called orbits that lie almost in the same plane(except that of  Pluto). The Earth makes its orbital motion around the Sun at a speed of 29.76 km per second and complete one revolution in 365.25 days or one year. For the other 8 planets, the speeds of revolution and  the days they take to complete one revolution are different. All the nine planets also rotate around their own axes in a perpetual manner while revolving around the Sun. The period of rotation around their axes also varies from planet to planet. The Earth, as we know, has period of rotation of 23 hours 56 minutes and 4 seconds. The planet Venus takes the longest time to complete one rotation : 88 days and the planet Jupiter takes the shortest time : 9 hours 50 minutes.

GROUPING OF PLANETS

On the basis of their distance from the Sun and other broad similarities, the nine planets conveniently fall in to two groups.

(a) The inner or terrestrial planets

      Four planets namely Mercury, Venus, the Earth and Mars constitute the terrestrial group. They are characterized with

      (i) relatively smaller masses

     (ii) higher densities

    (iii) slower speed of rotation

(b) The outer planets

       They are also called superior planets and include five members : Jupiter, Saturn, Uranus, Neptune. They are characterized with :

     (i) very large masses

     (ii) much lower densities

    (iii) faster rotation

    (iv) Higher number of satellites

 Moons

Except Mercury and Venus, all the other seven planets have natural satellites called moons revolving around them. The number of moons is different for different planets. Our Earth have one moon where as Mars have two moons as per the information released by planetary society, the number of moons , including ”moonlets” for other planets of solar system as on June 2003 stood as Jupiter (61), Saturn (31), Uranus (21), Neptune (11). These counts are likely to increase with time as more and more discoveries are announced periodically from observatories.

Our moon moves around the earth in an elliptical orbit at a mean distance of 3,83,403 km and a period of 27 days, 7 hours 43 minutes and 11.5 seconds.

Beside the planets and other satellites, our solar system is also populated with a belt of smaller planet- like masses called Asteroids that revolves around the Sun in the space between the orbit of Mars and Jupiter.

 Meteors commonly seen as shooting stars are merely broken fragments of planetary materials that enter in to our atmosphere almost every day. When such a mass of matter happens to enter our atmosphere at night, it meets enormous friction from the atmospheric gases and shines brilliantly for a few moments before it gets completely consumed and dissipated. Occasionally remaining parts of these meteors reach the surface of the Earth and strike with strong impact. These meteors reaching the surface are called Meteorites. Thousand of tones of meteoric material is believed to fall on the surface of the Earth every year. These materials have been extensively studied and grouped in to three categories : stony meteorites, intermediate meteorites, iron meteorites. Many craters(depressions) found on the surface of the Earth are believed to have resulted due to the impacts of meteorites in the recent geological past. Sometimes the meteoric materials can be actually traced near the crater leaving no doubts about its origin.

Comets are yet another group of planet-like masses having very elliptical orbit of motion around the Sun. They appear in the sky as dazzling tailed stars (having tail spread over thousands of kilometres) after regular interval of many years. The famous HAILLEY’S COMET has a period of 76 years and was last seen in 1986.