SOLAR SYSTEM

The earth is a planet. It is a member of the solar system. Nine planets and the sun are the main bodies of the "solar system". The planets in the order of increasing distance from the sun are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. Each planet moves around the sun and maintains an elliptical orbit. The orbits of all the planets are almost in the same plane which is close to the equitorial plane of the sun. The planets also rotate about their axes. About 99.85% of the mass of the solar system is contained within the sun while the planets collectively make up most of the remaining 0.15%. On the basis of their location, the planets can be divided into two groups:

1. Inner planets : which include Mercury, Venus, Mars and Earth
2. outer planets : which include Jupiter, Saturn, Uranus, Neptune and Pluto

The planets of the two groups differ markedly in size, density, composition and rate of rotation.

Size: The members of the inner planet group are small in size. On the other hand, the members of the outer planet group are so large that they are often called giants

Density: The inner planets are generally dense, their density being 4 x 10-3 kgm-3 or more. The density of outerplanets is much lower. For example, the Saturn has a density less than that of water. 

Composition: The low density of the outer planets suggests that they consist mostly of substances like hydrogen, helium, water, ammonia and methane. The high density planets consist almost entirely of silicates and metals. 

 Rate of Rotation: The planet Mercury is nearest to the sun, It has the fastest orbital motion (48 km/sec) and the shortest period of revolution (88 days). Pluto, the most distant planet, has an orbital speed of 5 km/sec and requires 248 years to complete one revolution.