22.1 SOLAR SYSTEM

• Our solar system is the only planetary system which is known to us at present though there may be numerous such systems scattered in the space.
• The earth is a member planet of our solar system.
• It is, therefore, necessary to have some elementary knowledge of the solar system to understand the origin of the earth.
• Planets are non – luminous bodies whereas stars are luminous bodies of the universe around us.
• In other words, the planets do not have their own light, rather they reflect the light of the stars while the stars have their own light and energy due to thermonuclear reactions wherein hydrogen nuclei combine under intense temperature and pressure to form helium nuclei which release vast amount of energy.
• The congregation of stars and planets is known as solar system.
• like shape consists of 8 planets (e.g. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune) and one star (the sun).
• Besides, there are numerous minor planets (Planetoids or asteroids). All these bodies revolve around the sun almost in the same plane and in the same direction along the near circular elliptical orbits.
• Most of the satellites of the planets also revolve in the same direction.
• The planetoids with some exceptions have their orbits between the orbits of Mars and Jupiter.
• The rotatory motion of the planets (except Venus and Uranus) is in the same direction as their revolution around the sun i.e. anticlockwise for one who looks at the earth from the North Pole to the South Pole.
• According to the Titius Bode Rule, the distance of each planet from the sun is approximately twice that of next planet close to the sun.
  • The diameter of the whole solar system is about 1,173 crore kilometres.
  • The earth is about 149,600,000 Km (1.496 away from the sun.
  • The diameter and average density of the earth are 12,742km and 5.52 respectively.
  • Average distances from the sun, period of orbital revolution, period of rotation, diameter, average density and number of satellites of the member planets of our solar system are presented.

22.2 PLANETS

1. MERCURY

• MERCURY is the smallest planet and closest to the Sun.
• It has the longest period of rotation and the shortest “year” taking only 88 Earth-days to orbit the sun.
• For a long time, astronomers thought that Mercury always kept the same face toward the Sun. But new observations show that the planet rotates once every 58 or 59 days.
• It has very thin atmosphere which cannot support life.

2. VENUS

• VENUS is the brightest of all planets because it reflects more of the sun’s light which falls upon it than other planets do.
• As its orbit is within the earth’s, it never gets very far from the sun and it is easily seen after sunset.
• It is called the “Evening Star” when it appears in the western sky and “Morning Star” when it appears in the eastern sky.
• The thick atmosphere that hangs over Venus traps the Sun’s energy and helps build up the most furnace-like heat yet found on any planet.
• So, with its very high surface temperatures, and its atmosphere consisting largely of carbon dioxide, Venus certainly cannot support life as it exists on earth.
• In general, the planets and their satellites rotate from west to east, but Venus rotates on its axis from east to west.

3. THE EARTH

• THE EARTH appears as a large greenish blue disc with its blue oceans and sparkling white polar ice caps.
• Our earth’s orbit lies between those of Mars and Venus.
• Its average distance from the Sun is 150 million km.
• It takes 23 hours, 56 minutes and 4 seconds to complete one rotation and 362 days, 5 hours, 48 minutes and 46 seconds (i.e. approximately 365 ¼ days) to complete its orbit around the Sun. This revolution around the Sun gives us our year.
•  For convenience, one-quarter of a day is ignored for three years and then an extra day is added to every fourth year to give us our Leap Year of 366 days.
• The Earth’s circumference is about 40,000 km and its diameter is 12,756.8 km across the equator and 12,713.8 km across the poles.
• Its surface area is about 510 million sq km.
• The size of the earth is therefore great but not vast in comparison with other planets such as Jupiter or Saturn.
• Our planet has one satellite, the Moon.

4. MARS

• MARS is “the Red Planet”, is smaller than the earth, and has very thin atmosphere and a very small amount of oxygen.
•  It is farther away from the Sun than the earth and so it is much colder.
  It takes 24 hours and 37 minutes to rotate on its axis and 687 days to complete one revolution.
• Mars presents its poles alternately to the Sun in the same way that the earth does because its axis is titled in a similar manner.
• Temperatures on Mars range from 2.670C to a low of -700C.
• About three-quarters of the planet’s surface is covered with bright reddish or yellowish patches.