The solar system is the name given to the family of the sun. Our solar system consists of the sun, nine planets orbiting around the sun, satellites (or moons) orbiting around the planets, asteroids, comets and meteors. The description of the various objects constituting our solar system is as follows:
1. Sun: It is the nearest star that we can see. It is heaviest body of our solar system around which all the planets revolve. The mass of the sun is 1.98x 10 30Kg and its diameter is 1.392x10 9m. It is about 109 times the diameter of the earth. The average distance of the sun from the earth is about 1.496x10 11m which is called one astronomical unit (A.U.). The sun rotates about is axis and completes one rotation in 25 days. The temperature on its surface is 6000 K. Sun is continuously emitting radiations both in visible regions and radio wave regions in all directions. The light emitted from the sun reaches the earth in 8.3 minutes.
2. Planets: A planet is a heavenly body, which revolves around the sun and does not emit its own light but reflects some of the incident light of the sun. There are nine planets, which are revolving around the sun in elliptical path. These planets in the order of increasing distance of the sun are:
(a) Mercury: It is the closest planet to the sun. Its mean diameter is 4820 km and its mass is 3.35x10 23 kg. It completes one revolution around the sun in 88 days. There is no atmosphere and water on the surface of the mercury and hence there is no possibility of life on this planet.
(b) Venus: It is a second planet of the solar system next to the mercury in order of increasing distance from the sun. Its mass, density and radius are comparable to those of earth. Hence it is called Earth's sister. Venus is covered with very thick atmosphere containing 95% carbon dioxide and pressure is about 100 times that on earth's surface. Its one side is always facing the sun and its surface temperature is always 450oC. There is no possibility of life on this planet. It is the brightest planet of solar system and also known as morning star and evening star. It does not possess any satellite. It can be seen with naked eye.
(c) Earth: It is the third planet of the solar system. It completes one revolution around the sun in 365.25 days. There is atmosphere and water on the surface of the earth and so life is there on the earth's surface.
(d) Mars: It is the fourth planet of the solar system in order of increasing distance from the sun. Its diameter is 6800 km. Its period of revolution around the sun is 1.9 years. There is a thin atmosphere mostly consisting of carbon dioxide. There are white patches on its surface, which shrink in summer and expand in winter. Mars changes colour in difference seasons. No life has been detected on it. It fact it is a reddish planet of solar system. It can be seen with naked eye. Mars has two moon, they are called Fobos and Dimos.
(e) Other planets: All the other planets of the solar system are far away from the sun and temperature on their surfaces is very low. The atmospheres of these planets are full of poisonous gases like methane and ammonia. There is no possibility of life on them.
3. Moon: It is the natural satellite of our earth. Its distance from the earth is 3.84x10 5 Km. Its mean diameter is 3480 km and its mass is 7.35x 1022 kg. The gravity of moon is 1/6 th that of the earth and it revolves around the earth in 27.3 days. It emits electromagnetic waves in the radio wave region, hence it is called radio moon. The moon always keeps the same face towards the earth. The temperature of the moon during the day is 100 o C and at night -150 o C. The surface of the moon has craters. There is no atmosphere and water on the moon's surface, hence there is no possibility of life on it.
Satellite: A heavenly body revolving around a planet is a stable orbit is called natural satellite. Thus, earth can be treated as natural satellite of sun and moon is the natural satellite of the earth. A satellite put to in its orbit around a planet by the man is called the artificial satellite.
4. Asteroids: These are group of more
than 1600 tiny rock pieces moving around the sun in betweenMars and Jupiter. The largest amongst them is called ceres with a radius of 350 km while the smallest one is just 50 m in radius. Asteroids are believed to be the pieces of a much larger planet, which broke up due to gravitational pull of Jupiter. The density and chemical composition of asteroids is similar to that of the moon. The largest asteroid completes one revolution around the sun in 4.6 years.
5. Comets: Comets are composed of rock like materials surrounded by
Halley comet was seen in early 1986 and is expected to be seen again in 2062 i.e. its period of revolution is 76 years.
When these pieces come close to the earth, they get attracted towards the earth and burn due to friction of the atmosphere and look like a bright line of fire. The light so produced is visible even from the surface of earth. Such bright meteors are called fireballs or shooting stars. Sometimes, the large pieces do not burn completely and reach the surface of the earth as stony, iron balls resulting in crators on the earth surface. These are called meteorites.
Chemical Composition of Atmosphere on a Planet
The nature of the gases present in the atmosphere of a planet can be determined by taking the spectrum of the radiations emitted by a planet. It has been found that the spectrum consists of number of dark lines. The dark lines correspond to particular elements present in the atmosphere of that planet and the intensity of the line tells about the relative abundance of the element.
From this study, it was concluded that the atmosphere of the earth consists of 78% nitrogen, 21% oxygen with some carbon dioxide, traces of hydrogen, helium and water vapor. Venus and Mars consist mainly of carbon dioxide, whereas the atmosphere of Jupiter consists of hydrogen, helium and methane etc.
Basic Conditions for existence of Life on a planet
The following basic conditions are required for the life to exist on a planet.
(i) There should be a suitable temperature range as required for life to exist on a planet.
(ii) There must be proper atmosphere free from poisonous gases.
(iii) There should be plenty of water.
The space exploration and other experimental observations have indicated that the above conditions are not prevailing on any other planet except our earth. Therefore, no life is possible on other planets as on earth.
The next constituent of universe is stars.