The Solar System
The solar system is dominated by the Sun, which accounts for almost 99.9% of the matter of the whole system. It is the source of light and heat to us. Scientists believe that the Sun has been formed from a moving cloud of gases, which is called Nebula. The Sun and the planets were born out of this cloud. The force of gravity has created them. For over millions of years, these balls of dust and gas are moving around the Sun. The Sun by virtue of its mass and weight, controls the movement of the planets. This force is called the force of gravity.
Sun is at the Centre of the solar system with eight planets (i.e. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune), satellites, asteroids, meteors and comets that move around the Sun.
Till 2006, there were nine planets in the solar system. Pluto was the farthest planet from the Sun. In 2006, the International Astronomical Union (IAU) adopted a new definition of a planet. Pluto and other celestial bodies like Ceres, 2003 UB313 does not fit this definition. It is no longer considered as a planet of the solar system.
It is believed that the Sun was born about 5 billion years ago. Since that time, it is continuously emitting huge amount of heat and light, and it is expected to glow in a similar manner for the next 5 billion years or so. The Sun is essentially a sphere of hot gases. The disc is also called the photosphere. The layers of gas above the photosphere are extremely hot, but their densities are extremely low. These layers are very faint and are not visible in the presence of strong light from the disc of the Sun. At the time of the total solar eclipse, when the light of the Sun’s disc is completely cut off, the outermost layer becomes visible. It appears like a crown round the Sun. This layer is called Corona.
The radius of the Sun is almost 100 times the radius of the earth and its mass is about a million times the mass of the earth. The Sun is the nearest star from us. The light of the Sun takes about 8.3 light minutes to reach us. The light of the nearest star (Proxima or Alpha Centuari) to the Sun reach earth in about 4.3 light years.
Planets are actually spinning around the Sun. They have their own elliptical path of movement known as the orbits. Movement around its own axis is called rotation and around the Sun is revolution. Planets, unlike stars, have no light or heat of their own. The word ‘planet’ comes from the Greek word “Planetai” which means ‘wanderers’. Planets keep changing their positions with respect to the stars.
• Mercury: It is nearest to the Sun. It is the smallest planet of the solar system, nearly of the same size and mass as the moon. It takes 88 days for one orbit around the Sun and 59 days for one spin on its axis. It has no satellite. There is no atmosphere on mercury. The surface of mercury is rocky and mountainous. One side of the surface facing the Sun receives maximum heat and light. The surface of this planet does not receive Sunlight or heat on its other side. One part of mercury, therefore, is very hot while the other part is very cold.
• Venus: It has no moon or satellite of its own. It rotates on its axis is somewhat unusual i.e. from east to west. The mass of Venus is nearly 4/5 times that of the earth. It takes 255 days for one orbit around the Sun and 243 days for one spin on its axis. Therefore it is often called a morning or an evening star. It has an atmosphere that consists of mainly carbon dioxide. It is the hottest planet in our solar system.
• The Earth: Our earth rotates from west to east. Our earth is more or less like a sphere, which is slightly flattened in the north and south. Slightly flattened or tapered at the poles, the earth is best to be described as geoids which mean earth like shape. It is the third nearest planet to the Sun. It is called blue planet due to presence of water and landmasses the earth appears blue-green in colour from the space. The earth is the only planet where some special environment conditions are responsible for the existence and continuation of life because it has the right temperature range, the presence of water, soil, minerals, suitable atmosphere and a blanket of ozone.
• Mars: It is almost half the size of the earth. It takes 687 days for one orbit around the Sun and 1 day for one spin on its axis. It appears slightly reddish and, therefore, it is also called the red planet. Mars has two small natural satellites named Phobos and Deimos.
• Jupiter: It is the largest planet of the solar system. It takes 11 years and 11 months for one orbit around the Sun and 9 hours, 56 minutes for one spin on its axis. It has 16 satellites. It also has faint rings around it. Its most distinguishing feature is the great red spot. Because of its large mass, it exerts a strong gravitational pull on other objects which pass by it. It is consists of hydrogen and helium in gaseous form. Its cloud like outer regions consists of methane in gaseous form while ammonia is present in crystalline form.
• Saturn: Beyond Jupiter is Saturn which appears yellowish in colour. What make it unique in the solar system are its three beautiful rings. It takes 29 years, 5 months for one orbit around the Sun and 10 hours, 40 minutes for one spin on its axis. It has 18 satellites. Saturn is the least dense among all the planets. Its density is less than that of water. It is similar in size, mass and composition to Jupiter. It is however cooler than the Jupiter.
• Uranus: It was the first planet to be discovered with the help of a telescope by William Herschel in 1781. Hydrogen and methane have been detected in the atmosphere of Uranus. It rotates from east to west. The most remarkable feature of Uranus is that it has highly tilted rotational axis. As a result, in its orbital motion it appears to roll on its side. It takes 84 years for one orbit around the Sun and 17 hours, 14 minutes for one spin on its axis. It has 17 satellites.
• Neptune: It takes 164 years for one orbit around the Sun and 16 hours 7 minutes for one spin on its axis. It has 8 satellites.