Planets are opaque heavenly bodies that also shine in the night sky, along with the Stars. But unlike the Stars, they have no light of their own, they only reflect the light of the sun. They do not twinkle like the Stars, but appear to give off a steady light, as they lie at comparatively smaller distances than the Stars.
The English word ‘Planet’ comes from the Greek word ‘Planetes’, meaning ‘wanderer’. As the planets appear to be moving against the background of the stars, the Greeks in ancient days believed that, the Earth is stationary at the center of the universe while objects in the sky, including the planets, revolve around it. In fact, the planets really wander, and wander systematically. Each planet has its own fixed path, and period of revolution. While spinning on their axes, they revolve around the Sun. While Venus and Uranus rotate from East to West, all the other planets rotate from West to East.
The path of revolution of each planet is elliptical, which is known as ‘Orbit’. The orbits of the planets are arranged concentrically with the Sun as the centre. With its gravitational pull, the Sun counterbalances the outward centrifugal force acting on the Planets due to their orbital velocities. Thus the Sun helps to keep the planets in their orbits and prevents them from moving away from their fixed path.
The planets, according to their distance from the Sun are – Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Till 2006, Pluto was also considered as a
Planet. But in 1992, the first Kuiper Belt Object was found, with more than 1,000 comparatively small icy bodies spotted in a disk-shaped region beyond the orbit of Neptune. Some of them are around the same size as Pluto. Following the discovery, some leading astronomers started to argue that Pluto looked more like a Kuiper Belt Object than a planet. Finally, in 2006, the International Astronomical Union (IAU) decided in 2006 to relegate Pluto from the status of a planet to a ‘dwarf planet,’ reducing the list of ‘real planets’ in our solar system to eight.
Among the planets, Mercury and Venus are termed as the ‘Inferior Planets’, as they are inside the Earth’ orbit and are closer to the Sun than the Earth. The other planets, viz., Mars, Jupiter, Saturn, Uranus and Neptune, are known as ‘Superior Planets’ as their orbits are outside the orbit of the Earth.
The first four planets, Mercury, Venus, Earth and Mars are called ‘Terrestrial’ or ‘Inner Planets’, as they are closer to the Sun and have similarity to the Earth. The four giant planets beyond the Mars, viz., Jupiter, Saturn, Uranus and Neptune, are called ‘Jovian’ or ‘Outer Planets’, because of their similarity to Jupiter.