A star is a luminous heavenly body, which emits light of its own. They are held at their place by the effect of their own gravitation. There are millions of stars. By the point of view of chemical composition, they contain 70% hydrogen, 28% helium, 1·5% carbon, nitrogen and oxygen and 0·5% other heavy elements. Stars appear, release energy and then are finished themselves. Their lifetime depends upon their mass and luminous intensity. The more brighter is a star, the lesser is its lifetime. Colour of a star depends upon its temperature. Sun is also a star. Its lifetime is 1010 years.
The group of various planets, satellites, comets, asteroids and several other heavenly bodies revolving around the sun is called the solar system. The solar system is considered to be originated from the sun. Its surface temperature is 6000°C. It is 150 × 106 km away from the earth. Its mass is 1·989 × 1030 kg (330,000 times heavier than the earth). Light takes 8 minutes to reach earth from the sun.
The names of the planets of the solar system in order of increasing distance from the sun are:
1. Mercury, 2. Venus, 3. Earth, 4. Mars, 5. Jupiter, 6. Saturn, 7. Uranus, 8. Neptune, 9. Pluto (Former planet).
Their brief description is given below:
Mercury: Mercury is the closest planet to the sun. It completes its one revolution around the sun in 88 days. There is no atmosphere on mercury. Days on it are too hot and nights are too cold. One day of mercury is equal to 90 days on earth and night is also of the same duration.
Venus: It is the second planet from the sun. It is almost equal to earth in size and weight. It completes
one revolution around the sun in 225 days. Venus is very hot planet with very high temperatures. It is
nearest to earth and the brightest object in the sky except for the sun and the moon. Since it is seen in
the east in the morning and in the west in evening therefore, it is called the morning star and the evening
Earth: Earth is the third planet from the sun. It is situated between Venus and Mars. It is the only planet where life exists. Its diameter is 12,756 km and the average distance from the sun is 14·96 × 106 km.
It completes one revolution around the sun in 365·25 days. Moon is the only satellite of earth.
Mars: Mars is the fourth planet from the sun. It is also called the red planet. No life has been detected on Mars till date, however, possibility of life on Mars in future is being worked on. Mars has a very thin atmospheric layer. Nix Olympia is the highest mountain on Mars. It is three times higher than Mt. Everest. It has two satellites namely Demos and Phobos.
Jupiter: It is the fifth planet from the sun and the largest of all other planets in the solar system. It takes 11·9 years to complete its one revolution around the moon. Jupiter radiates more energy than it absorbs from the sun. Its internal temperature is around 25000°C. It has 28 known satellites-the four large Galilean moons, 12 smaller named ones, plus 12 more small ones discovered recently but not yet named.
Saturn: It is the sixth planet from the sun and the second largest. It completes its one revolution around the sun in 29·5 years. Saturn is surrounded by rings which are composed of small particles. This planet appears as a yellow star in the sky. Saturn has 21 known satellites. Its largest satellite is ‘Titon’ which equal in size to mercury. Among other satellites Memas, Tethis, Deon, Riya, Hiparian, Iyapetus and Phobe are prominent. Phobe revolves in the direction opposite to that of Saturn. Saturn can be seen sometimes.
Uranus: It is the seventh planet from the sun and the third largest. It was discovered by Sir William Herschel in 1781. It appears green in colour when observed by the telescope. It takes 84 years to complete its one revolution around the sun. Uranus is the only planet Which remains in front of the sun on its revolutionary orbit while moving from one pole to another. Uranus has five rings called alpha, beta, gamma, delta and epsilon. It has 20 named moons plus one. recently discovered which as yet has not been given an official name. Its prominent satellites are Ariel, Umbrial, Titania, Oberon and Miranda.
Neptune: It is the eighth planet from the sun and the fourth largest. It was discovered in 1846 by a German Astrologer Johann Gale. It completes its revolution around the sun in 164 years and takes 16 hours to rotate on its axis. It appears yellow in colour. It has 8 known satellites; 7 small ones and Triton. The first satellite is Triton. It is larger than the moon of earth and is nearest to Neptune. Second to it is Marid.
Pluto (dwarf planet): Pluto is farthest from the sun and the smallest. It revolves once around the moon in
248 years and rotates on its axis in 6 days and 9 hours. It was discovered by Clide Tombag in 1930. It is
590 × 106 km away from the sun but its orbit is highly elliptical. Its orbit cuts the neptune’s orbit. Due to which its minimum distance becomes 270 × 106 miles and maximum distance becomes 460 × 106 miles. Due to this uncertain behaviour of moon Neptune becomes the farthest planet from the sun when pluto passes cutting its orbit. From 1979-1999 Neptune was farthest from the sun. After this pluto will remain farthest for 228 years.
Pluto lost its planet status: Pluto, on 24 August, 2006 lost its seven decade status as the ninth and outermost planet of the solar system. The decision was taken at an assembly of the International Astronomical Union (IAU), world’s top astronomical body.
Pluto’s status was contested for many years by some astronomers, who said that its tiny size and eccentric orbit precluded it from joining the other acknowledged planets. The eccentric orbit of pluto is highly inclined with respect to the planets. It is also smaller than several natural satellites in our solar system.
Pluto is a dwarf planet in the solar system, according to the recent IAU’s redefinition of planet.