A planet is one of the bodies in the solar system which revolve in elliptic orbits round the sun. The planets do not have light of their own but shine by reflecting the light which they receive from the sun.
The difference between a planet and a star is that a planet moves round the sun and has no light and heat of its own, whereas a star is a body like our own sun, and retains its own position on the celestial sphere with such accuracy that refined observations are necessary to detect its motion.
The planets known so far are nine in number. They fall into three distinct groups. The first group contains of four planets (Mercury, Venus, Earth and Mars). They are comparatively smaller and nearest to the sun.
They are known as terrestrial group. Beyond them with increasing distances from the sun, are the four major planets (Jupiter, Saturn, Uranus and Neptune). They are all at great distances from the sun and each of them is much larger than any of the terrestrial planets.
Mercury, Venus, Mars, Jupiter and Saturn were known to the ancient astronomers who observed them with naked eye. Uranus was discovered by Sir William Herschel in 1781 with a 6 ½ inch reflector of 7 feet focus of his own make. The discovery of Neptune in 1846 by Liverier ranks among the greatest achievements of mathematical astronomy.
Pluto, representing the third group was discovered in January, 1930 by astronomical photographs taken by the staff of Lowell Observatory.
The orbits of Mercury and Venus lie within that of the earth and for this reason they are also called the inferior planets and the others superior planets. Both Mercury and Venus have phases like that of the moon and can be seen through telescope.
Seen through a telescope, the Saturn is a most magnificent and impressive object with three layers of rings. A ring is not solid whole but most probably it is a constitution of myriads of small satellites of various sizes each following its own orbit about Saturn but so small that at the earths distance the individual constituents cannot be seen.
The Mariner 2 was guided so as to pass the planet Venus at a distance of almost 20,000 miles carried equipment to scan the planetary disc in microwave and infrared radiation as well as to measure the planetary magnetic field if at all appreciable.
The microwave measurements showed a limb darkening effect (darkening towards the edge of the disc) which excluded the possibility that the high microwave emission temperature must reach about 600oK. From the absence of a signal in the magnetometers it is concluded that the magnetic field of Venus is less than one tenth as strong as that of the earth.
Asteroids or minor planets
They are a numerous group of very small planets (planetoids) circulating in the space between Mars and Jupiter. They are thousands in number and are extremely small. The larger of them has a diameter of only 480 miles. Only four asteroids have diameters of more than 100 miles.
As regards the origin of asteroids, they are said to be the relics of a larger planet which broke up. Another suggestion is that the matter forming the asteroids was originally distributed uniformly in a ring about the sun, like the rings of Saturn and that perturbation by the planet Jupiter broke it up into many fragments.
Ultimately these fragments formed into what we called asteroids. But it can not be said how far these theories are correct.
These are the small planets revolving round other planets which control their motions, are the so called secondary planets.
They are small members of the solar system attendant on the larger planets, by which their motions are controlled. They revolve around the sun. The moon is also a satellite.
Life on planets
The possibility of life on mercury, Jupiter and other far off planets can be easily ruled out. Venus and Mars are favorably situated, have atmosphere, habitable summer and winter, moderate duration of years, days and nights, and gradual change of temperature, which are the prerequisites for life. The surface of Venus is enveloped by thick clouds. It is difficult to find out the details.
In 1877, Schiaparelli announced the existence of five lines over the orange colored surface of Mars and, in 881 he further announced that the lines first appeared single and then doubles.
He named them “canali” which means channel and which has been translated “canals”. This let to close observations of the planets and many observers claim to have seen the marking. Some deny the existence of the network of the five straight lines.
The surface of the Mars has many details, which can not be seen individually, but which the eye unconsciously integrates into lines and spots. Small telescope would bring out fine network of details, which is generally destroyed in large telescopes having greater resolving powers.
The argument for the existence of life rests mainly on the straightness of the lines which is rare in nature and implies artificiality and work of high intellect and intelligence.
Water is scarce on Mars and intelligent beings with engineering skill seem to have devised high projects for the supply of water. But the argument still rests on very insecure ground.
The existence of vegetation is definite and the possibilities of life can not be ruled out.
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