The birth of the planets

   The nine planets, including earth , that form our solar system are accompanied through space by 63 moons, dozens of comets, and hundreds of thousands of lumps of rock that never grew into planets. all of them are held together and guided on their jorneys by the sun, which weighs almost 1000 times as much as the rest of the solar system put together.

WHERE DID THE PLANETS COME FROM?
    five or six billion years ago,  a disc-shaped cloud of dust and gas in our milky way galxy bagan to collapse inwards towards the centre-forming the sun. one theory suggests that specks of dust in the gas cloud around the sun kept bumping and clumping into each other until they formed the planets. another theory claims that the gas cloud broken up into large blobs, which got smaller and harder, forming the planets.


SCIENCE FACT OR FICTION?
   mars has often been thought to be inhabited by alien beings. in 1894 the america amateur astronomer percival lowell mapped  networks of 500 'canals', joined by 'oases', over the surface of the planets. he thought that they must have been dug by martians to water their dry and dusty landscape.
    from the close observation of the space probe viking 1 in 1976, we know that there are 'canal like' structures on the surface of mars, like the huge 4500 km (2800 miles) long 'mariner valley'. we also know that these 'canals' formed  naturally (like our own river valleys).the  'oases' are in fact huge volcanoes. one, called olympus mons, is the largest yet discovered in the solar system at 25km (15miles) high.

IS THERE LIFE ON OTHER PLANETS?
    scientists reckon that the chances of life somewhere else in the universe are high enough to spend hundreds of millions of dollars searching for it, but they are more doubtful about extraterrestrial life in our own solar system. life depends on two-dozen chemical elements,of which carbon,hydrogen, oxygen , nitrogen , sulphur and phosphorus are the most important. for life to continue, light and warmth are needed. mars has the right chemical ingredients and may once have had the right conditions. A 4.5 billion -years-old meteorite from mars , found in antarctica, contains what some scientists think is a fossilied microbe that may have lived on the 'Red Planet'.
      Conditions now, including subzero temperatures, are not suitable for life on mars. they may, however, be kinder on Europa, one of Jupiter's moons. Satellites have photographed ancient river valleys on Europa, and there may still be water below its ice cover.

HOW WAS THE EARTH FORMED?

     Scientists believed that the earth formed from dust particles that clumped together to form lumps, which collided and larger lumps and clusters, gradually building into a planet. Temperatures rose to more than 5000 C (9000 F)- enough to melt the early earth into a churning cauldron. a rocky crust. but deep inside the earth, temperatures are still 4500C (8100F) and the rocks are molten-rising to the surface most spectacularly when a volcano erupts.