How did our world begin? Where did it come from? It the ancient times, people made imaginative stories about the beginning of the earth, but these did not really explain what had happened.
Some people did not satisfy with these stories. They watched or observed, the skies carefully. They had no telescopes, but they could observe many things without telescopes. They saw that all the stars moved through the night sky as a group, as if they were tied together. They also saw that a few objects seem to wander among the stars. They called these objects “planets”, which come from a Greek word meaning ‘wanderer’.
The earth is a part of the solar system —the system of the sun. The moon and the other planets and their moon also a part of solar system. The sun at the centre and the planets move round the sun in paths called orbits. The third planet, counting upward in the sun, is the earth.
We want to know how the earth began. To find out, we must learn something about the beginning, or the origin, of the solar system. Scientists have given similar explanations, or theories, of how this might have happened. But nobody knows for sure.
AN EARLY THEORY
One of the first of these theories was proposed by the great French astronomer and mathematician Pierre Laplace in 1766. It is called the nebular hypothesis. (A hypothesis is a guess that is based on facts and does not conflict with them.) Nebula is the Latin word for ‘cloud’; the universe contains many nebulae, or clouds made up of gas and dust.
Laplace believed that a sun was once a rotating cloud of a hot gas. The rotation caused the cloud to become round, like a huge ball. As the ball cooled, the cloud shrank in size and started to rotate faster. (A spinning skater spins faster, too, if he reduces his size by pulling in his arms.) The shrinking and spinning caused a ball to bulge to the equator and flatten out to the poles. A ring of gas and dust then formed round at the equator. This ring of material broke away from the ball, and the pieces joined together to form a planet. The planet orbited round the rest of the cloud. Meanwhile, the cloud went on spinning and shrinking faster and faster, producing more rings, each of them breaking away and forming a planet. Finally, a family of planets moving round a central globe of a hot gas-the sun was produced. The explanation seemed so reasonable that for a time everyone believe that this is how the earth and its sister planets began.
As more facts, we discover about the sun and the planets, the Laplace theory began to lose favour. The main problem was the fact that the sun spins slowly. If Laplace’s theory was true, it would spin quickly. Possibly the sun had slowed down, but no one could explain how this could have happened. So astronomers had to star all over again and look for new theories.
THE LITTLE PLANETS THEORY
About 1900, Thomas Chamberlin and Forest Moulton, two American scientists, put forward a new idea. It was called the planetesimal theory. (‘Planetesimal’ means ‘little planet’.)Some years later, two British scientists, Sir James Jeans and Harold Jeffreys, proposed another theory called the tidal theory. There were some differences between the British and American theories, but the mean theories were the same. At first, according to these theories, the sun had no planets. Then, ages ago, it came very close to another star. Each body exerted each pull of gravity on the other. This raised tidal wave of hot gas which swirled round the sun and the star. As they passed each other, streams of gas were pulled out into space between the two bodies. As these two bodies moved apart, some of the streams of gas condensed to form planets.
These, too, looked like very good theories. But astronomers have had discarded them. For one thing, the passing star could not have given the planets enough motion to keep them in orbit round the sun. They would have fallen back into the sun and burnt up. Also, both theories were based on the chance of a star having passed close to the sun.
THE DUST-CLOUD THEORY
Astronomers have since proposed more theories about the origin of the earth. All of these theories are of two main kinds. The natural, or evolutionary, theories suggest the planet form round a star as the star is created, or they form as part of a normal life. Laplace’s nebular hypothesis is of this kind. Other theories are called catastrophic theories. Such theories say that planets are created round a star only by an accident or a catastrophe, such as the approach of another star or a collision with it. The planetesimal theory is of this kind.
It is important which kind of theory is correct. If a natural theory is right, most stars should have planets. But if a catastrophic theory is the explanation on how the earth began, then the creation of out planet and the life on it is still unusual event in the history of the universe. Stars are so far apart that an accident involving two stars must be very rare. In fact, during the lifetime of the solar system which is 4,600,000,000 years-only one in every 10,000,000,000 stars could have gained planet in this way.
Astronomers can in fact detect planets around the stars. They cannot see them through the telescopes because the stars are so far away. But the planets disturbed the motion of the stars and this motion can be seen. A natural theory is therefore much likely than a catastrophic one. In fact, Laplace’s hypothesis is now thought to be correct in outline.
Most astronomers now believe that the sun and the planet were formed from spinning cloud of dust and a gas. This theory was first developed by a German astronomer Carl van Weizsacker in the 1940s. Since then, the astronomers have added his theory, which is called the dust -cloud theory.
Throughout the universe, there are huge clouds of dust and gas. More than 5,000,000,000 years ago, according to the theory, a large number of dust particles within such a cloud began to condense and formed a globule. As the globule became round, like a ball, it began to spin. This spinning caused is to flatten out. The centre of the disc, was spinning slowly than the outer parts. It began to condense even more. Eventually, the energy released by this process caused the temperature and the pressure in the globule to become high that thermo-nuclear processes began. The star-our sun-then began to shine in its own right.
The outer parts of the dust cloud were spinning too fast to condense into one mass. They broke up into smaller, swirling masses of gas and dust, which condensed to form a planet.
WHAT THE EARTH WAS LIKE
What was the newly formed earth like? Was it a ball of red-hot glowing rock? Was it cold? Or was it perhaps liquid? These questions are of great important to geologists who study the earth.
Most geologists now agree on a general picture of a young earth. The earth formed a dust particles gathered together in a cloud in space. The particles attracted together and formed a solid globe. As the force of the gravity of globe pulled in more and more particles, a new world grew larger and larger.
As the earth grew, it was not very hot. Its temperature is probably the same about the boiling water (100-degree celsius). But today, the inside of the earth has a much higher temperature. It is now so hot that the rock there is molten and flows from volcanoes in fiery and deadly rivers of the lava.
How did the earth get so hot? There were probably several reasons of the heating of the young earth. The materials of which the earth is made contains radioactive elements such us uranium. These elements are constantly producing heat-enough, in fact, to keep the centre of the earth as hot as it is now. This might seem to explain how the interior of the earth became molten. But the study of rocks showed that this melting happened more quickly than can be explained by the radio-activity alone.
As the new planet grew, solid particles would have rained down upon its surface. The impact of each one would have produced a little heat-enough perhaps to heat the world so that its interior began to melt. Then, enough heat would be gained by radio-activity to keep the interior hot when the bombardment of the particles stopped.
WHERE THE MOON CAME FROM
One event that did not disturb the surface of our young world was the formation of the moon. Many astronomers once thought that the moon came from the earth. Some even suggested that the Pacific ocean covered that the hole that remained when the moon left the earth. But the Apollo moon landing settled this argument. The moon is different enough from the earth to show that it was never part of the earth. The moon is about same age as the earth-4,600,000,000 years. So it probably formed as separate body like the earth and other planets were themselves formed, from the cloud of gas and dust.
HOW SPECIAL IS THE EARTH
There are many clouds and dust scattered throughout the universe. Astronomers believed that stars and planets are forming there just as our solar system formed long ago.They also believe that other planets with some forms of life must exist somewhere far away in the universe. It is impossible to guess how many planets contain life. But in case you may be thinking that our world is a special one in the universe, keep this in mind: the earth is just one of the nine planets circling the sun, and the sun is just of 100,000,000,000 stars that make up the great island of stars called the Milky Way or the Galaxy.
Astronomers believe that they can detect about 1,000 stars which have planets, but these are only nearby stars. Possibly, almost all of the stars in the Galaxy have planets. And, if these were not enough to think about, our most powerful telescopes can locate the distance gleam of about 1,000,000,000 other galaxies in the universe. Therefore, the total numbers of planets in the universe is probably many millions of millions of millions.