On the scale of the age of the universe, the creation of the sun is very recent indeed. It was only five billion years ago, on the periphery of our Milky Way galaxy, that an enormous mass of hydrogen was concentrated – held together by gravity – to form the sun. The pressure and temperature of this gigantic sphere of gas increased sufficiently to start nuclear fusion reactions between the hydrogen atoms. As a result enormous quantities of energy were released: the very heat and light that warms our planet and maintains life.
It is thought that the planets were formed at the same time as the sun, or a little after. The giant outer planets (Jupiter, Saturn, …) consist mainly of gas that did not attain sufficient mass to become a star. The interior planets close to the sun (Earth, Mars, Venus …) are said to be telluric because they are composed of rocky matter, agglomerations of cosmic dust, ice and heavy matter expelled from the sun. Everything happened under the effect of gravitation.
Our earth is 4.6 billion years old. Its primitive atmosphere was largely composed of nitrogen, carbon dioxide and water vapour. As it condensed, this water vapour gave rise to a primitive ocean (called Iapetus), surrounding a single continent (Gondwana). If we look at a map of the world, we can see that things have changed an awful lot since; that is the effect of plate tectonics. |