The earth revolves around its star, the Sun, and together with the eight other planets – Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto – and their moons, they form the Solar System.
But the Sun is only one of billions of stars that go to form yet another grouping, a galaxy. Our Galaxy, the Milky Way, consists of about 100,000 million stars. Many of these stars may also be circled by planets, some of which may perhaps have life on them.
Galaxies come in different shapes and sizes. They may be spiral or elliptical, or have no clearly defined shape at all. The Milky Way is a spiral galaxy, thick in the centre and tapering off to the sides, rather like a gramophone record. Our solar system lies about two-thirds away from the centre.
From edge to edge, the Milky Way measures 100,000 light years. Distances in space are measured in light years. This means, it would take a beam of light 100,000 years to travel from one edge of the galaxy to the other. Light travels at 300,000 km/s – the fastest speed we know of – and a light year is the distance a beam of light would travel in that time. Since there are at least another 100 billion galaxies in the Universe, this gives some idea of the almost unimaginable scale of our ultimate place in space.
Baptism of fire
Third world out from the Sun at an average distance of 150 million km, Earth measures just 12,756 km across and has a solitary moon. It receives just enough light and heat to make it suitable for life. Except at the poles, and on high mountains, the temperature is such liquid water can exist anywhere on the surface – a vital fact since it was in the oceans of the young Earth that life began.
Just after the Sun formed, about five billion years ago, it was surrounded by a great spinning pancake of gas and dust. Over the next few hundred million years, the loose material in this wide, rotating disc gradually lumped together to make the nine planets of the Solar System, along with a host of smaller objects like the various moon and asteroids.
Worlds, including the Earth that formed fairly close to the Sun, build up mainly from rock and metal. It was too warm for then to hold on to large amounts of lighter substances such as hydrogen and helium. At first, the Earth may have glowed orange-red, like a bright coal fire. It was heated intensely, both by the break-up of radioactive elements inside, and also by meteors of every size that constantly bombarded it from space.
Slowly, the topmost layer of the Earth cooled and hardened to become a thin, solid crust. At the same time, hot, molten rocks from inside the planet continued to well up to the surface through numerous cracks and volcanoes. As they did so, this material set free gases, including water vapour, that gathered around the Earth as a primitive atmosphere – a protective blanket against space.
The emergence of life
At some stage, it seemed that a thick ‘soup’ of chemicals steadily build up in the Earth’s newly-formed ocean, from gases washed out of the atmosphere, injected with energy by bolts of lighting and ultraviolet rays from the Sun, some of these chemicals became more and more complicated. Eventually, at least one of them started making copies of itself. From this amazing ‘duplicating’ substance, about three and a half million years ago, the first living thing developed.
The dynamic Earth
Powerful forces have shaped the Earth – and those same forces are still at work in the world today. Wind and rain scour rock away, rivers carve valleys and canyons, and ocean waves pound against shorelines, eroding their shape all around the globe. Yet even as old features on the surface are worn away, new ones slowly emerge to take their place. Mountain ranges are formed by those powerful forces, as are oceans and continents, which drift and change appearance over million of years. Slowly but surely, the planet Earth is constantly changing