Ask mr Brain...all will be explained
Why did life evolve on earth and not on another planet? In our solar system earth is the only planet which supports life. The on- going exploration of Mars is attempting to uncover evidence that the red planet once supported life, though probably only at the bacteria level rather than anything as complex as a human.
There is a relatively narrow - in astronomical terms - region around the sun where it is possible for life to exist on an earth-type planet. This ecosphere stretches from Mars to Venus. Any closer to the sun than Venus and the temperature would be too high, further than Mars and it would be too cold.
As it is, the surface temperature of Venus is 480 degrees Celsius, the result of solar energy being trapped by the planet's thick, sulphuric acid clouds. Mars is rather cold, at -23 degrees Celsius, with a thin carbon dioxide atmosphere - slightly better for life than Venus, but that's not saying much. The environment of planets such as Saturn is so hostile even space probes only last minutes.
The size of earth is another factor. If earth was much smaller, it would not be able to hold on to its atmosphere - in much the same way as the moon is unable to keep any atmosphere. If earth was a lot larger, it might have kept too much of its original hydrogen in its atmosphere - Saturn and Jupiter have - which would have prevented the rise of life.
Other factors which have allowed earth to sustain life include its rotation speed. Earth rotates every 24 hours and the Martian day is not much longer, although the Martian 'year' lasts 687 earth days.
On Venus a 'day' lasts 243 earth days and a 'year' lasts 225 earth days. Yes, on Venus a day is longer than a year! Calendar makers, for one, are probably relieved we evolved on earth rather than Venus.
If earth revolved much more slowly or orbited the sun at a vastly different rate, it is probable that life would not have evolved in the hostile environment and atmosphere that would result.
All in all, we're pretty lucky that earth is the right size, in the right place and moving at the right speed.
How did television become such a powerful medium of communication? With sound and moving pictures, television gives viewers the illusion of being direct observers of news and entertainment events.
The technical principles of television were developed in the 1920s and programmes were first broadcast by the British in 1937, but it was only during the 1950s that TV sets become part of most United States households.
With its colour, sound and movement, television is more dramatic than newspapers and radio, and as TV viewing has increased, many people are spending less time reading newspapers and books. This is especially bad news for young people who need to improve their reading but are tempted away by the television.
The advent of video cassette players, cable and satellite TV means that viewers now have even more choices.
