In 1969, when the astronauts Armstrong and Aldrin were on the stark and desolate landscape of the Moon, they were able to see the Earth as it had never been seen before. Just as we on Earth can look up at the Moon, so they were able to see the Earth rising over the barren lunar surface.

From this distance there is nothing to indicate that the Earth was any different from the lifeless Moon with its craters and plains, or from any other planet of the solar system.

However, those astronauts knew that beneath those reflecting clouds and alongside the shimmering oceans was a different world – one as full of beauty as the other is empty and sterile.

Yes, as far as we know, the planet Earth is unique. It is easy to forget that. There is no actual evidence that there is another place like Earth. What is it that makes Earth so different? Is it the mountains and valleys, its rocks, its minerals? No, other planets have these features. The supreme difference is that the Earth contains life. Wherever we look there are living things of amazing diversity and complexity – trees, plants, animals, birds, fish and insects. Chief among these living things is Man himself, with his unique ability to reason.

Earth from space

We might well ask, ‘What is the difference between the Earth and the other planets that enables this phenomenon of life to occur? Are such differences accidental?’

To answer this question we must note the conditions necessary for life to exist. The Universe as a whole is a dangerous place. Vast spaces, intense and powerful radiation, extremes of temperature from a little above absolute zero (minus 273 degrees Centigrade), to millions of degrees above, combine to make the Universe inhospitable to life. Living things are very delicate and even small variations from certain conditions mean death.

Here are some of the criteria that have to be met for life to exist:

• Temperature range

The range at which living things can function is small on a universal scale. At low temperatures all living processes stop around 0C, when water freezes and the upper limit for growth is around 45C. (Some forms of micro-organisms can grow at higher temperatures and others survive but do not grow in boiling water; even so the temperature range for growth is comparatively small).

• Water

All living processes take place in water. Our body consists of 70% water and many forms of life exist in water. Life cannot exist without water.

• Energy source

Living things stay alive by extracting energy from chemical reactions. In most cases this is done by breaking down food. The energy in food originally comes from the Sun. Plants capture the energy by means of a very special substance called chlorophyll and use the energy to make foodstuff that animals can eat, thus extracting the Sun's energy second-hand. Light is therefore essential to all the higher forms of life.

• Atmosphere

Most living things require oxygen in order for them to extract the energy contained in food.

• Correct force of gravity and atmospheric pressure

The astronauts on the Moon could jump higher and farther than on Earth because the Moon's gravitational pull is less. Conversely, on a large planet gravity would crush them into the ground. On Earth, the atmospheric pressure is about 14 pounds per square inch. If it were a lot more than this, living things would be squeezed to death.

The planet Mars from space. Inset: the barren Martian surface

• Freedom from radiation

Space is full of rays that are lethal to living things: gamma rays, x-rays, ultraviolet rays and cosmic rays have sufficient energy to break up complex life chemicals. Astronauts have to wear specially designed suits to protect them from this radiation when they venture from their spacecraft.

• Only Earth suitable for life

Of all the planets, only the Earth has all these things life needs. It is the correct distance from the Sun to give it the right temperature range and has plenty of water in liquid form. If it was only very slightly nearer the Sun, its water would boil off; a little further away and the oceans would freeze. It has an atmosphere containing oxygen and whilst allowing light through, is thick enough to prevent the dangerous rays in space reaching the Earth's surface. The atmospheric pressure is not excessive and the Earth is of a size that exercises a force of gravity that is compatible with living things.

A review of the features of other planets shows how unsuitable they are to sustain life:

MERCURY Moon-like surface – no water – very hot – no atmosphere
VENUS Moon-like surface – extremely hot (5000C) – atmosphere of carbon dioxide and sulphuric acid vapour – atmospheric pressure 100 times that of Earth
MARS Dry rocky surface – no water – ‘ice caps’ are solid carbon dioxide – negligible atmosphere – temperature generally very cold
JUPITER Not a solid planet – consists of liquid hydrogen at a temperature of minus 270o C – bathed in clouds of ammonia hundreds of miles thick