All the planets do not have life on them. Earth is the only planet in the solar system having life on it. This is because certain conditions or requirements are necessary before life can originate and flourish on a planet. The various conditions or requirements necessary for life to originate and survive on a planet are given below :
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The planet should have elements like carbon, hydrogen, nitrogen and oxygen, which are called building blocks of life. The atoms of these elements combine to form various molecules needed to form cells, tissues and organs of living beings, and hence help in the origin of life. The carbon should be present in the form of carbon dioxide gas, CO2; oxygen as O2; hydrogen as H2 and nitrogen as N2 gas.
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The planet should have sufficient oxygen gas in its atmosphere required for breathing and survival of living beings.
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The planet should have water. This water is to transport the nutrients; to regulate body temperature and to help in excretion.
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The temperature on planet should be neither too high nor too low. The temperature on planet should be suitable for carrying out the various life-sustaining (life-maintaining) reactions.
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The planet should have a protective blanket (like that of ozone layer) around it so as to prevent the harmful solar radiations from entering its atmosphere and destroying life on it.
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We shall now find out which of the four common planets like Mercury, Venus, Earth and Mars, fulfill above conditions for the evolution of life and its survival. We shall not discuss the remaining five planets like Jupiter, Saturn, Uranus, Neptune and Pluto, because they are very far off from the sun and hence too cold for the evolution and survival of life on them.