Life as we know it is possible only within a very narrow range of temperatures. Planets which meet this stringent requirement are few and far between in the solar system. In fact, there is only one: we call it Earth.
Earth is roughly in the middle of a zone in which conditions range from being too hot for life, in the case of Venus, and too cold for life, as is the case with Mars. The temperature of a planet is obviously influenced by its distance from the sun, but there is another more subtle influence at work. The temperature of each planet is also regulated by the strength of its own greenhouse effect, and in the case of Earth the balance, for the present, is exactly right.
It is a fear that this delicate balance may be upset that lies behind all the brouhaha at the Kyoto conference on climate change. But a few scientists take a more relaxed view of these matters, such as James Loverock, the originator of the socalled Gaia Hypothesis.
The Gaia Hypothesis, named after Gaia, the goddess of the ancient Greeks who symbolised our Mother Earth, is a theory which postulates a symbiotic bond between life itself and the environment.
Gaia asks the question: why is this planet of ours such an ideal spot for life to thrive? The temperature and pressure are just right; the air is breathable; and a benign atmosphere shields us from the worst effects of ultra-violet radiation. How did this happy circumstance arise?
There are two traditional answers to the question. One approach is that evolution has enabled life on Earth to adapt in the way best suited to any environment in which it finds itself. A complementary view has it that life has altered the environment to suit itself.
Plants, for example, absorb carbon dioxide by means of photosynthesis and create oxygen, while animals and humans survive by the reverse process: by this delicate relationship, and many others, life alters the composition of the atmosphere to achieve the conditions needed for survival.
Gaia goes further and tries to link the two. It suggests that life and the atmospheric environment are closely linked parts of a whole system, a system with elaborate feedback mechanisms which dictate the way in which the whole planetary scene evolves.
Supporters say that this combined system has developed in such a way that it can regulate and repair itself; if it is knocked off balance by catastrophic events, or more gradual, even human, influences, it can repair the damage, and life, although not necessarily any individual species, will survive.