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We seem to have found the perfect location. As third planet from the Sun, our Earth sits comfortably in the so-called circumstellar habitable zone: not too hot and not too cold. A single moon, 250,000 miles away, provides the twin stabilising influences of gravity and friction, keeping the planet at a steady tilt, and slowing the rate of rotation. A moonless Earth would not necessarily be a lifeless one. But it would certainly be wilder and windier, with climactic fluctuations so acute that even George W Bush would take notice. Vegetation would probably consist of the ground-hugging variety; animals would be burrowers; outdoor badminton would be out of the question.
The Moon is by no means the only body in our neighbourhood to exert beneficial effects on Earth. Two planets out sits Jupiter, a gas giant with a mass more than 300 times that of Earth. So strong is the gravitational pull of Jupiter that it acts like a cosmic goalkeeper, deflecting the comets that rain in from the outer reaches of our solar system. It seems certain that without Jupiter's protective influence, Earth's history would have been a much more violent one.
We know that life has recovered from the impact of interstellar debris before, most famously after the extinction of the dinosaurs 65 million years ago. But we also know that, in the short-term, these events were an ecological catastrophe. If they had occurred more frequently, they would surely have prevented any kind of evolutionary continuity, perhaps even life itself.
Evidently, life needs not only a suitable planet, but a suitable system of planets, perhaps even a suitable galaxy, if it is to flourish. Suitability, of course, is a difficult term to define, so long as we appear alone in the Universe. But if we can prove that our solar system typifies other planetary systems, then we are on our way to finding extra-terrestrial life. Unfortunately, the early signs are not good. Astronomers have already identified about 90 planetary systems, and none of them compare favourably to our own. But it's still far too early to make sweeping assessments. Predictions suggest that there are billions of planetary systems out there. We just need bigger and better telescopes to see them.
Life, the Universe and Everything | A Difficult Recipe | Location, Location, Location | The Realm of Possibility | Alien Titchmarsh | Find Out More
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