The Phoenix mission to Mars is very much in pursuit of liquid water, the key to life. Every watery place on our own planet, from the depths of the ocean to the tips of the highest mountains, supports life. The hardiest organisms, bacteria called extremophiles, can endure more or less anything the terrestrial environment can muster, from boiling acid baths to cold briny seas. But take away water, or freeze it or boil it to steam, and nothing grows. Life requires water and the water has to be in the liquid state for it to be useful.
Water is common in the universe, but is liquid only within a narrow window of temperatures and pressures. In our own solar system, temperatures range from 480C on the surface of Venus to -230C on Pluto. The habitable or "Goldilocks zone", as it is sometimes called, because it's neither too hot nor too cold, occupies a narrow band of our own solar system of less than 1% of the distance from the sun to the outer edges. This is precisely where the Earth orbits.
For another solar system to harbour life as we know it, it must have an Earth-like planet within its sun's habitable zone. This is why scientists are excited about Gliese 581c. The recently identified planet is the right distance from its sun to be capable of harbouring liquid water, and may be the best hope yet for alien hunters. Stéphane Udry and his colleagues from the Geneva Observatory were able to detect a tiny wobble in the parent star that betrayed the existence of a planet with a mass about 1.5 times that of Earth. They estimate the mean temperature of the surface of Gliese 581c to be a balmy zero to 40C, just right for life. A wobble is a long way from detection of life, but it is a start, and one of the Swiss team's other conclusions is that small Earth-like planets are probably common. So is life also common among the stars? And should it matter to us?
Since the dawn of civilisation, the status of the heavens has been central to man's concept of his place in the universe. The medieval "world" or cosmos consisted of a spherical Earth cocooned within a series of concentric spheres carrying the moon, the planets, the sun and, furthest away, the stars pinned on to the outermost firmament. Beyond that was God. The starry spheres had been invented to account for the nocturnal revolutions of the night sky. If all those stars were circling the Earth and yet maintaining their relative positions then it made sense to place them on the inner surface of a sphere. This was a cosy walled-in world with mankind occupying the centre.
But then Copernicus turned the medieval universe upside down when his heliocentric solar system allowed the Earth to spin and orbit the sun. Mankind occupied just one of several planets. But, even worse, the walls of the cosmos shattered. If, as Copernicus argued, the Earth rotated rather than the stars, there was no need for all those spheres. And without the outermost sphere the stars could recede backward into infinity. The universe got a whole lot bigger.
Copernicus received the first copies of his book Revolutions on his deathbed in 1543. But its implications were not fully appreciated. The clash with the Catholic church came more than half a century later, with the burning of Giordano Bruno at the stake in 1600 for claiming, among other heresies, the existence of a plurality of worlds, and then the infamous trial of Galileo in 1633.
The Catholic church has since retreated and even apologised to the long dead Galileo in 1992. Few would now defend an Earth-centred universe but, to many, mankind remains the central concern of an anthropomorphised God. This would be harder to maintain if Gliese 581c and other worlds are teeming with life. But it would make the universe a more exciting place to live in.
· Johnjoe McFadden is professor of molecular genetics at the University of Surrey and author of Quantum Evolution