Hope for the alien hunters

Nasa this week unveils a new emissary in the search for evidence of extraterrestrial life. The Phoenix Mars Lander, which launches next month, marks just the latest instalment in a quest that has exercised the imaginations of writers and scientists since long before the adventures of Jules Verne. In the 17th century Johannes Kepler, the architect of our modern understanding of the solar system, imagined a journey to a moon inhabited by serpent-like creatures called Prevlovans who endured the lunar night "bristling with ice and snow under the raging, icy winds". Regrettably, however, here is no reliable account of a real encounter with alien life-forms. Many doubt whether they exist at all. 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.