Alien life forms with liquid assets

Nigel Henbest and Heather Couper on vital signs from the 'new' planets

Last month two American astronomers announced their discovery of two planets, one in the Great Bear constellation and the other in Virgo, at a meeting of the American Astronomical Society in Texas. In the same breath came pronouncements about life there - according to the society, "the conditions indicate that life as we know it could exist on those planets".

It would indeed be ironical if the first life we found beyond Earth was in orbit around the obscure stars 47 Ursae Majoris or 70 Virginis. Astronomers have scoured the planets of our solar system for decades, in a hunt for even the tiniest living microbes.

Against this disappointing background runs one growing gleam of hope: the raw materials for life are turning out to be two-a-penny throughout our solar system and well beyond. In the Sixties, astronomers found huge dark clouds in our galaxy, the Milky Way, full of organic molecules. The Voyager 1 spacecraft discovered a moon of Saturn covered with an orange veil of organic material. And in 1986, Europe's Giotto spacecraft found that the icy nucleus of Halley's Comet was coated with black organic compounds.

No one doubts that the two newly discovered planetary systems must also be thick with the basic molecules of life. But how do they turn into living creatures? Astronomers are now focusing on one essential ingredient: liquid water.

What keeps us alive are chemical reactions in the watery interior of our cells. Astronomers think that simple one-cell organisms may well have formed in early oceans on our two neighbour planets.

Given water, the range of life on Earth is wider than anyone had anticipated even a few years ago. Most surprising are animals and plants that live miles below the ocean surface. Instead of living off light and oxygen, they rely on sulphurous gases and heat from volcanic vents, the "black smokers". This emphasis on liquid water has shifted attention in our solar system to Europa, one of Jupiter's moons. At this distance from the Sun, Europa's surface is frozen solid; but the moon's core is probably hot and may erupt black smokers into a deep ocean beneath the ice.

Although astronomers speak confidently of good conditions for life on the new planets, they have yet to see these new worlds. Geoff Marcy and Paul Butler, the planets' co-discoverers, were really observing the motion of their parent stars, using a large telescope at the Lick Observatory in California. They found that each star was wobbling very slightly backwards and forwards, as it was dragged around by an unseen companion. In both cases, the companion weighs in about the same as our solar system's largest planet, Jupiter.

The planet circling 47 Ursae Majoris is about as far from its star as Mars lies from the sun. Although Mars is frozen, this world is big enough to exude heat of its own, which may raise temperatures above freezing point. The other planet orbits 70 Virginis at roughly Venus's distance from the Sun. Its temperature works out to 85C. "That's cool enough to permit complicated organic molecules," calculates Marcy, "and because 85C is below the boiling point of water, this planet could conceivably have rain or even oceans."

But even if these worlds do not turn out to be the home of alien life, they may show the way. Any alien astronomers living some 35 light years away from our Sun would pick out the wobbles due to Jupiter, but would detect no sign of its smaller kin, including the Earth. In the long run, their role may be as signposts: "Here is a planetary system: look harder, and you may hope to find a second Earth."

What's Up

Venus is set for a long session holding centre-stage this spring. It is a dazzling object in the western sky after sunset, and this month shines at magnitude minus 4 - 10 times brighter than Sirius. Mid-month, it sets four hours after the Sun. Look for a stunning grouping of Venus and the crescent moon on 22 February.

On February 12, Earth passes through Saturn's ring plane. From now until 2009, we will see the south face of its rings illuminated. Saturn's next- door giant world, Jupiter, is now rising two hours before the Sun. Mercury, too, comes up an hour and a half before sunrise.

The constellations on view are starting to take on a spring-like appearance, with Orion and his entourage beginning to slip down towards the west. Gemini rides high in the south, and to the left is the dim and un-crab- like constellation of Cancer. Look - with binoculars - in the small triangle of stars at Cancer's heart to locate a large, faint cluster of stars. It is commonly called the "Manger" or the "Beehive", although ancient Chinese astronomers knew it as the "exhalation from piled-up corpses".

Diary (all times GMT)

11 9.00pm Mercury at greatest western elongation

12 8.37am Moon at last quarter

18 11.30pm New Moon

26 5.52am Moon at first quarter