Europeans united in the hunt for aliens

Over the next decade three probes will be searching space far beyond our solar system. If there is anything out there, says Ian Brown, it may not be on Mars

Europe may be in several different minds about a single currency, but at least its astronomers and scientists seem to be united on one thing – their desire to search for extraterrestrial life. At a meeting of European space ministers in Edinburgh this week, scientists outlined their plans to extend their search within and beyond the solar system.

Over the next decade, an advance guard of space probes is scheduled for launch by the European Space Agency (ESA). Mars remains the prime candidate for alien life, but unmanned missions will also explore the next most likely habitat: Titan, a huge moon of Saturn. In fact, a forerunner Euro-probe, Huygens, is already on its way to Titan, a world that ESA scientists describe as a "planetary-sized laboratory for pre-biotic chemistry". But the search will go much further. ESA is also proposing to send up three space-based observatories – Eddington, Gaia and Darwin – to scan the star fields for Earth-like planets orbiting alien suns that may signal the existence of life beyond our solar system.

No one expects to find animal life elsewhere in the solar system. But, buoyed by the realisation that life can survive, indeed thrive, in equally hostile environments on Earth, astro-biologists are hopeful of finding micro-organisms. It seems that where there's organic chemistry, water and an energy source, there's invariably life (at least on Earth) – no matter how harsh the conditions. And such essentials appear to be commonplace in space.

David Wynn-Williams, a Cambridge microbiologist with the Antarctic Astrobiology Project, is optimistic. "If there's life on Mars today, I reckon it's a couple of kilometres underground living in melted permafrost," he says. "Or it could be alive and well in pockets of water under the north polar ice cap."

A member of the ESA's Mars Express mission, Wynn-Williams points to similarities between Martian environment and sites in Antarctica, where microbial life exists in rocks and at the bottom of extremely salty frozen lakes. And, although Mars is too dry and cold for any form of life to exist there now, that may not always have been so. Millions of years ago, conditions may have been as warm and wet as Earth. Indeed, as it is smaller and would have cooled down quicker, life may even have arisen on Mars first.

But if water was once there, where did it go? Did some form of life ever evolve there? Does it still? We may know soon. Mars Express, due for launch by the ESA in 2003, will be the first probe specifically equipped to search for underground aquifers, while the small lander it will carry, the British-designed Beagle 2, will search for traces of life, past and present.

Even further out in space, beyond the asteroid belt and the giant gas planet of Jupiter, is Titan. One of Saturn's 30 known moons, the chemistry taking place in its dense atmosphere is thought to be similar to that of Earth's during its early phase of its atmospheric development some four billion years ago. The Huygens probe, half way there already, will enter Titan's atmosphere in early 2005, and will study this chemistry.

Huygens is hitching a lift aboard Nasa's larger Cassini spacecraft, which will also study Titan, but from orbit around Saturn. "It's a bit like going back in a time machine to Earth four billion years ago," says Helmut Lammer of the Austrian Academy of Sciences. "The atmosphere is a natural laboratory for studying pre-biotic chemistry on early Earth – the chemistry that led to life." Lammer concedes there's little probability of finding life. "It's far too cold and all the water is deep frozen on or below the surface. But the chemistry in the atmosphere may be very similar to the chemistry that preceded life on Earth."

A colleague, Jean-Pierre Lebreton of the University of Paris, agrees. "Methane in Titan's atmosphere is continuously destroyed by ultraviolet light," he says. "To explain the amount of the gas present in the atmosphere, we think there must be a large source either on or under Titan's surface. It could be in the form of lakes or oceans, or subsurface reservoirs."

Many question remain. Even if organic molecules containing carbon and nitrogen – the basic building blocks of life – do occur on Mars or Titan, how did they get there? Are they indigenous? If they're not, what brought them? The answer could be found in comets, which travel vast distances, occasionally dumping material on passing planets. Scientists believe comets have played a major role in seeding the galaxy with the chemical building blocks of life. Yet another ESA probe, Rosetta, will aim to catch up with comet Wirtanen in 2012. "An impacting comet has the power to destroy life on Earth, but we believe that comets may have also helped to create life in the first place," says Gerhard Schwehm, project scientist for the mission. Unlike planets, comets are thought to have undergone little chemical change since their birth with the rest of the solar system around 4.5 billion years ago. So preserved inside them may be the raw material out of which our solar system formed.

Organic molecules are also present in the immense void between the stars of our galaxy. "When we look at dust and gas in the interstellar medium, we see molecules that are found on comets and on Earth," says Pascale Ehrenfreund of Leiden Observatory, a co-investigator on the ESA's Infrared Space Observatory. "They are important components for building up more complex molecules. Of the 120 molecules so far detected in interstellar gas, about half are organic. The largest has 13 atoms, but we think there are much larger molecules out there," he says.

The ISO's most notable discovery was that water is just about everywhere in space. In 2007, the ESA's far infrared and sub-millimetre mission, the Herschel Space Observatory, will continue to unravel the complexities of interstellar chemistry. So, what's beyond our solar system? Over 70 "extra-solar" planets have already been detected since the first in 1995, but most are much larger and closer to their stars than Earth. Any planet capable of supporting life will be smaller and orbit at a distance where liquid water can exist on the surface.

Alan Penny of the Darwin study team at the Rutherford Appleton Laboratory near Oxford says about 1,000 stars are being searched for planets. About 5 per cent of these have closely orbiting worlds the size of Jupiter. "The reason we haven't detected small planets is because our methods aren't sensitive enough," he points out. Increases in the sensitivity of ground-based telescopes over the next 10 years are expected to take the size of detectable extra-solar planets down to one sixth of the size of Jupiter. Space-based telescopes, says Penny, would still be best for observations.

First to be launched, possibly in 2008, will be Eddington. This will search 500,000 stars for orbiting Earth-like planets. Next will be Gaia, which will watch for stars being pulled slightly in their orbits by the gravity of any accompanying worlds. Finally, and most ambitious of all, Darwin will use a sophisticated technique called "nulling interferometry" to detect Earth-like planets directly and determine the composition of their atmospheres through spectral analysis.

The ultimate aim is to detect something that indicates the unequivocal presence of living things. "If we see oxygen or ozone, we'll know that something is pumping it out continuously," says Penny. "Oxygen is very reactive and doesn't stay around long. People have wracked their brains to think of methods of getting a lot of oxygen into the atmosphere that have nothing to do with life – and they can't think of any. So, in all probability, if we see oxygen or ozone, it will mean life."