If we ever make contact with intelligent aliens, we should be able to build a universal translator to communicate with them, according to a linguist and anthropologist in the US.

Such a "babelfish", which gets its name from the translating fish in Douglas Adams's book The Hitchhiker’s Guide to the Galaxy, would require a much more advanced understanding of language than we currently have. But a first step would be recognising that all languages must have a universal structure, according to Terrence Deacon of the University of California, Berkeley, US.

How language develops is highly controversial. Some theories argue that the process has been built into the human brain through evolution, and that the sounds we use to communicate are arbitrary.

If that is true, there could be an infinite set of possibilities for expressing an idea through language. An alien race that developed through a completely different process of evolution would probably speak a language indecipherable to humans.

But Deacon argues that all languages arise from the common goal of describing the physical world. That limits the way a language could be constructed, he concludes.

Scented words

An alien race could use a strange medium like scents as their language, Deacon says, but the scents would still describe objects in their world. An odour that communicates "rock" or "tree" would be analogous to our words for the same objects. So there must be an underlying universal code that can be deciphered, as in mathematics.

"In Carl Sagan’s book Contact, aliens communicate to humans through prime numbers," says Deacon. "Why? Nature doesn't use prime numbers. But the numbers are intrinsic to the mathematical system, just as certain structures are intrinsic to language."

One of our most basic forms of communication is pointing, he says. Pointing directly references a physical object. When we invent a word for that object, that word is a symbol. Symbols can then convey meaning about objects even if they're not present in our immediate environment.

Abstract symbols

Deacon argues that no matter how abstract a symbol becomes, it is still somehow grounded in physical reality, and that limits the number of relationships it can have with other symbol words. In turn, this defines the grammatical structure that emerges from stringing words together.

If that is true, then in the distant future it might be possible to invent a gadget that uses complex software to decode alien languages on the spot, Deacon said. He presented his ideas on Thursday 17 April at the 2008 Astrobiology Science Conference in Santa Clara, California, US.

Testing the theory might be tough because we would have to make contact with aliens advanced enough to engage in abstract thinking and the use of linguistic symbols. But Denise Herzing of Florida Atlantic University in Boca Raton, US, points out that we might be able to test it by studying dolphins.

"Our work suggests that dolphins may be able to communicate using symbols," Herzing told New Scientist. "The word's not definitively in yet, but it’s totally possible that we might show universality by understanding dolphin language."

Nobody has yet seen an extraterrestrial, which may sound like a problem in establishing a science of astrobiology. But in the past 20 years or so, scientists have found clues that life may be quite common in the universe, and many are hopeful that they will soon find hard evidence of life beyond Earth.

Some hints come from terrestrial life. Biologists have discovered many species of extremophile - micro-organisms that thrive in extreme environments, such as alkali lakes and rock fissures deep underground. Life may have originated on the ocean floor around thermal vents or black smokers, which may be common features of other planets and moons.

And chemical traces of metabolism appear in Earth's rocks shortly after the planet’s ferocious Late Heavy Bombardment by meteorites, implying that life might be able to get started quickly and easily.

Meteorites from Mars occasionally hit Earth. Bacteria or their spores can probably survive the journey though space, despite the cold and intense radiation, which means that primitive life might once have been carried between the planets of the solar system, an idea called panspermia.

Martian microbes

In 1996, a team claimed that one Martian meteorite, ALH84001, contains fossilised Martian nanobacteria. Although much of their evidence has been discredited, sceptics cannot yet fully explain the crystals of magnetite found on ALH84001, which closely resemble crystals made by terrestrial bacteria.

Whereas Mars was once thought to be dry and barren, the latest evidence from ESA's Mars Express, and the NASA rovers, Spirit and Opportunity, shows signs of liquid water on or near the surface. It seems that billions of years ago, Mars was briefly warm and wet. And even now, water may occasionally flow on the surface.

Probably a better haven for microbes is beneath the Martian surface, where liquid water may be permanent and where the Sun's dangerous ultraviolet radiation does not penetrate. And newly discovered methane in the atmosphere of Mars may be the by-product of such deep-dwelling bacteria. A range of missions are planned to look for clearer signs of life, including a new version of the lost Beagle 2, and eventually an ambitious US sample-return mission.

Deep dark oceans

The other inner planets seem less promising for life. Although it has been suggested that Venus may once have been habitable, and that microbes could still survive in its clouds - 50 kilometres above the surface where the temperature falls below 70°C.

And Thomas Gold of Cornell University, US, has suggested that "deep hot biospheres" exist on Mercury and our Moon, with micro-organisms digesting the rocks as much as 10 kilometres into their respective crusts. He suggests this could also be true of most of the rocky planets and large moons in the solar system.

Orbiting the outer planets of our solar system, several large moons are thought to have a deep, dark ocean beneath their icy crusts. Life may feed off of heat and chemicals spilling into these oceans from volcanic vents on Jupiter's moons Europa, Ganymede and Callisto, as well as Saturn's Titan and Enceladus. The same may be true of Neptune's moon, Triton. And these oceans may be kept liquid with a dash of ammonia antifreeze. Scientists hope to send probes to explore Europa's ocean.

Meanwhile, Rosetta and other spacecraft are on a mission to find out whether comets carry a payload of complex organic chemicals, as scientists suspect. Comets might deliver this chemical "feedstock" to young worlds, giving life a head start. Many organic molecules are created by red giant stars, and detected by astronomers in interstellar clouds. Amino acids, the building blocks of proteins, were found on the Murchison meteorite.

Alien Earths

More than 150 planets have now been discovered outside the solar system. Many of these extrasolar planets are "hot jupiters" - massive planets that orbit blisteringly close to their stars.

More promising for life would be alien "Earths". These are small rocky planets like ours that orbit within a habitable zone - not too close or too far from the parent star, so the temperature is just right for liquid water to exist on the surface. Other options, such as ocean planets, might be more exotic. NASA’s Kepler mission could detect hundreds of alien Earths when it is launched, currently scheduled for 2008.

The next stage would be to sniff out life on these distant worlds. ESA's Darwin and NASA's Terrestrial planet finder will look for the spectral signature of oxygen, a suggestive but not conclusive sign of life. Later space telescopes will be able to image Earth-like planets many light years away, and search for less ambiguous biomarkers.

Then again, life might like much stranger habitats. David Stevenson of Caltech in Pasadena, US, has suggested that lonely planets, wandering far from any star, could remain warm under a blanket of hydrogen, and maintain liquid water and life.

ET calling?

How about intelligent life? The Drake equation predicts how many civilisations in the galaxy are currently trying to communicate with us, although some factors in the equation are almost pure guesswork.

Optimists face the Fermi paradox: if civilisations are common, then why have we not seen them? Scientists have spent more than 40 years on the search for extraterrestrial intelligence (SETI), using radio telescopes to listen out for broadcasts from the stars. And the Allen Array is being built specifically for the purpose. Other SETI astronomers are using optical telescopes to search for laser beacons. Some experts think we should be looking for giant space structures or a "message in a bottle" instead.

No luck so far, but we will probably find alien bugs before we tune into alien TV.

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