This computer generated montage shows Neptune (background) as it would appear from a spacecraft approaching Triton (foreground), which is Neptune’s largest moon at 2706 km in diameter (Image: USGS / JPL / NASA)

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.