Given the size of the universe - there are at least 100 billion stars in our home galaxy alone and perhaps 100 billion galaxies of much the same size scattered throughout deep space - few scientists believe that the Earth is the only home of life. But until quite recently, the field of exobiology - the study of extraterrestrial life also known as astrobiology - was almost moribund. It could come up with some interesting speculations but that was about all.
The robot planetary explorers that swept through the solar system in the 1960s and 1970s found no trace of life, or even potential life-supporting environments. Exobiology's most adventurous experiment - when the 1976 Mars Viking Lander tried to find biological activity in the soil of Mars - yielded discouraging results.
The Mars disappointment was the nadir for the hopeful new science. With the exception of the Earth, the solar system appeared to be barren. As for life beyond the solar system, the colossal distances involved made it simply unreachable, and in any case, no one knew for sure if other stars had planets at all, far less living planets.
Since those bleak days, exobiology's prospects have brightened enormously. A whole succession of discoveries have vastly increased the probabilities that life exists elsewhere in the solar system - as well as our chances of actually finding it.
Some of these discoveries have come from recent space probes and careful astronomical observation. For example, in the last few years, scientists have found evidence for planets around more than 60 nearby stars. The Galileo spacecraft has found what is almost certainly a liquid salty ocean beneath the surface of Jupiter's moon Europa. Mars most likely once had liquid water flowing on its surface. Scientists now believe that much of it is still there, locked beneath the surface.
The most vital 'exobiology' discoveries, though, were made right here on Earth. Biologists have learned that life is much more robust that most scientists believed 30 years ago. Earth micro-organisms have been found thriving in astonishingly hostile environments. Deep beneath the oceans, for example, near the volcanic vents known as black smokers, some microbes grow and multiply at temperatures above 110 degrees - according to some scientists, perhaps as high as 170 degrees.
Others thrive in acid conditions that would strip the skin from a human, while others still make a comfortable living in hot rocks kilometres below the ground. Some even prefer cold to heat: Antarctic life-forms can manage very well in what amounts to a permanent deep-freeze.
The existence of these so-called extremophile organisms radically changed our view of what might be called "the necessities of life". Extremophiles live happily without sunshine, without moderate warmth, without organic molecules to feed off and with no need for photosynthesis - many digest raw minerals and fuel themselves with basic chemical reactions.
Many share another fascinating characteristic, too. The genetic code of these creatures suggests that they are not recent adaptations that have moved away from 'mainstream life' into awkward niches shunned by their competitors. Instead, in evolutionary terms they are among the oldest living things on Earth - probably among the very first to appear.
The implications for life elsewhere in the solar system are huge. We know now that all life needs is liquid water - even a little dampness will serve - and some kind of energy source. Exobiology is back in business.
Mars remains the best candidate for the breakthough discovery of an extraterrestrial organism. In the early solar system four billion years ago, it may well have offered better prospects for life than the Earth. In 1998, NASA scientists found what may have been fossilised ancient Martian bacteria in a meteorite blasted from the planet's surface by a cosmic impact; the Mars rock drifted through space for millions of years before eventually crashing down in Antarctica.
Exobiology will be a major element in the Mars missions of the first half of the 21st century. ESA's Mars Express arrived in Martian orbit in December 2003; the Aurora Programme is looking at plans for a sample return mission and even a human visit. Some of Earth's Antarctic life-forms could probably live on Mars today. Perhaps below the Martian surface, the corresponding native organisms are just waiting to be discovered.
Europa, ten times more distant than Mars, is a more difficult proposition. But plans for Europa missions are on the drawing board, too. Exobiologists no longer restrict themselves to planetary environments. Comets, for example, are rich in organic material and certainly could be colonised by some kind of extremophile. Some theorists even think that life originated first in a cometary environment then reached Earth and, perhaps, other planets.
Last update: 10 May 2004