Three important lines of discovery have been fuelling the growing belief that we are not, after all, alone in the universe. Many organic molecules have been discovered in space, suggesting that the building blocks of life are widespread; planets have been found orbiting other stars, raising the possibility that some may harbour life; and living organisms have been found alive and well in habitats on Earth so hostile that survival on Mars, or even other Solar System bodies, seems quite feasible.
The first European workshop on exo/astrobiology ended last week with participants summarising the latest findings in these three areas and outlining where and how they would like to search for extraterrestrial life.
"More than 120 organic molecules have so far been detected in space and more are being detected all the time," Pascale Ehrenfreund, from the University of Leiden, the Netherlands, told the workshop. There's strong agreement that a lot of this organic matter landed on the Earth, although the role it played in initiating life here remains uncertain.
The discovery of exoplanets is no less dramatic than that of organic molecules. The first was discovered in 1995, but by April this year, 67 objects had been detected in orbit around other stars, 63 of them planets, according to Stephane Udry from the Geneva Observatory, Switzerland. All of these planets are large and more likely to resemble Jupiter than Earth.
Nonetheless, future space missions, such as the ESA cornerstone Darwin, will have the sensitivity to search for smaller Earth-like planets and even to determine from their spectral signatures whether they could be home to alien beings. Darwin will most probably look for the spectral signature of ozone, which is unlikely to exist in any quantity for any length of time in the atmosphere of a planet that is not home to life.
Darwin is unlikely to be launched before 2014 and, in the meantime, astrobiologists will have to rely on calculations to estimate the number of Earth-like planets. Such estimates, however, are prone to error! "The number of Earth-like planets in the Milky Way was put at 2.4 million this morning, but had dropped to 48 this afternoon," Malcolm Fridlund, ESA's study scientist for Darwin, told the meeting during a summing up.
One factor in the calculations involves estimating the 'habitable zone' around a particular star, which is the volume of space in which a planet could provide the sort of environment favourable to the type of life we have on Earth. Our concept of the 'habitable zone', however, is constantly changing as life turns up in ever more extreme environments on Earth.
"We are constantly being struck by the sheer breadth of environments in which microbes can live," said David Wynn-Williams of the British Antarctic Survey. Life has been found in such inhospitable places as rock inclusions in dry Antarctic valleys and around hydrothermal vents on the deep ocean bed. Hostile environments like these display temperatures, pressures, acidity, salinity or exposure to radiation previously thought to be incompatible with life. "We need to broaden our view of normality. There could be habitats on Europa or Mars that seem bizarre to us, but are perfectly normal for microbes," said Wynn-Williams.
Identifying where those habitats could be was the task of a splinter session. The best candidates were thought to be on Mars, Europa, Titan, the comets or asteroids, and on interstellar dust particles. Some new sites were suggested for Mars. Among them were the north polar periphery (where ice melts and re-freezes with the seasons), permafrost in the sub-surface (permafrost on Earth has been found to be teeming with microscopic life), and at the sites of extinct hydrothermal vents.
"The search should be for chemical and structural biomarkers as well as fossils," said Gerda Horneck from the DLR, Germany, during a summing up session. Chemical biomarkers could be pigments used by microbes to protect themselves from environmental stress factors, such as ultraviolet radiation. The instruments to detect these signatures could be carried on a series of minilanders, like those under discussion for ESA's new planetary exploration programme, Aurora, or on larger rovers such as those planned by NASA. The exploration effort should culminate in a sample return mission. "It would be very important to obey planetary protection guidelines. Human missions could interfere with the aims of robotic missions to look for life," warned Gerda Horneck.
The meeting closed and participants left in eager anticipation of the second Exo/astrobiology workshop which will be held in Graz, Austria, in September 2002.