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.

For more information please contact:

ESA Science Programme Communication Service
Tel: +31 71 5653183

USEFUL LINKS FOR THIS STORY

* Europe launches into astrobiology
http://sci.esa.int/content/news/index.cfm?aid=1&cid=1&oid=27228

IMAGE CAPTIONS:

[Image 1:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=1&cid=12&oid=27243&ooid=27253]
Ozone in a planet’s spectrum may indicate the presence of life. 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.

Note: The background image is a view of Earth seen by the Apollo 17
crew as they traveled toward the Moon. The white patch at the bottom
is Antarctica. Unobscured by clouds, Africa and the Arabian Peninsula
are visible at the top of this image. The large island off the coast
of Africa is the Malagasy Republic.

[Image 2:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=1&cid=12&oid=27243&ooid=24457]
An artist’s impression of the recently observed (from ground) transit of
the giant planet HD 209458 in front of the parent star. The discovery of
planets orbiting other stars has fuelled the belief that we may not be
alone in the universe. ESA’s Eddington mission will search for planetary
transits indicative of the presence of terrestrial, habitable planets,
providing for the first time evidence of the existence of other ‘Earths’.

[Image 3:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=1&cid=1&oid=27243&ooid=18420]
The Huygens probe will search for indications of habitats on Titan
compatible with life. After entering Titan’s atmosphere, Huygens’
parachute system will be deployed for the 2 – 2 1/2 hour descent, during
which most of the scientific measurements will be made. This artist’s
impression shows the Probe suspended from the stabiliser parachute passing
through the clouds that are expected at around 20 km altitude. The descent
will occur during daylight to provide the best illumination conditions for
imaging the clouds and surface.

—
Andrew Yee
ayee@nova.astro.utoronto.ca