Report - School of Physics

The possibility that O 2 and O 3 are not unambiguous identifications of Earth-like biology,
but rather a result of abiotic processes, has been considered in detail by Léger
et al. (1999) and Selsis et al. (2002). They considered various production processes
such as abiotic photodissociation of CO 2 and H 2 O followed by the preferential escape
of hydrogen from the atmosphere. In addition, cometary bombardment could bring
O 2 and O 3 sputtered from H 2 O by energetic particles, depending on the temperature,
greenhouse blanketing, and presence of volcanic activity. They concluded that
a simultaneous detection of significant amounts of H 2 O and O 3 in the atmosphere of
a planet in the habitable zone presently stands as a criterion for large-scale photosynthetic
activity on the planet. Such an activity on a planet illuminated by a star
similar to the Sun, or cooler, is likely to be a significant indication that there is local
biological activity, because this synthesis requires the storage of the energy of at
least 2 photons (8 in the case on Earth) prior to the synthesis of organic molecules
from H 2 O and CO 2 . This is likely to require delicate systems that have developed
during a biological evolutionary process. The biosignature based on O 3 seems to
be robust because no counter example has been demonstrated. It is not the case
for the biosignature based on O 2 (Selsis et al., 2002), where false positives can be
encountered. This puts a hierarchy between observations that can detect O 2 and
those that can detect O 3 .
Habitability may be further confined within a narrow range of [Fe/H] of the parent
star (Gonzalez, 1999b). If the occurrence of gas giants decreases at lower metallicities,
their shielding of inner planets in the habitable zone from frequent cometary
impacts, as occurs in our Solar System, would also be diminished. At higher metallicity,
asteroid and cometary debris left over from planetary formation may be more
plentiful, enhancing impact probabilities. Gonzalez (1999a) has also investigated
whether the anomalously small motion of the Sun with respect to the local standard
of rest, both in terms of its pseudo-elliptical component within the Galactic
plane, and its vertical excursion with respect to the mid-plane, may be explicable
in anthropic terms. Such an orbit could provide effective shielding from high-energy
ionising photons and cosmic rays from nearby supernovae, from the X-ray background
by neutral hydrogen in the Galactic plane, and from temporary increases in
the perturbed Oort comet impact rate.
1.3 Present Limits: Ground and Space
Figure 2 illustrates the detection domains for the radial velocity, astrometry, and
transit methods as a function of achievable accuracy. It also shows the location of
the exo-planets known to date, in a mass-orbital radius (period) diagram.
The fundamental accuracy limits of each method are not yet firmly established,
although such knowledge is necessary to predict the real performances of dedicated
surveys on ground and in space. Granular flows and star spots on the surface
of late-type stars place specific limits on the photometric stability, the stability
5