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