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Astron. Astrophys. 341, 304-311 (1999)

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7. Conclusion

Noll et al. (1997) have not stated that the O3 criterion was incorrect, they have stated that ifcomets contain few percents of free O2 per mass, their impacts on telluric planets would be an appreciable source of abiotic O2 and O3. They have proposed a mechanism for the presence of such species in comets: the chemical reactions induced by particle irradiation on pre-cometary icy grains.

We have shown that recent models of protoplanetary disks actually do not predict the production of a significant amount of O2 in comets. Though these models cannot be considered as final, their quality is indicated by their ability to predict several independent intriguing features observed in the Solar System (Shu et al. 1997).

In addition, Noll et al.'s hypothesis has no observational support in the Solar System:
(i) neither O2 nor O3 has been detected in comets (upper limit: [FORMULA] for Halley's comet)
(ii) the O2 input that would have resulted on the Young Earth, when cometary bombardment was heavy, did not build an oxygen rich atmosphere.

We conclude that the simultaneous detection of significant amounts of H2O and O3 (corresponding to an O2 pressure larger than 10 mbar, Léger et al. 1993b) in a telluric exoplanet, with a size comparable to, or larger than, that of the Earth and located in the Continuously Habitable Zone of its star presently stands as a criterion for a large photosyntheticactivity on a planet (see also Kasting, 1997).

Now, is this a real signature of a biologicalprocess? For G or later type stars, in the presence of a UV shielding by the planetary atmosphere, e.g. by O2 and O3, photosynthesis requires the absorption of several stellar photons and the storage of energy in between. A system that is able to do so has to be so complex a "factory" that it is difficult not to think of if as the result of a biological evolution (Raulin, private comm.).

Considering the fact that the H2O - O3 criterion is considered as a criterion for remote detection of life (and is the base of most present mission concepts developed to achieve this goal), it is highly desirable to keep on developing a continuous and vigorous conceptual and modelling activity, in order to qualify or falsify it.

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© European Southern Observatory (ESO) 1999

Online publication: November 26, 1998