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

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1. A new field opens in astronomy

In 1995, a new chapter of astronomy opened when Mayor and Queloz reported the discovery of a Jovian extra-solar planet (exoplanet) around the main sequence star 51 Peg. This discovery, rapidly followed by others (Butler & Marcy, 1996), was not that of the first exoplanet. Wolszczan (1991) had already unambiguously identified planets around the pulsar PSR 1257+12, but the observation of 51 Peg opened the field of observational Bioastronomy out of the Solar System.

The next steps in this quest for extra solar life are the search for exoplanets that are:
(i) telluric,
(ii) habitable,
(iii) inhabited by micro-organisms.

Strategies for items (i) to (iii), aiming at obtaining "yes or no" answers at each step have already been proposed. In this paper, we concentrate on item (iii).

Owen (1980) showed that exo-biological activity is most likely based on carbon chemistry with H2O as a solvent (see also Léger, 1997). He showed that if biological activity on a telluric planet develops on a large scale, it necessarily produces a large quantity of O2. As this gas is very reactive with reducing rocks and gases emitted by volcanoes, it would disappear in a short time in the absence of a continuous production (4 Myrs on Earth; Kasting, personal comm.; Holland, 1978). So he concluded that the abundant presence of O2 in the atmosphere of an exoplanet demonstrates the presence of life.

Another step was made when Angel et al. (1986) showed that O3, whose detection is easier, is a tracer of O2. The spectral signature of ozone occurs in the IR (9.6 µm) whereas that of molecular oxygen is in the visible (e.g. 760 nm). For a solar-type star, the ratio of planet to star luminosities improves by almost 3 orders of magnitude when one moves from the visible to the mid-IR (Bracewell, 1978).

IR interferometric missions in Space have been proposed to search for telluric exoplanets around nearby stars and perform a spectroscopic analysis of their atmosphere in the mid-IR, searching for the signatures of CO2 (15 µm), H2O (6-8 µm), O3 (9.6 µm) and CH4 (7.7 µm) (DARWIN, Léger et al., 1993a, 1996; OASES, Angel & Woolf, 1997; TPF, Beichman, 1996).

If the preceding statements are correct, such missions should be able to answer questions (i) to (iii). However, Noll et al. (1997) have recently claimed that the detection of O3 is not an unambiguous identification of Earth-like biology because abiotic processes may cause O3 to be present.

Considering the major scientific significance of this topic, and the high cost of the missions required to perform the search, careful study of the validity of this objection is needed. This is the goal of the present paper.

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© European Southern Observatory (ESO) 1999

Online publication: November 26, 1998