Before addressing the problem of how to find these intelligent societies, we have to discuss the distances in the galaxy. As shown in Fig. 10.3, our Milky Way has roughly the shape of a disk, with a radius of 50 000 Ly and a thick ness of 1300 Ly, in which the Sun (dot) orbits at a distance of 26 000 Ly from the center (McNamara et al. 2000). If 4 million life-carrying Earth-like planets (dots) were distributed at random over this vast volume of space, then the average distance l between two planets would be about 170 Ly, while the mean distance between the 2 million planets, where in the course of galactic history intelligent life has presumably formed, would be about 210 Ly. As we have assumed that most of these civilizations have already perished, the average distance to the surviving 4000 intelligent societies would be about 1700 Ly.
Mean spatial distance 100 000 Ly
Fig. 10.3. The mean distance between life-carrying planets
These estimates have important consequences for our search efforts. In Chap. 8 we have discussed how, in the not too distant future, we will be able to directly observe terrestrial planets and detect traces of life by observing, for example, H2O - and particularly O3, in their atmosphere (Fig. 8.13). Detecting such life-carrying planets at the relatively small distance of about 170 Ly would constitute a huge advance in our understanding of how and under what conditions life forms.
Moreover, as the demise of an intelligent civilization is not expected to be fatal to all forms of life on a planet, it might well be possible that some eukaryotic cells or primitive multicellular life forms, perhaps surviving deep in the rocks of the planetary crust, would come forward and rebuild a situation similar to Earth 1 billion years ago, complete with an oxygen atmosphere. By the incessant working of evolutionary forces, intelligent life might then in due course appear for a second time, another billion years later. Because of this, the number of 4000 existing civilizations could be an underestimate -possibly even a big underestimate - and the nearest intelligent civilization may exist much closer than 1700 Ly. From a discrepancy between the age of the parent star and that of the planetary atmosphere, we might even be able to judge, from great distances, whether such life-carrying planets have undergone violent disturbances in the past. In any case, as about 2 million planets are supposed to have suffered a catastrophic demise of their intelligent society, an interstellar trip to such a planet at a distance of about 210 Ly would certainly be very interesting, as one could study then in detail the archaeology of this lost advanced civilization.
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