Planet Formation and Panspermia. Группа авторов

emerging in physical conditions similar to those on Earth.⁹

Capabilities of advanced technological civilizations are, of course, unknown at present. However, it is exactly the reason why we are justified in using a general heuristics like the extended continuity thesis in thinking about them. Together with other general principles guiding our thinking (e.g., scientific realism, naturalism, and Copernicanism), we are free to modify or abandon them as the empirical data comes in—whenever that occurs. To insist that because we do not possess such data at present, in an early epoch of both our own and cosmic evolution, we should censor our thinking, is to succumb to chronocentrism of the worst sort.¹⁰

      The other key input is the rate of habitat formation in the course of the Galactic history. The pioneer study of Charles Lineweaver [3.42]—and the subsequent improvements by [3.9, 3.43, 3.64]—indicates that the median age of Earth-like planets in the Galaxy is

      which is significantly greater than the Earth/Solar System age. Since roughly that epoch, the rate of formation of terrestrial planets in the Milky Way has steadily decreased. While this can be used to strengthen Fermi’s Paradox (as argued at length in [3.14]), there are other interesting speculative applications of the Lineweaver timescale.

      Notably, this timescale justifies our previous conclusion that we are living in an early epoch of evolution of our own brand of intelligent observers. The fact that we have discovered these timescales now and have our, even very crude and simplistic, models of the relevant processes implies that older and more advanced civilizations have much better and precise insight into the same. Thus, a Kardashev’s Type 2.x civilization is likely to be able not only to survey all sites for abiogenesis in the Galaxy but is likely to be able to predict the emergence of future habitable sites. Moreover, an implication of the research of Lineweaver and others is that, as far as terrestrial planet formation is concerned, the Milky Way is past its prime: the rate of such habitat formation already decreased and will continue to decrease in the future. A clear implication is that the rate of local occurrences of abiogenesis is decreasing and will continue to decrease.

This happens due to the processes of chemical evolution and star formation which we have studied. However, it also happens due to another, intuitively obvious, but so far not quantitatively studied process: the emergence of life and intelligence itself at individual sites in the Galaxy implies that the available number of sites for future emergencies of this kind is decreasing. In other words, if life emerges somewhere and persists, there are less “slots” left for such future emergencies in the Galaxy as a whole. This effect may or may not be very small so far—its magnitude depends on the intrinsic probability of local abiogenesis (averaged appropriately). While the continuity thesis tells us that this intrinsic probability should not be 10⁻¹⁰⁰, we cannot say whether it is 10⁻⁶, 10⁻³, 0.1, or 0.9, which the magnitude of the “filling the slots” effect, as manifested so far, hinges upon. However, what we may be rather certain is that in the future the effect will gain in importance. Especially as the number of habitable “slots” intrinsically falls off.

      Of course, we need to keep in mind the point emphasized by Freeman Dyson (e.g., [3.24, 3.25]) that not all potential habitats are Earth-like planets; however, formation rates of other potential habitats generally follow the same pattern, which follows from the general chemodynamical evolution of the Milky Way. This was valid for the Galactic past, on which our evidence is based. Therefore, we are justified in using Earth-like planets as synonymous with habitats for origination and evolution of life and intelligence so far. Why is the temporal qualification important here? This is the key point here: because directed panspermia, in contrast to other processes leading to the expansion of life, is not bound by a narrow class of Earth-like planetary habitats.

It is obvious why this is so. The design space of advanced technological evolution (or what has been dubbed postbiological evolution) is many orders of magnitude larger than the design space of natural, biological evolution; for instance, even a primitive technological civilization such as this on Earth has already created more diverse ways of locomotion or perception than the animal evolution has managed to do since the Cambrian Explosion. Again, barring an existential disaster, all researchers in the futures studies agree that the transformative technologies of the near future, such as nanotechnology and AI will immensely expand the reach of (post)human design [3.11]. There is no reason to doubt, in accordance with the principles of exploratory engineering ([3.5, 3.22, 3.63], esp. Figure 3.1 in [3.5]), that what applies to the future of humanity will apply to other Galactic civilizations as well, notably the older and more advanced societies [3.19, 3.20]. After all, we share the same laws of physics and therefore the realm of the possible, although huge, is limited by the same constraints of physics (and, presumably, economics in a generalized sense; cf. [3.60]). Therefore, advanced societies are likely to be capable of creating life forms capable of surviving in other habitats, similar to the “Dyson tree”: a hypothetical plant (perhaps resembling a tree) capable of growing on the surface or inside a cometary nucleus (e.g., [3.25, 3.62]). Gradually, such organisms could be able to create the critical amount of complex organics to establish the basis for subsequent human colonization of objects such as those in the Kuiper Belt, which contain huge amounts of volatiles such as water, methane, or ammonia. Even more resistant organisms could be engineered to survive and thrive in the atmospheres of gaseous giants, along the classical suggestions of Sagan and Salpeter [3.55]. Of course, that visionary study suggested searching for naturally evolved ecologies in atmospheres of Jupiter and other gas giants. If such ecologies are physically possible, there is no reason to doubt that, even if they did not evolve naturally due to contingency and opportunism of evolution, they could still be engineered by sufficiently advanced civilizations.

Such nonstandard habitats—like cometary nuclei and gas giants’ atmospheres—are immensely larger than surfaces of Earth-like planets by any meaningful metric (e.g., number density per unit Galactic volume or the amount of available resources). Therefore, if some life forms could enter those niches, they are likely to dominate the overall cosmic tally of life forms—in the fullness of time, even if not up to now.¹¹ In other words, a randomly chosen living being in our universe is mostly likely to have directed panspermia in its phylogenetic past. Taking into account both the decreasing rate of the terrestrial planet formation with cosmic time, the perspective of those nonstandard habitats for engineered, if not naturally emerging biospheres, and the ideas of visionaries and optimists like Constantine Tsiolkovsky or Gerard O’Neill about entirely artificial habitats [3.4, 3.58]—all those point in the direction of a radically different astrobiological landscape than is conventionally assumed. As in many other stages of history of science and philosophy, it is the lack of imagination which is by far the bigger problem than its excess.

3.4 Conclusion and Prospects

We have discussed a speculative account of the extended continuity thesis and its consequences in the fullness of space and time. Overcoming chronocentrism is difficult but offers several promising directions for further research and for methodological grounding of astrobiology. (This comes in addition to other, and arguably more important roles, the rejection of chronocentrism plays in the domain of environmental ethics, space ethics, or political philosophy.) In affirming the extended continuity thesis as a usable heuristic, we have to avoid pitfalls to which chronocentrism invariably leads in our dynamic, evolutionary universe which is always in change and flux.

The extremely speculative discussion above seems to suggest that directed panspermia becomes more and more important with the passage of cosmic time. Eventually—rather soon on cosmological and evolutionary scales—it will become the main channel of transitioning from non-life to life. This will be true entirely independently of whether directed panspermia ever occurred