Following the overall organization of matter in the universe, we discuss the panspermia prospects from stellar to cosmological levels.
4.2 Three Levels of Habitability and Panspermia
Without much of an approximation, we could consider that life as we experience it is powered by the Sun. The chemical elements that constitute both, living and other matter, are forged in stars. Also, the planets and their stellar hosts are condensed from gaseous matter that accumulated in galaxies. The conglomerates of galaxies are the building blocks of our universe. Within these clusters, the galaxies mutually interact and merge, which can heavily change their interiors. Galactic habitability would thus incorporate habitability phenomena between the level of individual stars/planetary systems and the cosmological level. The basic habitability features of these levels and their possible relation to the panspermia hypothesis are outlined, which, in essence, is a discussion of a potential for matter exchange. Figure 4.1 presents a sketch of these basic levels and their relations which are described in the remainder of this section.
4.2.1 Stellar System Level
To the limits of our ignorance, it is the level where life manifests itself. The modern habitability models (including galactic ones) have their roots in the concept of the circumstellar habitable zone [4.15]. Our present knowledge on exoplanets implies that there should be a multitude of Earth-like worlds within the habitable zones of their host stars. Their habitability would depend on the stability of orbital configurations, the evolution of the stellar radiation output, their atmospheres, and the possibility of them being impacted by the small bodies that orbit within the stellar system.
Figure 4.1 Sketch of the levels of influences of matter and their inter-relations in regard to panspermia.
Planetary systems can emit panspermia material on their own, during their intrinsic evolution that consists of planetary migrations and behavior of asteroid belts. However, this is likely to take place during the early stages of the stellar system’s evolution. Upon depleting the initial reservoirs of small bodies, e.g., such as the possible scenario in TRAPPIST-1 [4.11] and reaching stable planetary configurations, the emissions of this kind are likely to become significantly smaller. However, even such small emissions could be significant over longer time periods, increasing the “background” galactic panspermia potential. This implies an underlying assumption that dormant forms of life, within their carriers, are not affected by the harsh space conditions during these time spans. The nature of such a panspermia process is inherently diffusive and it would likely boost the appearance of life in general. In an epistemological sense, it is similar to an in situ appearance of life, given favorite galactic conditions. On the other hand, sporadic events such as close stellar flybys can perturb planetary systems and increase the emissions of material. As such, they offer a handle to relate the panspermia process to galactic parameters, especially dynamics.
4.2.2 Galaxies: Cosmic Cradles of Life
Similar to [4.8], the following formalism is usually used to estimate the stellar collision cross-section [4.23]:
(4.1)
where the right-hand side variables are stellar density, velocity, and collision cross-section, respectively. Following the simulations from [4.20], the author uses <σ> = 100 AU2, as the average value for the collisions that have a disruptive potential for terrestrial planets. We can consider it likely that an order of magnitude larger values, for a collision distance (parameter), of b = 100 AU ≈ 5 × 10−4pc, would be sufficient to disturb the small bodies of the target stellar system, consequently scattering some of them into interstellar space. This relates to the small bodies that are relatively close to the inner terrestrial planets, such as the asteroid belt in the Solar system. However, such objects can be found more loosely bound and in much larger numbers in the outer Solar system. Given that the Oort Cloud could stretch as far as 105 AU [4.27], a nearby stellar passage at even 0.5 pc, could send swarms of icy and rocky objects into interstellar travel. However, these more distant objects are likely to be the pristine remnants from the stellar system formation, and less likely to have contact with the inner parts of a stellar system, where complex biochemistry could take place and aggregate the organics, originating from molecular clouds, into actual living forms.
The existence of such an extensive structure of small objects around the Solar system would imply that our Earth did not suffer much from flyby perturbations throughout its history. The dynamically unexcited Kuiper belt implies that the Solar system did not have encounters with other stars within 240 AU [4.6] in its birth cluster as well as in its current galactic environment. An estimate of the collision parameter value that is most effective in terms of ejecting the panspermia material is likely to be a matter of fine tuning. However, a caution should be made here, since a radically different (unknown) form of life (e.g., Dyson’s “cometary trees” or Lem’s cloud of Y-shaped microscopic components) might be preferentially propagated by different kind of ejection events, with different ejection parameters, creating a kind of natural selection between these different “panspermias”.
Encounters within 100 AU can capture or eject a Neptune from its orbit [4.22], which would be captured by the star flying-by in 1 in 12 encounters. The planets around binary or multiple stars appear to be more vulnerable to stellar flybys [4.19]. While planets hosted by a single star have a significant probability of being disturbed only in longed-live stellar clusters, the ones having a binary host are much less stable. No wonder then we have found ourselves orbiting a single star. Although binary systems are more numerous than single star planetary systems, they offer less stable habitability conditions since the orbits of their planets are more frequently perturbed, when compared to single stars for a fixed value of the collision parameter.
The material ejected from binary systems should, then, on average, have less evolved living matter. The more potent panspermia material should be expected to originate from single star planetary systems that had flybys as close as ~102 AU. Flybys at ~103 AU are likely to scatter around only the debris of small bodies that did not change their biological complexity much since the formation of the given stellar system. This might still be important for the overall astrobiological landscape, depending on the timescales involved. Even if it consists just of prebiotic complex chemicals, it might delineate the part of the GHZ containing a set of biospheres of a particular kind.
Capture time scales and collision cross-sections for single and multiple stellar systems are