Over long-time scales, galaxies should have increasing amounts of “living” material spread across them, if it is sufficiently resistant to everpresent radiation hazards. While the flyby induced ejections from “mature” planetary systems can offer better panspermia prospects, it is a nonnegligible probability of planets that are ejected by giant planets from the same stellar system at early stages of planetary system evolution. Six such planets are estimated to be recaptured at the Solar galactocentric distance within ~ 200 Myr [4.12]. This result is obtained based on the assumed present epoch stellar density at the present Solar galactocentric distance.
4.2.3 Cosmological Level: Interactions of Galaxies
As an extension of galactic habitability, the supergalactic habitable zone was described in [4.25]. They argued that near the center of large clusters and superclusters of galaxies, the intensive interactions of galaxies create inhospitable conditions for life and that life should rather thrive at the outskirts of these structures. Large massive galaxies are often found near the centers of galactic clusters and can have many minor mergers with smaller galaxies during their lifetime. The authors also argue that small galaxies might offer the best habitable conditions. Other papers have argued that dwarf galaxies and satellites of large galaxies might offer the best conditions for life [4.13, 4.28]. Massive clusters of galaxies are also likely to contain galaxies with an extremely active galactic nuclei, notably quasars, that could have a significant habitability impact. A consideration of habitability near the supermassive black holes is considered in [4.32], while habitability near the centers of spiral galaxies is considered in [4.7].
The plethora of galactic interactions within superclusters of galaxies, that account for the visible part of the fabric of our universe, including most obvious ones such as mergers and flybys, can significantly influence the habitability conditions in individual galaxies. In example, the energetic explosions of dying stars or mergers of stellar remnants could eject gas and dust well outside the plane of the galactic disk where it can get picked-up by a satellite galaxy such as the one of the Magellanic Clouds. Apart from incorporating and exchanging gaseous and stellar components of one another, interacting galaxies can also mix their populations of interstellar small bodies and rogue planets. In addition, the galactic mergers are usually not catastrophic events as far as stellar systems are concerned, but rather the individual stars can find themselves orbiting within new galactic hosts without significantly disturbing their planetary systems. If humanity survives long enough, then a likely scenario is that we will find ourselves inside the Andromeda Galaxy in the distant future. Unlike star orbiting planets, the stars can readily change their galactic host after completing only a handful of galactic orbits that last ~108 yr. Given the ~1010 yr age of the universe, the galaxies appear as a relatively more dynamical environment in this respect than individual stars, with far more pronounced matter exchange.
A first discovery in this respect is a meteor, with velocity parameters consistent with an extragalactic origin [4.3]. In addition, more voluminous prospects should stem from the Milky Way and its rich history of interactions with other galaxies. Perhaps, even our Sun originated in other galaxies interacting with ours during the last 10 Gyr. Future habitability studies that will integrate Milky Way stellar orbits using data from GAIA space mission are likely to offer a probability for such a radical conjecture.
4.3 Conclusions
Incorporating the panspermia hypothesis might significantly improve the existing models by expanding them to include phenomena from the stellar level to the cosmological. As we have estimated, life need only occurs in 1 out of 2,000 galaxies to meet any assumption that life occurs throughout the universe [4.16]. The foundation of panspermia, a form of matter exchange, has a potential to improve habitability models by making them connect the habitability relevant phenomena from the stellar level to the cosmological scales of galactic clusters.
From all habitability aspects that appear to be relevant for the panspermia hypothesis, the most interesting appears to be the consequence of stellar flybys. Although the flybys can disrupt the habitability of the planets in the system, they can, in turn, fill the galaxy with scattered rogue planets and small bodies, possibly carrying living organisms.
At present, there is much evidence of organic matter almost everywhere in the universe [4.30]. The thousands of exoplanets that we have discovered so far should thus not suffer from the lack of building blocks for life. Small bodies of those systems are likely to supply such material in pristine form to respective planetary hosts. The possible evidence for abiogenesis is discussed in [4.18], such as the prevailing homochirality of dust and other interstellar matter, from which the planets are formed. Whether life first appears in the molecular gas clouds and dust or within the fully formed planetary systems is yet to be ascertained. Regardless, the latter scenario also has a panspermia potential to spread life, especially within the crowded regions of the galaxy where flybys of stellar systems are common. This might offer a goldilocks solution. While in crowded areas of a galaxy, there is much possibility for launching life carriers out into galactic space, there is less chance for life to develop in often disturbed planetary systems. Panspermia seems likely between stars in the same stellar cluster while “open” interstellar space seems to offer less prospects. On the other hand, the less crowded environments are more habitable in terms of less potential for disruption of planetary orbits and have less background radiation than stellar clusters.
If the Solar system migrated from the inner parts of the galactic disk, then it might have gathered a collection of organic/life material in addition to the budget available at its time of birth. While floating to areas less populated with stars, this material could encounter better conditions to form a biosphere on one of the Solar system planets.
It is evident that matter undergoes significant mixing, within stellar systems, galaxies, and also between galaxies. If this proves irrelevant for the appearance of life, then it should be significant and influence the evolution of life over cosmic timescales. Most of the matter in the universe mixes with its environment and could not be considered isolated for long periods of time. As suggested by one of the most prominent panspermia proponents [4.33], life should be viewed rather as a cosmic phenomenon and not a phenomenon related to Earth. Organic and possibly living material should be available across galactic space. The panspermia hypothesis appears to have no contradictions with our current knowledge and it offers a bunch of clear predictions for future empirical testing, in accordance with the best traditions of modern epistemology and the scientific method. The question that arises then is not whether life can spread/appear at some point in the galaxy but when. Future models of habitability should take this into consideration in order to be more versatile and realistic.
Acknowledgements
BV acknowledges the financial support by the Ministry of Education, Science and Technological Development of the Republic of Serbia through the contract number 451-03-68/2020/14/200002. The authors also thank Milan M. Ćirković for insightful comments on this manuscript.
References
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