5 5 [3.3], p. 118.
6 6 There are exceptions to this view sometimes considered in the domain of science fiction literature. For instance, in his brilliant novel Blindsight, Peter Watts describes how the human kind of intelligence might be an exception, rather than the rule, among all star-faring species [3.67]. We cannot discuss this fascinating and disturbing scenario here in more detail, but see [3.15].
7 7 Note that this conclusion applies to epistemology and philosophy of science; extrascientific motivation clearly present in the rejection of the continuity thesis by creationists does not and cannot invalidate this heuristic usage, since their supernaturalist metaphysics lies outside science [3.28, 3.31].
8 8 Not everyone agrees, as expected. Although even most non-biocentric moral philosophers would agree that an inhabited planet possesses some additional value, even if purely instrumental, in comparison to a dead one, a small but surprisingly vocal group of philosophical pessimists and anti-natalists has argued for a diametrically opposite view. In a cartoonishly simplified form, life (both human and non-human) is mainly suffering and so is of negative net value, and human duty is to actually prevent its proliferation and procreation. Drawing on some traditional sources, like Buddhism and Schopenhauer’s philosophy, some of the modern representatives of this view have been Peter Wessel Zappfe, Emil Cioran, David Benatar, Ray Brassier, and Eugene Thacker (a popular overview is given by Thomas Ligotti in [3.41]).
9 9 Notably, the temperature at which living/intelligent systems operate is perhaps the key factor in both subjective time flow and in overall evolution, as per Dyson’s scaling hypothesis [3.23]. See also [3.56].
10 10 A popular version of this cop-out is to state that such thinking invariably belongs to the domain of science fiction. What a conservative mind fails to perceive is that there is no sharp boundary between the two discourses; what has been a science fiction concept of chain reaction for H. G. Wells or smart dust for Stanislaw Lem has long ago smoothly transitioned into the scientific/technological discourse. See [3.15] for an extended discussion of this fascinating topic.
11 11 Admittedly, some additional assumptions are required here. Notably, the cosmologically allowed future will last longer than the history of the universe so far—which is firmly confirmed by physical eschatology [3.2]. Also, the conclusion hinges on the, possibly also geocentric, assumption that a typical extraterrestrial biosphere is a system of many individual life forms as agents—and not a single biospheric-size superorganism as, for example, Lem’s Solaris (see Lem [3.39]).
12 12 Compare this with the artistic vision of Clarke and Kubrick in 2001: A Space Odyssey (cf. [3.27]).
4
Life in the Milky Way: The Panspermia Prospects
Branislav Vukotić1* and Richard Gordon2,3†
1Astronomical Observatory, Belgrade, Serbia
2Gulf Specimen Marine Laboratories, Panacea, FL, United States
3C.S. Mott Center for Human Growth & Development, Wayne State University, Detroit, MI, United States
Abstract
We discuss the panspermia hypothesis within the context of recent findings on Milky Way habitability. Galactic habitability is the key to understanding the phenomenon of life within the cosmological framework. It is the middle of the three levels of habitability: planetary system, galactic, and extragalactic. Incorporating the panspermia hypothesis might significantly improve the existing models by expanding them to include phenomena connecting stellar and cosmological levels.
Keywords: Galactic habitability, panspermia, Milky Way, galactic dynamics, astrobiology
4.1 Introduction
Over the last two decades, studies of galactic habitability have matured significantly. From rudimentary pioneering works, primarily based on metallicity gradients, to sophisticated numerical models that consider a multitude of parameters, from cosmological to stellar scales. However, there is still much room for improvement, given the significantly different conclusions from various models, as well as overall uncertainty about the underlying parameter distributions and averaging procedures.
One of the main factors affecting continuous habitability is the movement of stars within the host galaxy. Apart from secular changes in galactic environment, the stars move and experience different parts of the galaxy with different habitable parameters, during the lifetime of the star. While the general habitability parameters at a certain galactocentric radius might be considered as constant, the stars with substantial eccentricity of their galactic orbits, vertical oscillations, or even galactic rotational speeds that are different from the speed of the spiral pattern might endure significant changes of their galactic environment. In addition, recent works have pointed to a significant amount of stellar radial migrations. This can produce stellar trajectories passing through various parts of the galactic disk, characterized by large variations in habitability.
Potential interstellar panspermia material would also be moving in the same galactic potential as the stars. In addition, the smallest bodies could also move under other, non-gravitational forces such as light pressure (suggested in early work of Swante Arrhenius [4.4]), magnetic fields, and possibly even something analogous to the Yarkovsky effect. Various kinds of small bodies and rogue planets are likely wandering around through galactic interstellar space [4.29]. With a potential to carry panspermia material, such bodies could affect life in stellar systems by getting picked up by the stars that are found in the vicinity. At the start of the panspermia process, the panspermia material is thought to be blasted off in rocks from the host body. Compared with the galactic scales, processes on life hosting sites (planets and smaller bodies) happen on small scales. Introduction of panspermia to galactic habitability models can therefore couple the life hosts to the galactic environment and large-scale galactic processes. So far, most models have considered that life hosting sites are only oneway dependent on galactic processes. Once life arises, its survival depends on the frequency of nearby stellar explosions, stellar passages, activity of the galactic nucleus, etc. However, panspermia could modify such models with positive habitability feedback. When life arises, besides enduring the galactic environment, it could also contribute to the spread of life in that environment. This is similar to the well-known Huygens–Fresnel principle in classical wave optics, where every point on a wavefront is treated as a source of interfering wavelets. Similar parallels to other type of processes have been demonstrated [4.24].