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].
The discovery of live 250 Myr old halophilic bacteria in a grain of salt [4.31] implies the possibility that panspermia could be viable even on galactic time scales. The Galactic Solar orbit lasts a little under a quarter of a billion years. On even shorter timescales, there is a possibility that neighboring stellar systems could exchange material. The recent discoveries of the Oumuamua [4.26] asteroid-like object and comet 2I/Borisov [4.17, 4.21], that show hyperbolic orbital characteristics of extrasolar bodies, have demonstrated that external objects can traverse the Solar system, which makes it possible that some of them could eventually be captured or sampled. The galaxies are the main form of organization of matter in the universe. Galactic evolution and underlying processes link the evolution of matter on stellar/planetary scales and smaller, to the overall evolution of the universe. In a similar way, stars with their planetary systems can be considered as pivotal objects for the evolution of life. While their evolution and appearance are influenced by galactic scale processes, they provide suitable conditions for biochemistry to take place. Some of these processes might arguably take place in the interstellar molecular and dust clouds, but these are also the birthplaces of stars and planets.
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
