We stand in need of a more sophisticated conceptual framework, and with it a more sophisticated vocabulary, with which to better understand and explore the possibilities of panspermia. The development of the idea of panspermia has been impeded by its association with marginal and fringe science. This is unfortunate. As we will see below, the problems of panspermia are central to astrobiology.
The question of panspermia is really the question of the distribution of life in an astrobiological context. In so far as astrobiology is, “...the study of the origin, evolution, distribution, and future of life in the universe,” panspermia represents the third of the four modalities of the study of life named in this definition, viz. the distribution and future of life. Thus the study of panspermia, or whatever panspermia becomes when it becomes assimilated to astrobiology, ought to have equal billing with origins of life research and evolutionary biology.
Alternatively, we can think of panspermia as the cosmological equivalent of biogeography -- biocosmography, if you will. This would cover all the possibilities of the distribution of life in the universe, as biogeography studies the distribution of life on Earth. Indeed, biogeography would, in this context, become a special case of biocosmography, which latter would be the more comprehensive discipline that provides an umbrella under which biogeography is to be found.
However we conceptualize panspermia in the big picture of intellectual inquiry, we need to recognize and to unambiguously state the gradations of panspermia that are possible. What follows is my own imperfect and off-the-cuff typology of the gradations at which astrobiologically significant distinctions might be made:
Life begins on a planetary surface and remains confined to a single region on that planet’s surface, never expanding to a planetary scale. On a tidally-locked planet closely orbiting a red dwarf star, for example, a living planet might not have a biosphere, but rather a bioring at the interface between light and darkness. In this case, life would never become planetary in scale.
Life expands to a planetary scale, whether after beginning at one location on a planetary surface and expanding from that point of origin, or beginning at several locations and growing together. We don’t think of life attaining a planetary scale as panspermia, but it is, we could say, the minimum case of panspermia, in which life establishes a biosphere around a single planet where that life originates.
Life expands to include the immediate neighborhood of a planet with a biosphere, which would include any naturally occurring moons in orbit around the planet.
Life that originates on a planet within the frost line expands to other planets and planetary bodies within the frost line, which in planetary systems like ours means that the small, rocky planets within the frost line share a common biological community. Alternatively, if life originates in or around gas giants beyond the frost line (such as in subsurface oceans of gas giant moons), this life also belongs to a common biological community.
We know that planets in the inner solar system do in fact exchange material with each other, so that lithopanspermia is a possibility. There is probably also some exchange of matter between the inner and outer planets, though also probably less in this case.
All or most planets and moons within a given planetary system share the same biological community. In this case, life is pervasive within a planetary system and takes root wherever it finds reasonably clement conditions.
Multiple planetary systems associated with multiple stars share the same biological community. Until quite recently this would have been thought to be quite a stretch, but the fly through of our solar system by Oumuamua has made us aware that even interstellar lithopanspermia is a possibility. A paper has been written about this possibility, Implications of Captured Interstellar Objects for Panspermia and Extraterrestrial Life by Manasvi Lingam and Abraham Loeb. Also, it seems that Oumuamua was not unique; the asteroid 2015 BZ509 may be of extra-solar origin -- cf. “An interstellar origin for Jupiter’s retrograde co-orbital asteroid” by F. Namouni and M. H. M. Morais.
An entire galaxy shares the same biological community. If interstellar lithopanspermia is possible, given sufficient time (and I acknowledge that sufficient time may not yet have elapsed for this eventuality in our relatively young universe, but it is at least possible), an entire galaxy might share life from the same origin of life event by way of lithopanspermia borne by Oumuamua-like objects.
A local galaxy group (i.e., a gravitationally bound group of galaxies) share the same biological community. Recent discoveries regarding the number of hypervelocity stars (cf. Revisiting hypervelocity stars after Gaia DR2 by Douglas Boubert, James Guillochon, Keith Hawkins, Idan Ginsburg, N. Wyn Evans and Predicting the hypervelocity star population in Gaia by T. Marchetti, O. Contigiani, E. M. Rossi, J. G. Albert, A. G. A. Brown, A. Sesana and “One of the Milky Way’s fastest stars is an invader from another galaxy” by Joshua Sokol) points to the possibility of hypervelocity stars with associated planetary and subplanetary bodies, which could transfer life from this planetary system to other planetary systems which they pass.
A local galaxy cluster shares the same biological community.
Multiple galaxy clusters share the same biological community.
The entire universe shares the same biological community. While it does not seem likely that this case holds in our universe, since other galaxies are already passing beyond our cosmological horizon, so that there wasn’t much of a chance for life on Earth to have spread to them even through the mechanism of hypervelocity stars. However, if life began first in galaxies already beyond the cosmological horizon, and made its way, galaxy by galaxy, and came to Earth from elsewhere, rather than originating here, it may be possible that this is a case.
Roughly, this scheme for the gradation of panspermia is based on the ability of life to overcome progressively greater gravitational thresholds. Some time ago I suggested that spacefaring civilizations could be graded according to their ability to overcome gravitational thresholds, and similar considerations hold for life.
It really doesn’t matter what we call these gradations of panspermia, or if we find it is better to make the distinctions at different locations, but it is worthwhile that distinctions like this be made.
