On possible life-dispersal patterns beyond the Earth

Abstract

AbstractThe assumption that exoplanets are ‘in equilibrium’ with their surroundings has not given way to life's transmissivity on large spatial scales. The spread of human diseases and the life recovery rate after mass extinctions on our planet, on the other hand, may exhibit spatial and temporal scaling as well as distribution correlations that influence the mappable range of their characteristics. We model hypothetical bio-dispersal within a single Galactic region using the stochastic infection dynamics process, which is inspired by these local properties of life dispersal on Earth. We split the population of stellar systems into different categories regarding habitability and evolved them through time using probabilistic cellular automata rules analogous to the model. As a dynamic effect, we include the existence of natural dispersal vectors (e.g. dust, asteroids) in a way that avoids assumptions about their agency (i.e. questions of existence). Moreover, by assuming that dispersal vectors have a finite velocity and range, the model includes the parameter of ‘optical depth of life spreading’. The effect of the oscillatory infection rate ($b( t,\; \, d)$) on the long-term behaviour of the dispersal flux, which adds a diffusive component to its progression, is also taken into account. The life recovery rate ($g( t,\; \, d)$) was only included in the model as a link to macrofaunal diversity data, which shows that all mass extinctions have a 10 Myr ‘speed rate’ in diversity recovery. This parameter accounts for the repopulation of empty viable niches as well as the formation of new ones, without ruling out the possibility of genuine life reemergence on other habitable worlds in the Galaxy that colossal extinctions have sterilized. All life-transmission events within the Galactic patch have thus been mapped into phase space characterized by parameters $b$ and $g$. We found that phase space is separated into subregions of long-lasting transmission, rapidly terminated transmission, and a transition region between the two. We observed that depending on the amplitude of the oscillatory life-spreading rate, life-transmission in the Galactic patch might take on different geometrical shapes (i.e. ‘waves’). Even if some host systems are uninhabited, life transmission has a certain threshold, allowing a patch to be saturated with viable material over a long period. Although stochastic fluctuations in the local density of habitable systems allow for clusters that can continuously infect one another, the spatial pattern disappears when life transmission is below the observed threshold, so that transmission process is not permanent in time. Both findings suggest that a habitable planet in a densely populated region may remain uninfected.