Galactic Panspermia

Abstract

We present an analytic model to estimate the total number of rocky or icy objects that could be captured by planetary systems within the Milky Way galaxy and result in panspermia should they harbor life. We estimate the capture rate of objects ejected from planetary systems over the entire phase space as well as time. Our final expression for the capture rate depends upon the velocity dispersion as well as the characteristic biological survival time and the size of the captured object. We further take into account the number of stars that an interstellar object traverses, as well as the scale height and length of the Milky Way's disk. The likelihood of Galactic panspermia is strongly dependent upon the survival lifetime of the putative organisms as well as the velocity of the transporter. Velocities between 10 − 100 km s −1 result in the highest probabilities. However, given large enough survival lifetimes, even hypervelocity objects traveling at over 1000 km s −1 have a significant chance of capture, thereby increasing the likelihood of panspermia. Thus, we show that panspermia is not exclusively relegated to solar-system sized scales, and the entire Milky Way could potentially be exchanging biotic components across vast distances.

Persistence of Technosignatures: A Comment on Lingam and Loeb

“…Hence, it is conceivable that the relative likelihood δ intermittently scales even like L 4 . Since this is analogous to panspermia -as a kind of "natural settlement"a similar conclusion has been reached by Ginsburg, Lingam, and Loeb (2018, see in particular their eq. 11).…”

On the Habitable Lifetime of Terrestrial Worlds with High Radionuclide Abundances

“…Thus, over a period of ∼ 100 Myr, they can traverse distances of 1 kpc. During the course of this journey, they may be captured by three-body gravitational interactions into binary systems (Ginsburg et al 2018). Once they have been captured, this opens the possibility of seeding other worlds in that system via interplanetary panspermia.…”

Possible Transfer of Life by Earth-Grazing Objects to Exoplanetary Systems

“…Polyextremophiles such as D. radiodurans are estimated to die on an exponential timescale of ∼ 10 5 yr with minimal radiation shielding (Ginsburg et al 2018;Phillips et al 2013;Burchell et al 2004;Mileikowsky et al 2000). While the abundance of polyextremophiles in the atmosphere is yet unconstrained, in our analysis we assume 1 polyextremophile colony per transporting object.…”

“…We adopt the following three dimensional velocity dispersions for local stars, each corresponding to the standard deviation of a Gaussian distribution about the LSR: σ U = 35 km s −1 , σ V = 25 km s −1 , σ W = 25 km s −1 (Bland-Hawthorn & Gerhard 2016). Tight stellar binary systems have the largest capture cross sections, so we exclusively consider these systems in this analysis (Ginsburg et al 2018;Valtonen 1983). We apply the scalings in Valtonen & Innanen (1982); Valtonen (1983) to estimate the capture cross section of a stellar binary system of two solar-mass stars (such as Alpha Centauri A & B) as a function of relative speed of the system and the object, assuming that the orbital speed of the bound orbit is near zero,…”