One thing that I've wanted to quantify since reading Intelligent Life in the Universe, an outstanding book by Carl Sagan and I.S. Shklovskii, is the idea of exogenesis. Exogenesis is the hypothesis that life formed elsewhere in the universe and was somehow transferred to earth in the form of some small seed or spore. Now since E.T. E. coli presumably do not have little tiny jetpacks or other means of active transport, they would need to traverse the cosmos in some passive way. One such way would be solar sailing. Way back in Solar Sails I, we derived equations describing the maximum speeds and time-of-travel for various distances for a given solar sail. Each of these equations was a function of the surface mass density of the sail, which is just the amount of mass per unit cross-sectional area. All we need to know is the cross-sectional area and mass of a given object and we can apply these equations to just about anything! Assume we have some spherical blob with the density of water (1g/cm^3). The effective sigma of this blob would just be the mass divided by the cross-sectional area. In other words, $$ \sigma = \frac{m}{Area} = \frac{\frac{4}{3}\pi r^3 \rho}{\pi r^2} = \frac{4}{3}\rho r .$$ Rearranging to get r in terms of the other variables, we have $$ r = \frac{3\sigma}{4\rho} . $$ Plugging in our density of 1g/cm^3 and a suitable sigma (10^-4 g/cm^2), we get $$ r \le 0.75 \times 10^{-4} cm = 0.75 \mu m .$$ Check out this fun site to see what kind of critters can fit in this blob. From the previous post, we saw that for a sigma of 10^-4 g/cm^2, our sail would get to the nearest stars on a timescale of order 10,000 years. Thus if our blob has a radius of less than about a micron, it could spread to hundreds of stars in around 10,000-100,000 years. Even if it would take millions of years, that would be almost no time at all on the cosmic scale. Just based on this calculation it all seems fairly feasible. In making these calculations I have neglected several important aspects of the problem. First, in no way have I actually calculated any sort of probability of this happening. Additionally, I would have to see how likely it is for some blob to reach planetary escape velocity (presumably just by riding that tail of the Boltzmann distribution). Finally, and perhaps most important of all, I have not given any sort of motivation or mechanism by which some living body could survive hundreds of thousands of years in the vacuum of space with constant radiation exposure. But I have heard that some forms of life are totally extreme (especially if they drink this). Even though such a process seems possible, it certainly doesn't seem like the easiest way to get life on earth. I prefer the much more satisfying "amino acids + lightning + magic = life" model. But it does offer some interesting possibilities. Suppose we as people think that people are super awesome and therefore people should be everywhere. We do some bio magic and put whatever DNA we want into viruses, which we then pack into as many micron spheres as we can make. We then point them at the nearest stars and have them disperse. What would the probabilities be that they land somewhere habitable? Are there any ethical considerations in doing this? Is it a galactic faux pas?

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