Tau Ceti has always been an interesting star, one of two (the other being Epsilon Eridani) that Frank Drake chose as targets for his pioneering Project Ozma SETI observations. The astrobiological interest is understandable. We’re dealing with a Sun-like star relatively close (11.9 light years) to Earth. But recent thinking downplays Tau Ceti as a potential home for life. Ponder this: The dust disk around the star seems vastly larger than what we find in our own Kuiper Belt, with deadly implications.
Or are they? Let’s look more closely. A model of Tau Ceti’s disk shows that the mass of small objects up to ten kilometers in size may total 1.2 Earth masses. Compared to our Kuiper Belt’s 0.1 Earth masses, this is one massive disk, with ten times the amount of cometary and asteroidal material found in our own system. This despite the fact that Tau Ceti seems to be twice the age of Sol. You might reasonably assume that any Earth-like planet in this system has been bombarded far more often than Earth itself.
Image: Does severe bombardment rule out intelligent life around Tau Ceti? Hint: Be careful what assumptions you bring to bear. Credit: PPARC/David Hardy.
The question is whether this is a valid indicator, another feather in the cap of those who favor the ‘rare Earth’ hypothesis. The reasoning, as Milan Ćirković (Astronomical Observatory of Belgrade) points out in a recent paper, would go something like this: Our Earth was hit by an object just large enough to kill off the dinosaurs (or, at least, to assist in their destruction), the Chicxulub impact of some 65 million years ago.
A little larger and even the mammals might not have survived. Smaller and the mammals might not have become ascendant. Here’s how Ćirković puts it:
…micrometeorites bombard Earth all the time, and larger particles create beautiful meteor showers apparently without threatening the biosphere or in any known way influencing the evolutionary processes. On the other hand, studies of early history of the Solar System suggested that collisions with bodies hundreds of kilometers in size remaining at that epoch had caused repeated meltdown of the entire planetary crust and perhaps even complete atmosphere blow-off…Thus, only a finite — and quite small—range of impactors at the fixed epoch of K-T boundary could have caused the evolution of modern humans.
Our emergence as an intelligent species, then, seems predicated on the Chicxulub strike being of a rather precise size. But as the author points out, we must be cautious about this ‘fine-tuned catastrophism.’ Rather than saying that our emergence is contingent upon a particular type of impact (and thus a vanishingly rare occurrence), we should be saying that our emergence in the present epoch is contingent upon that impact.
And that has wide implications. For what we are now doing is removing a bit of anthropocentrism from our thinking. We simply cannot know what forms of life might have emerged had the Chicxulub impact occurred at a different time, or been of a different magnitude. Ćirković goes on to apply Bayesian methods to the question of our understanding of catastrophic risk. Interestingly, we tend to underestimate the chances of catastrophe:
It is intuitively clear why: the symmetry between past and future is broken by the existence of an evolutionary process leading to our emergence as observers at this particular epoch in time. We can expect a large catastrophe tomorrow, but we cannot — even without any empirical knowledge — expect to find traces of a large catastrophe which occured yesterday, since it would have preempted our existence today.
This is interesting stuff, noting our reliance on Earth-specific records that cannot be translated into analysis of catastrophe in other systems. And its look at observation-selection effects leads to the conclusion that only continued astrobiological study will help us understand what biospheres and potentially intelligent communities might be out there, on Tau Ceti or elsewhere in the Milky Way. The author’s conclusion: “No amount of armchair theorizing can escape the observation selection effects related to the evolutionary development of intelligent observers on Earth.”
Life amidst the debris around Tau Ceti? Maybe so. This is a short but densely argued paper; I’ve only managed to summarize a subset of its major points. For more, the reference is Ćirković, “Evolutionary Catastrophes and the Goldilocks Problem,” accepted for publication in the International Journal of Astrobiology (abstract).