Does complex life emerge at a gradual, uniform rate? If so, we can come up with one answer to the Fermi paradox: We have not detected signs of extraterrestrial life because the time needed for complex life to appear generally exceeds the life of a star on the main sequence. But the assumption that intelligence appears over time with a gradual inevitability — a key tenet of work by Brandon Carter, Frank Tipler and others in the 1980s, may not in fact be true. Solar system-wide events connect life with its stellar environment, while galaxy-wide events provide yet another context.
Punctuated Evolution Among the Stars
Milan Ćirković (Astronomical Observatory, Belgrade) and colleagues have much to say about this in a new paper in Astrobiology. It’s a rich treatment of our older assumptions and newer thinking about punctuated evolution, the idea that life actually evolves in spasms rather than smooth ascents. Species remain relatively stable for long periods but endure sudden changes that can create evolutionary innovation. The paper relies not only on studying the frequency of extinction events on Earth but also on the idea that a system of stars interacts strongly with its galactic environment.
Here the theories abound. Consider just one of the discontinuities that could affect life on a galaxy-wide level. Gamma-ray bursts (GRBs) caused by merging neutron stars or collapsing, supermassive stars could cause massive extinctions. In one calculation, the collapse of η Carinae could deposit in Earth’s upper atmosphere energy equal to the simultaneous explosion of one kiloton nuclear bombs per square kilometer over the surface of the hemisphere facing the blast. Other calculations show galactic GRBs being fatal to eukaryotes up to a distance of 14,000 parsecs, creating a vast ‘zone of lethality.’
But such events change over time. From the paper:
Since the regulation mechanism exhibits secular evolution, with the rate of catastrophic events decreasing with time, at some point the astrobiological evolution of the Galaxy will experience a change of regime. When the rate of catastrophic events is high, there is a sort of quasi-equilibrium state between the natural tendency of life to spread, diversify, and complexify, and the rate of destruction and extinctions. When the rate becomes lower than some threshold value, intelligent and space-faring species could arise in the interval between the two extinctions and make themselves immune (presumably through space-faring technology) to further extinctions.
A Broader View of Catastrophe and Change
But GRBs are only one possible mechanism illustrating a star system’s dependence on the galaxy around it. Ćirković and team list several others:
- Cometary impact-causing ‘galactic tides’
- Neutrino irradiation
- Clumpy cold dark matter
- Climate changes induced by spiral arm crossings
All of these have been studied in recent astrophysical literature. And in focusing on them, this paper homes in on a crucial point. We’re beginning to realize that it’s unrealistic to restrict the study of astrobiology to a closed system involving a single star and its planets. The Galactic Habitable Zone (GHZ), first introduced by Guillermo Gonzalez and colleagues in 2001 and followed up by Charles Lineweaver, has come into its own. And as Ćirković notes:
The GHZ constitutes an annular ring several kpc wide, comprising the solar circle at a galactocentric distance of 8.5 kpc; and, although its definition does not mention intelligent beings, it should clearly be the main target of SETI studies. In both astrobiology and Earth sciences, a paradigm shift toward an interconnected, complex view of our planet has already been in place for quite some time in both empirical and theoretical work…
A Galactic Phase Transition
The paper goes on to speculate that the factors above could each play a role in creating an astrobiological phase transition within the galaxy that indicates we may, contrary to common assumption, be living in a galaxy aswarm with intelligence, but most of it at stages of evolution not terribly different from our own. This would be the result of what Ćirković calls ‘galactic neocatastrophism,’ a galactic punctuated evolution. And it leads to the conclusion that there is no reason to reject the existence of extraterrestrial intelligence in the Milky Way.
A rare Earth? Maybe only in time. It’s possible we’re just early for the dance.
The paper is Ćirković et al., “Galactic Punctuated Equilibrium: How to Undermine Carter’s Anthropic Argument in Astrobiology,” Astrobiology Vol. 9, No. 5 (2009), pp. 491-501 (abstract). For more on the GHZ, Ćirković points to the excellent review of recent thinking on the galactic habitable zone in Gonzalez, “Habitable Zones in the Universe,” Origins of Life and Evolution of Biospheres 35 (2005), pp. 555-606.