Life's Evolution May Depend on Galaxy

Intelligent life beyond Earth might not be as dim a hope as many scientists think, according to a new study challenging a widely held anti-ET argument.

Many skeptics tout an idea called the anthropic argument that claims extraterrestrial intelligence must be very rare because the time it takes for intelligent life to evolve is, on the average, much longer than the portion of a star's existence that is conducive to such life.

But now astrobiologist Milan M. Cirkovic and colleagues say they've found a flaw in that reasoning.

The anthropic argument, proposed by astrophysicist Brandon Carter in 1983, following on his pioneering work on anthropic principles in 1970s, is built on the assumption that the two timescales - the lifecycle of a star and the time required for evolution of living and intelligent creatures - are completely independent. If this is true, Carter argued, it's extremely unlikely that these two windows of possibility would last roughly the same amount of time, and would occur at the same time.

But that mode of thinking is outdated, Cirkovic claims. In fact, he says the relevant timescales are not independent; they are deeply entwined. "There are many different ways in which planets in our solar system are not isolated," Cirkovic said. "We must not regard habitable planets as closed boxes. If you abandon that assumption of independence, then you have a whole new background in which you can set up various models of astrobiological development."

Cirkovic points to gamma ray bursts, nearby supernovae, and perturbations of comet clouds as possible events in the astrophysical environment of the star that can influence the biological environment on a planet. For example, when a star travels through one of the dense spiral arms of the Milky Way, both its own development and that of its planets could be disrupted by higher levels of interstellar electromagnetic radiation and cosmic rays, due to the higher frequency of star-forming regions and supernova explosions.

All these connections conspire to rule out the independence suggested by Carter and connect the life of a star and the evolution of life on a planet, Cirkovic argues link...