Watch too many episodes of Star Trek and you might think that every solar system hosts some sort of talkative life-form. But a standard argument holds that despite the high probability that other hospitable planets exist, intelligent life is exceedingly rare. Now a paper accepted for publication in The Astrophysical Journal puts better odds on the possibility that we have smart neighbors in the universe.
The original case was put forward in 1983 by British cosmologist Brandon Carter, who is now at the Paris Observatory. Carter noted that the first life-forms on Earth took almost as long to evolve into humans (some 4 billion years) as the sun, a middle-aged star, had been shining (5 billion years). He argued that since intelligence took so long to appear in the only case we know, it's very unlikely that intelligence generally evolves much faster than stars. In the more likely case that it typically evolves more slowly than stars, it would not have time to develop before a star burned out, Homo sapiens being a lucky exception. Carter's reasoning implied that smart aliens must be a rare breed indeed.
Mario Livio of the Space Telescope Science Institute in Baltimore argues that Carter's conclusion is "at best premature," since it assumes that the pace of biological evolution is completely independent of the star's lifetime. But that's not necessarily the case, he says. For instance, oxygen production, which leads to an atmospheric ozone layer that protects evolving organisms from ultraviolet radiation, depends on how much radiation the star emits, which is a function of its mass and lifetime.
If the star's own development paces the development of life, Livio argues, the near-equality of the two timescales, far from being a lucky accident, could well be the typical cosmic situation. If so, intelligence in other planetary systems would generally have time to develop before the star in the system dies, implying that extraterrestrial civilizations might not be as rare as first thought.
Carter could not be reached for comment, but cosmologist Martin Rees of Cambridge University calls Livio's argument "rather obscure and a bit hard to understand." He agrees that stellar evolution may influence life, but probably only in early stages of evolution. "I genuinely think we don't know enough to say whether or not the universe is teeming with [intelligent] life," says Rees. "I'm completely agnostic about it."