Extraterrestrial Civilizations: Coming of Age in the Milky Way
If civilizations exist around other stars they are likely to be just emerging across our Galaxy right now: like an apple orchard suddenly maturing and ripening in the autumn sun. So concludes Space Telescope Science Institute theorist Mario Livio, in a paper to be published in the Astrophysical Journal. Livio emphasizes that his theoretical work doesn't necessarily mean extraterrestrial civilizations really do exist, but it shows they cannot be dismissed either.
If civilizations exist around other stars they are likely to be just emerging across our Galaxy right now: like an apple orchard suddenly maturing and ripening in the autumn sun. So concludes Space Telescope Science Institute theorist Mario Livio, in a paper to be published in the Astrophysical Journal.
Livio emphasizes that his theoretical work doesn't necessarily mean extraterrestrial civilizations really do exist, but it shows they cannot be dismissed either. We would be a lonely, isolated quirk of cosmic evolution if intelligent life forms appear on a planet at some random time in the parent star's life, say some theorists. Instead, Livio makes the case for a possible causal link between the sun's lifetime and the appearance of intelligent life on earth. This link should hold true for sun-like stars elsewhere in the universe: offering an equal opportunity for intelligent life to arise elsewhere in space.
The second part of his case is based on the possibility that carbon - the fundamental building block of life as we know it - may not have been widely available until the universe was about 1/2 its present age. This means that, given the added billions of years more required for biological evolution, intelligent carbon-based life didn't make an appearance any earlier than roughly 3 billion years ago.
He points out that before the universe could make life like us, it has to make carbon atoms. The carbon was created by nuclear fusion in the hearts of early stars, and then ejected when the stars lost their outer gas layers and left their cores behind as white dwarfs.
Livio calculates that carbon production may have peaked only two billion years before the sun and earth formed, based on estimates of the star formation rate made with Hubble Telescope and other ground-based telescopes. Though life first emerged on earth a few hundred million years after its formation, it took a vastly longer time - nearly 3 billion years - for the first multi-celled organisms to appear. It took almost another billion years before life emerged from the sea onto the land. The earliest humans appeared less than 4 million years ago - at about the halfway point in our sun's lifetime. If this were purely coincidental, other theoreticians have argued, then it would take much longer than the life of a star for most civilizations to arise. And so it would be unlikely extraterrestrial civilization would come about at all. We would be alone in the universe: reduced to a novelty - or accident - of the cosmos.
Because sunlight provides far more energy for life than other chemical processes, biological evolution is intimately linked to the sun's behavior, Livio maintains. For example, the complex evolution of our atmosphere is interrelated with the sun.
Our planet's atmosphere had to develop ozone to block out destructive UV radiation from the sun before animals could emerge on the land Likewise, he says, other civilizations should have emerged not much sooner or later than about halfway through their parent star's life cycle. That is, around stars like our sun, or slightly cooler, that live healthy long stable lives and release enough energy to nurture life on accompanying planets.
If Livio is correct, and the Galaxy may be blooming with new civilizations, then where are they? Why haven't they visited us?
Livio cautions that his work does not prove the existence of extraterrestrial civilizations, but points out that earlier conclusions that they do not exist may be premature.
He says that that it's also risky to think civilizations would colonize the Galaxy. "This assumes we have even the vaguest understanding of the psychology of extraterrestrial civilizations."
He adds: "It's impossible to imagine the thinking of a civilization which might have evolved a million of years ahead of humans.We could be about as uninteresting to them as an amoeba is to us. Actual proof will have to await advances in biology and astronomy."