Concerning the evolution of intelligent life, the main open questions include:
—What are the geochemical constraints on the evolution of complex life?
—What are the timescales that those constraints dictate?
—Are there evolutionary “filters” or bottlenecks that make it extremely hard to make the transition to intelligence?
On Earth, for example, it took about three billion years for the most basic multi-cellular life forms to appear. It took four and a half billion years (and a series of contingencies such as plate tectonics and asteroid impacts) to reach even the most rudimentary capability of interstellar communication (That is, via radio reception and transmission). These considerations demonstrate that it is important to first establish whether planetary systems that are older than the solar system are common in the Milky Way.
A recent study that examined planet formation history concluded that the solar system formed close to the median epoch for giant planet formation, and that about 80 percent of currently existing Earth-like planets may already had been formed (pdf) at the time of Earth’s formation. This gives us great leverage for probing extrasolar intelligent life.
Even though no single biosignature would be absolutely compelling, an atmosphere that is very rich in oxygen (say 20 percent or more) would probably be the most promising target initially. Wheras non-biological processes (such as the splitting of carbon dioxide by intense ultraviolet radiation) can produce oxygen in a planetary atmosphere, only under rare circumstances would these create such high levels of enrichment. Only in combination with other potential biosignatures, however, such as methane, would the credibility of a life-based origin for the oxygen be significantly strengthened.
One would ideally like to go beyond biosignatures and seek the clearest sign of an alien technological civilization. This could be the unambiguous detection of an intelligent, non-natural signal, most notably via radio transmission, the aim of the SETI (Search for Extraterrestrial Intelligence) program. Yet there is a distinct possibility that radio communication might be considered archaic to an advanced life form. Its use might have been short-lived in most civilizations, and hence rare over large volumes of the universe. What might then be a generic signature? Energy consumption is a hallmark of an advanced civilization that appears to be virtually impossible to conceal.
The two most plausible, long-term energy sources available to an advanced technology are through commanding stellar luminosity with a construction known as a “Dyson sphere”, possibly including harvesting the starlight from many stars, not just one, or even from an entire galaxy; the other is by controlled fusion of hydrogen into heavier nuclei. In both cases, waste heat would be an inevitable outcome, producing a detectable mid-infrared (MIR) signature.
Other potential signatures of advanced civilizations that have also been suggested, such as various forms of atmospheric industrial pollution, or short-lived radioactive products, are necessarily transitory. (Basically those aliens either clean up their act or destroy themselves). Infrared emission, on the other hand, seems almost unavoidable. A recent large survey by the Wide-field Infrared Survey Explorer (WISE) satellite did identify five red spiral galaxies whose combination of high MIR and low near-ultraviolet luminosities are inconsistent with simple expectations from high rates of star formation. A conventional explanation for these observations, such as the presence of large amounts of internal dust, has not been ruled out, however. Such peculiar objects deserve follow-up observations before we explore whether they might represent the signatures of galaxy-dominating species.
More pessimistically, biologically-based intelligence may constitute only a very brief phase in the evolution of complexity, followed by what futurists have dubbed the “singularity”—the dominance of artificial, inorganic intelligence. If this is indeed the case, most advanced species are likely not to be found on a planet’s surface (where gravity is helpful for the emergence of biological life, but is otherwise a liability). But they probably must still be near a fuel supply, namely a star, because of energy considerations. Even if such intelligent machines were to transmit a signal, it would probably be unrecognizable and non-decodable to our relatively primitive organic brains.
The key point is that for the first time in human history, we are only two or three decades away from being able to actually answer the “Are we alone?” question. Because the answer may affect nothing less than our last claim for being special in the cosmos, its importance cannot be overemphasized.
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