Our intuition that life is far more widespread in the universe than technology may be misleading us. Reconsideration could affect how resources are prioritized in one of science's great quests.
The search to find life beyond the Earth has followed two broad paths – seeking signs of intelligent life such as radio signals, or hunting for the effects of biological activity. Having so far not succeeded on either account, it's hard to be definitive as to which is more likely to work.
Nevertheless, a study published in The Astrophysical Journal Letters carries the somewhat counter-intuitive case for technology over biology.
The argument for seeking biological signs is simple. Not all worlds with life will spawn civilizations whose technology we can detect, indeed it is likely the vast majority won't. A star like Alpha Centauri could easily have lifeforms on orbiting planets, the chance of something high-tech is remote. The idea is implicitly encoded in the famous Drake equation, an attempt to calculate the number of technological civilizations in the galaxy.
However, Dr Jason Wright of Penn State University and co-authors argue that this simple reasoning needs to be set against four factors which may collectively outweigh it.
The most obvious of these is the ease of detection. If a radio signal is powerful enough, we could spot it across the galaxy, whereas biological signs are likely to be noticeable only around nearby stars.
There is also the fact technological life forms may spread their products far wider than they themselves will travel. As far as we know, only Earth hosts life within our solar system, but human technology can be found on Mars and the Moon and in orbit around Venus and Jupiter. It's possible some will stay functional long after not only humanity, but all life on Earth, has gone.
The possibility technology could go on self-replicating far beyond its original makers (whether through their design or by accident) also needs to be considered, the authors argue.
Finally, life is tied to planets or at least moons, while technology can exist between worlds and even between star systems.
The authors state that while those involved in the Search for Extraterrestrial Intelligence are familiar with many of these arguments, they're less familiar to other astrobiologists.
To weigh these arguments, the authors modify the original Drake equation to produce two “Drake-like” versions that estimate the numbers of technological or biological signatures to be found. As with the original version, each requires estimating the chance of various events, often with little to go on.
Will one in a hundred planets that host life eventually evolve a technologically advanced civilization, or one in a million? No one knows, but many people have opinions. Where spaceflight is achieved, will it on average lead to the colonization of a handful of planets, or millions? The answer is equally uncertain. Depending on which numbers you pick, the authors note one could conclude biological signatures far outnumber technological ones, or the reverse.
“An objective, quantitative comparison of the actual relative abundances of technosignatures and biosignatures is difficult because it depends on details of extraterrestrial life that we cannot know for certain until we have some examples to learn from,” the paper notes.
On the other hand, we can probably be more certain that where technology exists, it will be easier to find than signs of life at least as long as it is still operating, rather than a colossal wreck. Technological signals are also less likely to be ambiguous.
When astronomers such as Frank Drake were first contemplating the question, they had little choice in the matter. We could search for signs of biology on Mars and perhaps elsewhere in the Solar System, but anywhere further afield would depend on radio signals. We couldn't even detect more distant planets, let alone examine their atmospheres for gasses indicative of life.
Today we know of thousands of exoplanets, some potentially capable of supporting life. Forthcoming telescopes may allow us to find evidence if it is there, at least in closer examples. Much as the JWST will transform other areas of astronomy, the authors think we'll get more value for money when it comes to finding life from the Square Kilometer Array, with its massively enhanced capacity to detect radio signals.