If intelligent, technological extraterrestrial life exists in the galaxy, then it is conceivable that such a civilization might embark on a similar exploration strategy. Extraterrestrial intelligent (ETI) civilizations may choose to pursue astronomy and search for planets orbiting other star systems and may also choose to follow-up on some of these targets by deploying their own remote exploratory spacecraft. If nearby ETI have observed the Solar System and decided to pursue further exploration, then evidence of ETI technology may be present in the form of such exploratory probes. We refer to this ETI technology as “non-terrestrial artifacts”, in part to distinguish these plausible exploratory spacecraft from the flying saucers of science fiction.

In a recent paper titled “On the likelihood of non-terrestrial artifacts in the Solar System”, published in the journal Acta Astronautica (and available on arXiv.org as a preprint), Jacob Haqq-Misra and Ravi Kopparapu discuss the likelihood that human exploration of the Solar System would have uncovered any non-terrestrial artifacts. Exploratory probes destined for another star system are likely to be relatively small (less than ten meters in diameter), so any non-terrestrial artifacts present in the Solar System have probably remained undetected. The surface and atmosphere of Earth are probably the most comprehensively searched volumes in the Solar System and can probably be considered absent of non-terrestrial artifacts. Likewise, the surface of the moon and portions of Mars have been searched at a sufficient resolution to have uncovered any non-terrestrial artifacts that could have been present. However, the deep oceans of Earth and the subsurface of the Moon are largely unexplored territory, while regions such as the asteroid belt, the Kuiper belt, and stable orbits around other Solar System planets could also contain non-terrestrial artifacts that have so far escaped human observation. Because of this plenitude of nearby unexplored territory, it would be premature to conclude that the Solar System is absent of non-terrestrial artifacts.

Although the chances of finding non-terrestrial artifacts might be low, the discovery of ETI technology, even if broken and non-functioning, would provide evidence that ETI exist elsewhere in the galaxy and have a profound impact on humankind. This is not to suggest that the search for non-terrestrial technology should be given priority over other astronomical missions; however, as human exploration into the Solar System continues, we may as well keep our eyes open for ETI technology, just in case.

Background: The Fermi Paradox
According to a simple but powerful inference introduced by physicist Enrico Fermi in 1950, we should expect to observe numerous extraterrestrial civilizations throughout our galaxy. Given the old age of our galaxy, Fermi postulated that if the evolution of life and subsequent development of intelligence is common, then extraterrestrial intelligence (ETI) could have colonized the Milky Way several times over by now. Thus, the paradox is: if ETI should be so widespread, where are they? Many solutions have been proposed to account for our absence of ETI observation. Perhaps the occurrence of life or intelligence is rare in the galaxy. Perhaps ETI inevitably destroy themselves soon after developing advanced technology. Perhaps ETI are keeping Earth as a zoo!

The ‘Sustainability Solution’
The Haqq-Misra & Baum paper presents a definitive statement on a plausible but often overlooked solution to the Fermi paradox, which the authors name the “Sustainability Solution”. The Sustainability Solution states: the absence of ETI observation can be explained by the possibility that exponential or other faster-growth is not a sustainable development pattern for intelligent civilizations. Exponential growth is implicit in Fermi’s claim that ETI could quickly expand through the galaxy, an assumption based on observations of human expansion on Earth. However, as we are now learning all too well, our exponential expansion frequently proves unsustainable as we reach the limits of available resources. Likewise, because all civilizations throughout the universe may have limited resources, it is possible that all civilizations face similar issues of sustainability. In other words, unsustainably growing civilizations may inevitably collapse. This possibility is the essence of the Sustainability Solution.

Implications for the Search for Extraterrestrial Intelligence (SETI)
If the Sustainability Solution is true, then we may never observe a galactic-scale ETI civilization, for such an empire would have grown and collapsed too quickly for us to notice. SETI efforts should therefore focus on ETI that grow within the limits of their carrying capacity and thereby avoid collapse. These slower-growth ETI may possess the technological capacity for both radio broadcasts and remote interstellar exploration. Thus, SETI may be more successful if it is expanded to include a search of our Solar System for small, unmanned ETI satellites.

Implications for Human Civilization Management
Does the Sustainability Solution mean that humanity must live sustainably in order to avoid collapse? Not necessarily. Humanity could collapse even if it lives sustainably—for example, if it collides with a large asteroid. Alternatively, humanity may be able to grow rapidly for much longer—for example, until we have colonized the entire Solar System. Finally, the Sustainability Solution is only one of several possible solutions to the Fermi paradox, so it is not necessarily the case that all civilizations must grow sustainably or else face collapse. However, the possibility of the Sustainability Solution makes it more likely that humanity must live more sustainably if it is to avoid collapse.