Extraterrestrial technology may exist in the Solar System without our knowledge. This is because the vastness of space, combined with our limited searches to date, implies that any remote unpiloted exploratory probes of extraterrestrial origin would likely remain unnoticed. Here we develop a probabilistic approach to quantify our certainty (or uncertainty) of the existence of such technology in the Solar System. We discuss some possible strategies for improving this uncertainty that include analysis of moon- and Mars-orbiting satellite data as well as continued exploration of the Solar System.
If extraterrestrials exist in our galaxy, then might they try observing us? This question was raised by Ronald Bracewell [1] shortly after Cocconi and Morrison's suggestion [2] to search for extraterrestrial broadcasts. It is certainly possible that extraterrestrials could be observing us remotely; after all, near-future human technology includes the prospects of observing atmospheric spectra of extrasolar terrestrial planets [3] and exploring nearby star systems with unpiloted remote exploratory probes [4,5,6]. One possible scenario for human exploration of space begins with the discovery of an Earth-like planet around a nearby star, with an exploratory probe sent as a follow-up mission. It is at least plausible that extraterrestrials might also adopt a similar strategy [7,8]. If so, then extraterrestrial technology could be hiding in our own Solar System.
We have obviously not yet discovered any technology of extraterrestrial origin, but how sure can we be that the Solar System contains none of these artifacts? In this paper we develop a probabilistic approach to this question. Certain regions of space have been searched sufficiently to discover such technology, if it exists, but much of the Solar System remains unexplored. It is completely possible that extraterrestrial technology could be actively observing us without our knowledge, while it is also possible that defunct extraterrestrial technology exists on planetary surfaces or interplanetary space. Therefore, it is of interest to calculate a likelihood that a given region of space is in fact absent of extraterrestrial technology. We begin our argument by first clarifying the types of extraterrestrial technology we consider. We then develop our probabilistic analysis and discuss its implications for future searches of the Solar System.
The Fermi paradox [9], also known as the Great Silence [10], asks the question: if intelligent life is common in the galaxy, then why have no technological civilizations been observed yet? Many resolutions have been proposed to this question [11,12]. These solutions include the possibility that life is rare in the galaxy [13] or that intelligent civilizations inevitably destroy themselves [14,15,16]. However, the absence of extraterrestrials could simply be because long-lived technological civilizations expand too slowly to have yet arrived at Earth [17,18,19]. Still other resolutions to the Fermi paradox suppose that extraterrestrials are actually widespread but have purposefully refrained from making contact [20,21,22,23]. In both of these cases, there is nothing to preclude the possibility that extraterrestrials might remotely explore our Solar System using unpiloted probes. We refer to such technology as non-terrestrial Artifacts (NTAs) to indicate their extraterrestrial origin but also to distinguish realistic remote exploratory probes from piloted flying saucers in the realm of science fiction.
The zoo hypothesis [20] is a term describing the possibility that extraterrestrials might be concealing themselves from us for one reason or another. We often imagine that a benevolent extraterrestrial civilization might have our best interests in mind and avoid interfering with our primitive culture until we cross a certain threshold, such as the discovery of light-speed travel [20,21] or our initiation of conversation with a nearby intelligent space probe [24]. An even more omnipotent extraterrestrial civilization might engineer a virtual planetarium that forces us to observe an empty universe [23], which would give us almost no chance at discovering their presence until they choose to reveal themselves to us. More mundane versions of the zoo hypothesis simply invoke the vastness of space to suggest that extraterrestrials or extraterrestrial technology could be hiding in the asteroid belt [25,26,27,28] or other places in the Solar System [29,30,31] without us noticing. This range of possibilities for the zoo hypothesis can be rather confusing for SETI (search for extraterrestrial intelligence) researchers because some of these hypotheses are testable whereas others are not. We therefore sub-divide this zoo hypothesis into two categories: the testable zoo hypothesis and the untestable zoo hypothesis.
The untestable zoo hypothesis describes any attempt by an advanced extraterrestrial civilization to willfully conceal its presence from us. We typically assume that any extant extraterrestrial civilization is probably much more technologically advanced than us [1,2,14,15,16,32,33], so there could be almost no limit to the ways in which superior extraterrestrials could keep us in the dark. In this sense, the untestable zoo hypothesis is tantamount to an unknowable faith-based system because we can ascribe to the extraterrestrials any mechanism imaginable that maintains their unobservability. Such scenarios are popular in science fiction, perhaps in part because they allow us to maintain belief in a Galactic Club [34] of ext
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