Exoarchaeology, also known as xenoarchaeology, is the theoretical study of remains and artifacts left by extraterrestrial civilizations. The field parallels astrobiology (the physical study of alien species). Since no actual extraterrestrial cultures have ever been identified, this field is currently a theoretical one, which can be divided into two broad groups: paranormal researchers who believe they possess evidence of alien intervention in Earth's history (so-called "ancient astronauts"), and scientists and novelists who have speculated about the types of large, noticeable artifacts that an actual alien culture might leave behind for us to find.
Many popular science fiction books and TV shows refer to exoarchaeology as though it will one day become a legitimate field of study. Assuming we ever do encounter other intelligent civilizations, this is no doubt true. In Star Trek, Jean-Luc Picard's much-referred-to hobby in archaeology usually involved alien cultures, and would properly be preferred to as exoarchaeology.
- Theoretical Exoarchaeology -
Unlike Picard, we have no alien artifacts to work with. Instead, exoarchaeology today is a theoretical exercise, an attempt to think about with what sorts of artifacts an alien culture might leave for us to find. For a century, this has involved attempts to identify possible evidence of alien visits to the Moon and Mars, beginning with the infamous mis-labelling of natural features on the Martian surface as artificially constructed "canals." However, what we now know about Mars suggests that an advanced civilization almost certainly never existed there, and certainly does not exist there now. This does not rule out the search for artifacts left by visitors, in either place.
However, the theoretical search for artifacts has always gone much further. If other civilizations do exist, but some of the basic laws of the universe are as unbreakable as we think, then it will always be tremendously expensive and time-consuming to travel between the stars. This means that alien civilizations, and perhaps future humans, will not be racing about the galaxy in warp-capable starships, but will instead be content to explore the universe through large numbers of unmanned probes, perhaps sending a bulky colony ship here and there when something sufficiently interesting is uncovered.
Such probes would almost certainly have to harvest raw materials as they went, in order to sustain their very long-term missions, and are therefore known as von Neumann probes, after the writer who first described such a theoretical space probe. A particular type of von Neumann probe, known as the Bracewell probe, might be sent by alien cultures with the explicit purpose of interstellar communication. A Bracewell probe would contain an artificial intelligence able to make first contact and then transmit the results back to its home culture. It is possible that one or more Bracewell probes is already in our solar system, monitoring human development and waiting for the appropriate time to make contact (or simply spying on us, with no intention of making open contact). However, so far no searches have turned up solid evidence of such a probe in operation.
In addition, sufficiently advanced cultures might decide to take advantage of the most inexhaustible resource we have, solar power, but on a scale far greater than a few crude panels on the surface of a single planet. The most ambitious possible method of harnessing solar power, named the Dyson sphere after inventor Freeman Dyson, would be to construct a shell entirely around a star, perhaps at a distance somewhat greater than the current orbit of the Earth, and coat the entire interior of that shell with panels. Essentially this would allow an alien culture to harness the entire power of the star.
However, finding an alien civilization by searching for Dyson spheres is more difficult than it sounds. For one thing, an actual sphere might be too immense a task for any culture to bother undertaking; most might prefer simply to launch an exorbitantly large number of solar panel satellites into orbit around their star. Moreover, if anyone has constructed a Dyson sphere in our galaxy, then by definition it emits much less light than its star does - and therefore would be harder to find. When our methods of observing the galaxy grow advanced enough, we might be able to pinpoint possible Dyson spheres by finding sources of gravitational influence which do not correspond to a bright enough light source, and then send our own von Neumann probes to investigate.
Assuming that alien cultures (if they exist) have not perfected some sort of real-life equivalent of the science fiction warp drive, then the extraordinarily long periods of time necessary to travel between the stars introduces an interesting problem for potential exoarchaeologists: even if we do uncover evidence of alien intelligence, it may be quite some time before we know whether those artifacts belong to a surviving culture or whether those who left them are long dead. Indeed, even messages sent at the speed of light - those which SETI projects are currently searching for - would likely have been sent so long ago (potentially centuries, millennia, or more) that in any other context they would be considered archaeological artifacts.
- Ancient Astronaut Theories -
There are a growing number of so-called "ancient astronaut" theories, which hold that Earth's cultures were visited in the distant past by aliens from a variety of possible extraterrestrial origins. The most ambitious of these theories, like Zecharia Sitchin's theory of planet Nibiru, go further, arguing that these alien astronauts not only visited ancient Earth but conducted large-scale genetic engineering projects here, including the creation of the modern, intelligent human race through refinement of its less intelligent hominid or primate ancestors.
Thus far, no credible direct evidence of the physical presence of alien astronauts on Earth has ever been brought forward and accepted by the mainstream archaeological community. Instead, arguments tend to rely on two sources of circumstantial evidence. The first is creative interpretation of myths and legends of ancient cultures, arguing that certain references to supernatural phenomena are in fact early observations of extraterrestrial spacecraft or even aliens themselves. The second is to argue that certain artifacts are evidence of extraterrestrial visitation because they could not have plausibly been produced by the culture with which they are associated, given what we know about that culture's technology and knowledge of the natural world.
The first response of many professional archaeologists and astronomers alike is to laugh off these theories as conspiracy theories - and it is certainly true that credible physical evidence is lacking. Nevertheless, panspermia - the idea that the first seeds of life on Earth came from somewhere else - is still a viable theory. Noted astronomer Carl Sagan once pointed out that unless panspermia were rejected out of hand, scientists must give at least some consideration to the question of whether ancient astronauts played a role in the evolution of human life and civilization on Earth.