The rate of growth of new technologies is often proportional to past knowledge, leading to an exponential advance over time. This explosive process implies that very quickly after a civilization reaches technological maturity, it will develop the means for its own destruction through climate change, for example, or nuclear, biological or chemical weapons. Developments of this type, over mere hundreds of years, would appear abrupt in the cosmic perspective of billions of years. If such self-destruction is common, this could explain Fermi’s paradox, which asks “where is everybody?”—and could imply that relics of dead civilizations should be abundant in space.

When exploring habitable worlds around other stars, we might therefore find planets with burnt-up surfaces, abandoned mega-structures or planetary atmospheres rich with poisonous gases and no sign of life. Even more intriguing is the possibility that we will find technological relics flying through our solar system with no detectable functionality, such as pieces of equipment that lost power over the millions of years of their travel and have turned into space junk.

How much debris exist in interstellar space would depend on the abundance of technological civilizations and the scope of their aspirations for space exploration. Based on Kepler satellite data, we know that about a quarter of all stars host a habitable Earth-scale planet. Even if a small fraction of all habitable Earths led to technological civilizations like our own during the lifetime of their stars, there might be plenty of relics out there in the Milky Way for us to explore.

This opportunity establishes a potential foundation for a new frontier of space archaeology, namely the study of relics from past civilizations in space. Instead of using shovels to dig into the ground, this new frontier will be explored by using telescopes to survey the sky and dig into space.

Naively, one might consider this research horizon as completely futuristic. But interestingly, the first artificial relic might have just been discovered over the past year when the Pan STARRSsky survey identified the first interstellar object in the solar system, ‘Oumuamua. The abundance of interstellar asteroids with ‘Oumuamua’s kilometer-scale length was estimated a decade to be vanishingly small, making this discovery a complete surprise. 

In addition, ‘Oumuamua is more elongated than any known asteroid in the solar system. But most intriguing is the fact that ‘Oumuamua deviated from the orbit one would have expected based on the sun’s gravitational field. Although such deviations could be associated with the rocket effect associated by outgassing due to heating of water ice by the sun, there was no sign of any cometary tail behind ‘Oumuamua, and calculations imply, contrary to observations, that its spin period should have changed significantly by any cometary torque. Might ‘Oumuamua have an artificial engine? Even if it happens to be a piece of natural rock as indicated by its lack of radio transmission, this rock appears to be very unusual by many counts. 

The discovery of ‘Oumumua should motivate us to keep searching for interstellar debris in the solar system. Interstellar objects may not be strictly onetime visitors. A small fraction of them may get trapped by the gravitational “fishing net” cast by the sun and Jupiter. Objects passing close enough to Jupiter could lose orbital energy through their gravitational interaction and stay bound to the solar system subsequently. Indeed, an asteroid occupying an orbit indicative of such origin, BZ509, was identified recently in a retrograde orbit around Jupiter. 

It is impossible to use existing chemical rockets to chase down ‘Oumumua because of its high speed, but one can contemplate missions to land on interstellar objects that are bound to the solar system. Although they represent a tiny minority of all the asteroids or comets in the solar system, their interstellar origin can be identified based on their unusual orbits around Jupiter or, in the case of comets, through their distinct (extrasolar) isotope abundance of oxygen, detectable by spectroscopic observations of their cometary tail. 

Finding evidence for space junk of artificial origin would provide an affirmative answer to the age-old question “Are we alone?” This would have a dramatic impact on our culture and add a new cosmic perspective to the significance of human activity. Finding a civilization dead due to war or climate change will hopefully convince us to get our act together and avoid a similar fate. But it would be even more remarkable if radar imaging or flyby photography near an interstellar relic within the solar system would show signs of an advanced technology that our civilization had not mastered as of yet. There is no better lesson to learn than that of civilizations that had developed advanced technologies up to the point of self-destruction.