Low-Hanging Fruits of Extraterrestrial Intelligence

From a large interstellar distance, a technological civilization like ours appears like an engine fueled by starlight and nutrients on a rocky planet and spewing electromagnetic signals and rockets to outer space.

Identifying primitive forms of life based on the composition of a planet’s atmosphere or its appearance as a blue-green dot, is an extremely challenging task that the mainstream of astronomy chose to focus resources on. The products of technological engines might be easier to detect if they exist in sufficient abundance. It would be a strategic mistake to train our telescopes on small fruits at hard-to-reach tree tops if low-hanging fruit might be available for us to pick nearby.

In 1960, the visionary astronomer Frank Drake — who recently passed away, pioneered Project Ozma, a year after the seminal paper by Giuseppe Cocconi and Philip Morrison proposed a search for interstellar radio communications. Project Ozma was named after the queen in L. Frank Baum’s 1904 book, The Land of Oz, a place “very far away, difficult to reach, and populated by strange and exotic beings.” Drake’s approach and related equation, established the foundation of the Search for Extraterrestrial Intelligence (SETI), which focused since then on electromagnetic signals as the primary signature of technological engines. In 2021, the Galileo Project added to it the search for interstellar objects of technological origin near Earth.

Whereas electromagnetic signals propagate at the speed of light and escape from the Milky-Way disk of stars within tens of thousands of years, rockets move at a speed that is ten thousand times smaller and remain gravitationally bound to the Galactic disk. With chemical propulsion, the characteristic spacecraft speed of tens of kilometers per second is an order of magnitude lower than the local escape speed from the Milky-Way, which is five hundred kilometers per second. As a result, chemically-propelled spacecraft would accumulate over time within the Milky-Way disk. Viewed this way, the Galactic disk is a basket that has been collecting the fruits of technological engines on Milky-Way planets for billions of years. Most of these engines stopped working by now, leaving no other way to learn about their existence than through interstellar archaeology.

Extraterrestrial technological objects divide into two categories: space trash and functional devices. The five probes that NASA launched so far towards interstellar space: Voyager 1 & 2, Pioneer 10 & 11 and New Horizons, will all become space trash in less than a million years from now. But in the coming decades, our technological civilization is likely to launch much smaller and more resilient spacecraft, equipped with artificial intelligence (AI) and 3D printers, that can repair themselves and last much longer. In fact, self-replicating probes might be the most abundant population in interstellar space, as they could multiply exponentially in time by using the raw materials available on planets throughout the galaxy. If another civilization reached the tipping point of launching self-replicating AI probes billions of years ago, it would be difficult to detect the faint electromagnetic signals from its early phase of radio communications, as these signals propagated billions of light-years away. However, we can find their probes as they enter the Solar system from interstellar space.

Ten years before Project Ozma, Enrico Fermi asked: “where is everybody?” This was in 1950, whereas humanity developed survey telescopes and satellites that allow it to detect interstellar objects bigger than dust only over the past decade.

The survey capabilities enabled by recent technological advances, enabled the discovery of the first three interstellar objects. They include the interstellar meteor CNEOS 2014–01–08 in 2014, the interstellar space object `Oumuamua in 2017, and the interstellar comet Borisov in 2019. Among these three, the first two appear to be outliers relative to the population of space rocks within the Solar system. The interstellar meteor was tougher in material strength than all other 272 meteors listed in the CNEOS catalog, namely rarer than 99.6% of the familiar space rocks. Moreover, it was faster than 95% of all the stars in the vicinity of the Sun. In addition, `Oumuamua was inferred to be flat, pushed away from the Sun without a visible cometary tail and its initial velocity was closer to the Local Standard of Rest than 99.8% of all nearby stars. Borisov appeared to be a typical comet.

The realization that two out of the first three interstellar objects appear weird, motivates the Galileo Project research team to plan an expedition that will retrieve fragments from the first interstellar meteor. This meteor could be natural in origin but from an astrophysical source never imagined before. For example, X-ray imaging of the Vela supernova remnant revealed bow shocks from bullets flying out of the explosion site. Perhaps CNEOS 2014–01–08 was a bullet tougher than conventional iron meteorites, which was shot out of an exploding star.

On the other hand, if some interstellar objects are technological relics, their inferred abundance depends on whether they are functional or space trash. In collaboration with my Harvard undergraduate student, Carson Ezell, we calculated that probes targeting the habitable zone of stars would be less abundant by 16 orders of magnitude than space trash on random trajectories, for the same detection rate near Earth.

During the next decades, humanity could find a functional device from another technological civilization as long as it will invest similar funds in this search as in the search for primitive life or dark matter. It is well known from dating experiences that you are more likely to find the mate that you are looking for if you are committed to the search. There are numerous daters who searched in the wrong places, gave up on the search and fulfilled their own prophecy that they are alone.

In order to harvest the fruits of other technological civilizations, we must search our “back yard” in the Solar system for objects that came from outside of it. Some of the objects we find may be intelligent while others would burn up in the Earth’s atmosphere.

Here’s hoping that finding technological products from afar will inspire us to focus on an ambitious future in space rather than the traditional past on a worn-out rock we call Earth.


Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He chairs the advisory board for the Breakthrough Starshot project, and is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021.



Get the Medium app

A button that says 'Download on the App Store', and if clicked it will lead you to the iOS App store
A button that says 'Get it on, Google Play', and if clicked it will lead you to the Google Play store
Avi Loeb

Avi Loeb is the Frank B. Baird Jr Professor of Science and Institute director at Harvard University and is the bestselling author of “Extraterrestrial”.