At what distance could a “twin Earth” detect our signals?

Just over 60 years ago, Frank Drake turned a radio telescope at two stars and “listened” for signals of an alien civilization. That experiment marked the beginning of the Search for Extraterrestrial Intelligence or SETI. The stars Drake chose were nearby, residing just about 10 light-years away. He made that choice for a good reason: His calculations showed that this was the distance Earth’s own signals, emitted by human technology at the time, could be detected with his state-of-the-art (at that time) radio telescope. Using present-day Earth technology as a standard for both a source and receiver was good science and showed that SETI could be founded on solid scientific assumptions: Begin with what you know is possible. 

How much has changed since Drake ran those epoch-making experiments? Six decades later, progress in astronomy has allowed the long-marginalized SETI to be transformed into the robust field known as “technosignatures.” With the search for civilizations among the stars heating up, what are the current standards for detecting an alien planet with similar technological capacities? In other words, if Drake were to redo his experiment now, how far out should he have searched — and what if he used different kinds of telescopes? 

In a paper published this week, a team of scientists has answered exactly that question. The answers show just how far the field of technosignatures has come (full disclosure, I was happy to be part of the effort).

Updating Drake’s experiment

The paper is titled “Earth Detecting Earth: At What Distance Could Earth’s Constellation of Technosignatures Be Detected with Present-day Technology.” The project was led by Sofia Sheikh, a postdoctoral fellow at the SETI Institute and a member of our NASA-funded Categorizing Atmospheric Technosignatures (CATS) project. The paper was part of the second round of funding for CATS (the first NASA effort to look for signatures of technology from exoplanets and their atmospheres). Its publication marked a major milestone in setting the field of technosignatures on firm footing. 

There are many ways human civilization on Earth could be detected from a distance: city lights; atmospheric pollutants detected in our atmosphere; the presence of orbiting satellites; radio signals from cell-phone networks; radar signals used to study other planets in our own solar system. Each of these could be considered a technosignature. But how far out in space could each be detected if you only had the same technologies we possess today? Do you need to be actually orbiting the Earth looking down to find them or could you see them from the Sun’s nearest neighbor star, Proxima Centauri (which is 4.2 light-years away)? By conducting a comprehensive study of Earth’s many technosignatures, the Shiekh-led study established an important new baseline for the field.

So, what did we find?

Planetary radar, meaning radio signals bounced off planets in our Solar System, remains the king of Earth’s technosignatures, detectable out to distances of 10,000 light-years (using our current most-sensitive radio telescopes). There are at least a few hundred thousand stars within that distance. So theoretically there are a lot of worlds that will someday be bathed in detectable planetary radar signals from Earth if civilizations of comparable development to us exist on those planets. (Of course, a civilization 10,000 light-years away won’t detect a signal we send now for 10,000 years). 

Next comes the radio signals we use to communicate with our distant space probes. The Deep Space Network (DSN) is a set of radio transmitters around the world that stay in contact with missions like the Voyager probes (now more than 15 million miles from Earth). In their calculations, Sheikh and co-author Macy Huston found that DSN signals sent to deep space missions could be detected by Earth-level aliens out to about 60 light-years. 

The next entry in the list shows how far the field of technosignatures has come. We humans have been pumping out radio signals powerful enough to be detected across interstellar distance by our own technology for quite some time (that was the basis of Drake’s research strategy). But when Drake carried out his work, he had no idea whether the stars he was searching had orbiting planets (a prerequisite for life). He just had to hope for the best. It was not until 1995 that we discovered the first exoplanet orbiting a Sun-like star. Since then, we’ve discovered that our galaxy is rife with alien worlds. Even more important, we’ve learned how to peer into the atmospheres of those worlds to look for chemical clues of life. Using these techniques, our CATS team was the first to show that atmospheric “pollution” (i.e. gases emitted by technology) could be detected across interstellar distances. Based on that work, we found in the new paper that atmospheric technosignatures could be detectable at the distance of Proxima Centauri. In other words, we could, in principle, detect evidence of a technological civilization at our level on a planet orbiting the Sun’s nearest neighbor. It doesn’t mean there is one there, but that is an exciting milestone for our species to cross.

Another remarkable finding of the study was that the radio “leakage” — meaning signals meant for terrestrial purposes (like cell-phone towers) — could be seen out to more than 4 light-years. This is just under the distance to Proxima Centauri. Given how rapidly mobile technology is growing, it’s easy to imagine that such leakage could be detected out to true interstellar distances in the future.

The point about future growth for cell-tower leakage also holds for many of the other technosignatures investigated in the paper. Artificial illumination from urban areas (i.e. city lights) could be seen out to just 1/300^th of a light-year (about 2,000 times the distance from Earth to the Sun). That means someone would have to get pretty close to the Sun to see city lights. But as coauthor Thomas Beatty has shown, if we were looking at a much more urbanized planet and if we did so with the next generation of space telescopes, such artificial illumination could be seen across from interstellar distances. That means a city-world — also called an “Ecumenopolis,” like Trantor from Asimov’s Foundation series and Coruscant from Star Wars — might be detectable in the next 30 to 50 years.

You can see how all the technosignatures line up by looking at the figure above. Obviously, at this point in our history, many of the marks of our civilizations (like satellites) are not visible from across interstellar space if using our present-day technology. But our detection technologies will get better over the coming decades. That means we will increasingly be able to see civilizations at different levels of “advancement” relative to us. What the new paper shows is exactly where we are now — so we can see exactly where we need to go in the future.