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Want to Find Aliens? Look For Their Detritus

In this illustration, debris along the outer reaches of a planet-forming disk orbits in the glare of a distant sun.
T. Pyle
In this illustration, debris along the outer reaches of a planet-forming disk orbits in the glare of a distant sun.

From science fiction movies, we all know how it happens. Astronomers working with huge telescopes detect an object at the edge of the solar system. It's coming our way and it's moving fast. Working feverishly, they apply the latest image-enhancement techniques, revealing super-sharp pictures of interstellar garbage.

No, wait. That's not how the movies work.

The astronomers get images of vast interstellar starships, right? That is how we first detect the aliens. We see their starships, not garbage. Well, in real life, it's more likely that our first evidence of intelligent life in space will come from detecting their radio waste.

The idea of receiving "unintentional" waste signals from an extraterrestrial intelligence goes back a ways. More than 30 years ago Woody Sullivan, one of my professors at the University of Washington, looked at radio leakage from the Earth and how it would appear to distant aliens.

The idea was simple. The Earth, or perhaps any planet inhabited by a technologically adept species, is constantly leaking "waste" radio signals into space that could be detected from quite a ways out in the galaxy. From TV broadcasts to powerful military radar, radio energy meant for some other purpose escapes into space as waste. By analyzing the properties of our own waste radio signal, Sullivan found that an eavesdropping race could develop a relatively substantial account our civilization on Earth.

Since that time, other researchers have taken up the idea of alien waste and its visibility. Given news that a team of billionaires are drawing up plans to mine the asteroids, a recent study by astronomers Duncan Forgan and Martin Elvis seems of particular importance.

Their topic? "Extrasolar Asteroid Mining as Forensic Evidence For Extraterrestrial Intelligence."

Yup. You read that right. Forgan and Elvis are looking at mining tailings as possible evidence of high-tech cultures in space. Now, before you laugh too loudly, consider our own culture's mining waste. We have, after all, leveled whole mountains in search of coal, tar-sands and gold. Space-faring civilizations will likely be doing the same resource-harvesting in space. But things would get a whole lot messier in space.

Forgan and Elvis's basic premise rests with a fundamental problem of doing anything destructive in space. Debris doesn't just sit there. It spreads out as each pebble and dust grain goes wandering off on its own trajectory. That means, if you are chewing up asteroids to the tune of billions of tons a year (or much more) to harvest metals, you will also be left with dusty debris that spreads out to form a ring or disk.

Using infrared telescopes we can already observe such "debris disks." But these are associated with newly formed planetary systems (collisions between asteroids and planetary embryos are the source of the dust). Forgan and Elvis wanted to see if a debris disk formed by purposeful asteroid mining would be detectable and if its observed properties could point to exactly that purpose.

Their answer? Yes and No. As they put it:

For TAM to be detectable, it must be prolific and industrial-scale, producing a large amount of debris and disrupting the system significantly to be detected.

The big problem, they found, will be distinguishing between natural and asteroid-mining-debris disks. Detection was, however, theoretically possible. That's very cool. But whether Forgan and Elvis are right about the disks in particular is of less importance than what may be a broader point about technological civilizations.

Or, put another way, by their crap you shall know them!

You can keep up with more of what Adam Frank is thinking on Facebook and on Twitter: @AdamFrank4

Copyright 2021 NPR. To see more, visit https://www.npr.org.

Adam Frank was a contributor to the NPR blog 13.7: Cosmos & Culture. A professor at the University of Rochester, Frank is a theoretical/computational astrophysicist and currently heads a research group developing supercomputer code to study the formation and death of stars. Frank's research has also explored the evolution of newly born planets and the structure of clouds in the interstellar medium. Recently, he has begun work in the fields of astrobiology and network theory/data science. Frank also holds a joint appointment at the Laboratory for Laser Energetics, a Department of Energy fusion lab.