Humans have wondered whether we are alone in the universe for millennia, but the question has taken on new dimensions in recent decades with the discovery of thousands of exoplanets. These worlds, which have been observed since the 1990s, orbit other stars and suggest that the Milky Way is teeming with planets, some of which may be habitable or host alien life.
A clear detection of an alien civilization would be one of the most consequential discoveries in history, which is why the search for extraterrestrial intelligence (SETI) community has rigorous standards about what qualifies as evidence when looking for “technosignatures.”
Technosignatures tend to evoke visions of extremely advanced alien civilizations that can harness the power of their stars with giant structures, such as Dyson spheres or gargantuan solar sails. If such megastructures exist within the Milky Way, they could be detectable from Earth because they would occult stars or other sources of light. Likewise, astronomers have scoured the skies looking for strong radio signals that aliens might be beaming out to contact other societies.
But there’s another category of technosignature, poised to be within our observational sights in the coming decades, that would hint at a civilization more technologically comparable to our own. It’s called “leakage.”
Leakage is a signal that would be emitted by technologies such as radio or television broadcasting, which send radio waves rippling isotropically out into space. A panel of scientists elaborated on leakage during a recent presentation at the meeting of the American Association for the Advancement of Science (AAAS) in Seattle, Washington.
“The really exciting thing I think, for me, is this ability to detect leakage,” said Andrew Siemion, an astrophysicist at UC Berkeley and director of the Berkeley SETI Research Center, at the AAAS presentation. “At SETI, we always talk about I Love Lucy or some ancient TV broadcast but the truth is that none of our current generation of experiments is sensitive enough to detect a signal that’s that weak.”
Radio leakage has been recognized as a technosignature of our own civilization for decades, and has been chewed over by everyone from astronomer Carl Sagan to the writers of the animated comedy Futurama. On Earth, this leakage is weak and not directed at any particular interstellar target, so it’s unlikely that aliens can tune into our early 20th century serials.
But if any aliens are leaking radio signals from nearby exoplanets, we may soon be able to eavesdrop on them. This is because a new generation of radio observatories is poised to dramatically boost the sensitivity and speed of observations, enabling scientists to finally screen nearby worlds for radio leakage.
In particular, radio astronomers are eagerly anticipating the construction of the Next Generation Very Large Array (ngVLA), a network of antennae that will span North America, as well as the Square Kilometre Array (SKA), which will stretch from South Africa to Australia. Both projects are highly ambitious and will take at least a decade to build before they see first light.
But once these enormous arrays prick up their ears, we will have entered a new era of observation. NgVLA is expected to be 10 times as sensitive as the best observatories in operation right now. SKA will be a 50-fold improvement, and will be the most sensitive radio telescope ever built.
“This new generation of telescopes also have very wide fields of view, which means they are great for performing surveys of large numbers of objects over large areas of the sky,” Siemion noted.
Once NgVLA is operational, it could detect isotropic radio buzz to within about five light years. That means we would be able to hear any aliens broadcasting from Alpha Centauri, the nearest star system, which we know has at least one Earth-scale planet.
Siemion said it was trickier to estimate how far the full SKA project might be able to detect leakage, because its design is still being developed. But it may well end up with triple the distance range of ngVLA, which would allow it to screen about 10 star systems within 15 light years for leakage.
“In other words, isotropic leakage is really hard to detect!” Siemion emphasized.
Fortunately, there is another type of “directed” leakage that is slightly more powerful than isotropic waves. If a nearby alien civilization has developed satellite technologies, for instance, it might communicate with its spacecraft with targeted radio messages. These direct messages could skim past the spacecraft and end up being picked up by the new crop of radio telescopes on Earth.
“We believe most other planetary systems to be like our solar system, in which most of the planets are more-or-less co-planar,” Siemion said. “This means that if another civilization is exploring their exoplanetary system, some of this directed leakage will be confined to the plane of their system, which would be aligned relative to us if we see planets in transit. This would give us a greater chance of detection.”
Fortunately, he added, nearby systems such as TRAPPIST-1 and HD 219134 are aligned this way, so they would be perfect targets to look for directed radio leakage. We also know that these systems host planets, some of which are Earth-sized and within the habitable zone of their star.
A leakage detection is a thrilling prospect for SETI researchers because it would be a highly credible sign of an alien civilization. The giant distances between the antennae of ngVLA or the SKA, which are known as baselines, will enable the observatories to pinpoint the whereabouts of a signal with sharp accuracy. As a result, it is unlikely that scientists would pick up false positives of signals that actually originate from our own radio leakage.
“With telescopes like the ngVLA or the SKA, in addition to being more sensitive and having wider frequency coverage, they also have longer baselines which allows you to very carefully map where the signal is,” Siemion said. “That means that for us to be fooled, the transmitter would have to be really far away. If it were closer, like within our solar system, we would see it move during the duration of the observation.”
Moreover, while a detection of ozone or methane in an exoplanet’s atmosphere could potentially be caused by either biological or geological activity, narrowband radio leakage does not have any obvious abiotic source.
“All of that said, if we detected a narrowband radio signal, I am certain a wide variety of possible explanations would be proposed—some involving intelligent life and others involving exotic new physics,” Siemion said.
A detection of alien radio leakage would also validate decades of rumination about the possibility that a brief period of messy, primitive broadcasting may be a common phase for emerging technological societies. If our own civilization is any example, this phase could dwindle with the adoption of more sophisticated tools such as fiber-optic communications.
The cosmic odds are weighted heavily against capturing a civilization in this tenuous radio blitz. But if we did, it would hint at an extraterrestrial society with a similar level of technical achievement to us. Much though it would be mind-blowing to be contacted by advanced star-hacking aliens, it might be more comforting to stumble across a relatable technological peer.