For the past century, we’ve inadvertently broadcast our presence to an estimated 75 nearby star systems. About a quarter of those stars have confirmed exoplanets orbiting in the habitable zone. A few may be harboring intelligent life capable of receiving our long-ago leaked transmissions. But what kind of emissions are seeping into space now? Could extraterrestrials detect our satellite pings around the globe, and could they be listening in on our cellphone conversations? 

To figure out our latest leakage, a radio astronomy team is building a model of Earth’s technosignatures, the technological byproducts we radiate into space. The last time scientists simulated Earth’s emissions, Farrah Fawcett chased TV criminals on Charlie’s Angels, and ABBA debuted on Top of the Pops. Nowadays, billions of mobile devices stream gigabytes of content across continents, beaming an undulating pattern of broadband signatures as our planet cycles through night and day.

The Search for Extraterrestrial Life (SETI) embodies the science of looking for life beyond Earth, including evidence of extraterrestrial (ET) technology. For decades, SETI radio astronomers have focused on scanning the cosmos for a narrowband radio burst that intentionally signals “Hey, we are here”. Likely, alien logic suggests that a prominent narrowband frequency sticks out in the universe’s natural cacophony of broadband buzz. 

If a star system 10 light-years away has an exocivilization with a SKA Observatory, its alien astronomers can detect our Earth civilization right now.

However, Sullivan, Brown, and Wetherill proposed an alternative SETI methodology in their seminal 1978 research paper, arguing to eavesdrop on unknowing extraterrestrials who might be leaking their everyday radio technologies. 

To prove their point, they calculated Earth’s most powerful radio leakage at the time, narrowband TV transmissions and antiballistic missile systems, as seen from three nearby star systems. Factoring in planetary angles and alien receivers comparable to ours, the team concluded that we shimmered enough radio emissions to be detectable by star systems at least 25 light-years away.

“At the time, their paper electrified the SETI community. SETI was no longer some vague, science-fiction idea, not if lowly us could detect an extraterrestrial civilization as lowly as us,” explained Jason Wright, an astronomy professor at Penn State and director of its Extraterrestrial Intelligence Center, to IFLScience.

While the paper validated the concrete science behind SETI, radio astronomers, often short on funding and telescope time, returned to their standard narrowband search, waiting for a wow signal to pop out from the universe’s broadband din. Plus, our technologies were evolving into more energy-efficient systems. Cable TV took off in the 1980s and home computers in the '90s, changing the architecture and power of our transmission signals. Lower emissions, then less likely to be detected, so the thinking went.

“SETI scientists kept saying we were going radio-quiet,” recalled Michael Garrett, an astronomy professor at Manchester University, to IFLScience. “But then all these low-powered signals from our Wi-Fi and mobile devices keep multiplying. My mobile [phone] is maybe one watt of power, but now there are billions of them all over the planet. It’s kind of a nightmare, living in this cocoon of constant radio noise.”

Depending on your perspective of alien contact, we may be living in a nightmare if malicious ETs can sense our new modern-day emissions. But Ramiro Saide, a radio astronomer hailing from Mozambique, thinks otherwise. “Detecting an extraterrestrial signal would be an amazing opportunity to learn from another culture. There’s some kind of fulfillment to discover we are not alone, that this emptiness of space is not actually empty,” he told IFLScience. 

So Saide and his postdoctoral supervisor, Garrett, along with Nalini Heeralall-Issur from the University of Mauritius, decided Earth’s radio leakage needed an update to its 1978 profile. 

Incorporating geolocation data from OpenCelliD’s crowd-sourced software, the team mapped out 30 million cellphone towers around the globe. Averaging each cell site’s frequency range and wattage, they collated peak power output from the mobile towers’ antenna beams that stretch sideways toward our horizon. Then, factoring in some theoretical alien observers from three nearby star systems, what emerged is a wavy pattern of radio leakage as the mobile towers rise and set with the Earth’s rotation.

“Detection is not just about the power of the transmission. We take into account factors like frequency, timing, and bandwidth. Of course, there’s also the sensitivity of the receiving telescope, its direction and distance,” explained Saide. “So a northern star observing Earth would detect most of our cellphone leakage because most of our tower transmissions are in the northern hemisphere.”

The team’s findings revealed that a red dwarf about 8 light-years away, labeled blandly HD95735, receives our most potent emissions, 4 gigawatts gleaming on the horizon from mobile towers in China and the US. Staring at our equator is Barnard’s Star, a red dwarf about 6 light-years away with a confirmed exoplanet, and glaring at our southern hemisphere is Alpha Centauri A, a sun within a three-body star system about 4 light-years away. The team determined these stars might pick up about 3 gigawatts of tower transmissions leaked from parts of Europe, Asia, Africa, and Australia.

While these emissions are hardly detectable by a theoretical Green Bank-type telescope in the neighborhood of 10 light-years, the team’s estimates are incomplete. Not only are the cell tower numbers likely low, but the team’s calculations are based on frequencies emitted by nascent mobile technologies. Upcoming 5G technologies promise high-frequency, broadband architectures, translating to more powerful narrow-beamed radiation across a wider breadth of the electromagnetic spectrum. 

“We’ve established this preliminary model, so we can build upon it and extrapolate into the future,” noted Garrett. “Where will we be in the next decade with our 5G, 6G, or 7G technologies? What about the thousands of satellites yet to be launched? I suspect we’re fast becoming a red-hot potato detectable by any advanced civilization with the right technology.”

The team is factoring in additional open-source data to get a more accurate estimate of Earth’s radiation. Saide has mapped out the megawatts and frequencies emitted from commercial airports worldwide. Then, there’s the 1 or 2 watts radiating from each handheld device, a relatively easy measurement to resolve since most of humanity owns a cellphone. Add to the model an estimated 200 million Wi-Fi routers around the globe. Satellites, however, present a challenge. 

Elon Musk’s expanding Starlink constellation accounts for over half of the 8,000 satellites actively swarming our planet. Amazon intends to compete with Starlink’s broadband satellite service while China and the EU develop their fleets. According to the International Astronomical Union, a whopping 100,000 satellites will be launched in the coming decade.

“We know Starlink operates on broadband at higher frequencies, around 10 gigahertz, the part of the spectrum that interferes with our radio telescopes,” Garrett confirmed. “But we still don’t know about the company’s ground base stations, satellite transmission cycles, power wattages, or how the constellation will evolve. We’ll have to do some homework to add these parameters to our model.”

Additional transmissions are buzzing within our Solar System and beyond. NASA’s Deep Space Network, an array of powerful radio antennas that talk to its spacecraft, has a live website that details real-time frequencies and wattages of its multiple orbiters, landers, and flybys. The team wants to extrapolate from NASA’s numbers, averaging spacecraft transmission rates to factor in communication networks from other space agencies. The end product will simulate artificial radio signals from our entire Solar System.