What Earth Would Look Like to Alien Observers

What would Earth look like to a distant alien civilization? In a study, scientists have worked out what intelligent extraterrestrials—should they exist—might detect if they were to observe our planet from some of our neighboring stars.

Researchers involved in the search for extraterrestrial intelligence (SETI) have yet to find evidence of such alien civilizations. But the intriguing possibility remains that they might exist somewhere in our universe.

"I believe that there's every chance advanced civilizations are out there, and some may be capable of observing the human-made radio leakage coming from planet Earth," Nalini Heeralall-Issur, one of the authors of the latest research from the University of Mauritius, said in a statement.

The possibility of "eavesdropping" on the everyday radio transmissions inadvertently leaking into space from other technical civilizations was first considered by scientists in the 1970s.

The planet Earth
Stock image: Illustration of the Earth in space. Would a distant alien civilization be able to detect us? iStock

These researchers examined the specific case of our own planet, Earth, and concluded that the most likely form of radio leakage that might be detected by another intelligent civilization was associated with military radar systems and television stations.

They created a model of the Earth's radiation and showed that an external observer would see variations in the signal power as different regions of the Earth rotated in and out of view.

But the nature of Earth's radio leakage has changed significantly since the 1970s as technology has advanced.

"We decided to carry out the research because this topic hadn't been touched since the late 1970s and a lot has changed since then," Michael Garrett, another author of the study from the University of Manchester, United Kingdom, told Newsweek.

"Also, I heard many colleagues say that the Earth had gone radio quiet—because the big radio and TV transmitters were now on cable. But I felt that wasn't right—that we needed to take into account the new developments of mobile phones, and WiFi—although these are not very powerful, there are just so many of them that it all adds up. Our paper was to try and work out just how much it adds up to—and it's significant."

In their study, which was published in the journal Monthly Notices of the Royal Astronomical Society, the researchers examined the overall contribution of mobile towers across Earth to the radio leakage coming from our planet.

The team created a model of the leakage and then calculated how this would be seen by any alien observers located at three different viewing points—the "nearby" stars HD 95735, Barnard's star, and Alpha Centauri A. These stars are all located within a few light years of Earth.

Their results show that alien observers in these locations would detect our signals in slightly different ways. For example, an extraterrestrial civilization around Barnard's star—which lies around 6 light-years away—would see the largest peaks in detectable signals when either Western Europe or East Asia first comes into view or finally disappears around the limb of the Earth from its frame of reference.

From HD 95735—a red dwarf located 8.3 light-years away—meanwhile, the main contribution of these signals would come mainly from the east coast of China, followed by the west and east coasts of North America.

Finally, from Alpha Centauri A—located around 4.2 light-years away—the primary contribution would come mainly from the west of Asia and Central Europe, with East Africa and Australia also making significant contributions.

Overall, the results show that the Earth's radio leakage signature is periodic and very dependent on the location of the hypothetical observer. This is due to the rotation of our planet and the non-uniform way mobile towers are distributed. The team calculated that the largest leakage emissions would be detectable from northern stars, such as HD 9573, because most towers are located in the northern hemisphere.

The study also highlights that the leakage signature of Earth has changed significantly over a relatively short time scale. In the 1970s, the radio emissions from TV transmitters and radar were dominated by a few thousand, very powerful transmitters located in specific areas of the planet—mostly Western Europe and North America.

Now, millions of mobile towers—operating at higher frequencies and more modest powers—are the norm. And these are spread out more equitably thanks to the technological advancements in developing countries around the world.

Any alien civilization located within 10 light-years of Earth with detection capabilities similar to our own would not currently be able to detect our planet's mobile tower leakage. But with large, "very sensitive" systems they would be able to, Garrett said.

"Whether they could make sense of the data is another question—but surely digital data with video and images would make some kind of sense to them. Also maths. Language would be more difficult to decode," he said.

Any given alien technical civilization is likely to be much more advanced than we are, Garrett said, in which case detecting our signals would be feasible.

"We've only been a technical civilization for 100 years so very likely any other random technical civilization will have been around for a lot longer—their technology would appear to be like magic to us. So their capabilities would be way beyond our own—certainly detection of these signals from nearby stars seems reasonable to assume."

In addition, the researchers said the Earth is getting brighter in terms of radio leakage and that trend looks set to continue for the decades ahead.

"For an alien civilization...they are likely to measure substantial increases in years to come," Garrett said. "This would be the most obvious sign to them that something was going on here on Earth—that another technical civilization, albeit a fairly primitive one, was on the rise."

These radio transmissions could, in theory, be used to infer certain properties about our planet, such as its rotation rate, orbital motion, details regarding the atmosphere, and possibly even information related to human culture.

Currently, the team's leakage model is somewhat limited. But in future, they plan to develop their model of mobile tower leakage to include more powerful and broader 5G emissions, as well as other radio sources, such as radar systems and communication satellites, among others. These technologies will likely make us more detectable.

"Another development is the satellite constellations being developed to provide global WiFi to every person on the planet," Garrett said. "They use radio waves to connect people to the internet—no matter where they are."

"The Earth will be embedded in a cocoon of radio waves as these satellites do their work. So yes, we're going to get a lot brighter but also very likely at other wavelengths—for example, the Infrared part of the spectrum."

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