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Why could extraterrestrials already know about the existence of humans?

Extraterrestrial astronomers may be looking in our direction from distant planets. What would they see?

We have been searching the skies for some time now. Despite decades of listening for control radio signals and searching for signs that other worlds might be even remotely habitable, the selection has been sparse so far. While astronomers have identified a few potential candidates where life could exist elsewhere in the universe, as well as a mysterious and puzzling signal, there is still no concrete evidence of extraterrestrial life out there.

This article is part of a special week-long focus on extraterrestrial life, marking the upcoming 60th anniversary of the most famous extraterrestrial life form, Doctor Who, by the BBC.

But what if it were true? What if they were looking back, trying to find us? Would they know that there is life on Earth?

This is a question that scientists have been grappling with in recent years, as we continue to unintentionally broadcast our presence into the galaxy. "Hold a mirror up to yourself in space, and what would they see of us?" says Jacqueline Faerdi, an astrophysicist at the American Museum of Natural History in the United States. "We are searching, and that means other worlds may be searching too."

As of today, we have discovered over 5,500 planets orbiting other stars in our galaxy, known as exoplanets. However, these observations are still in their infancy, and trillions of worlds likely scatter throughout the Milky Way. On some of these worlds, we have started hunting for chemical signatures in their atmospheres that could indicate biological activity, as well as technosignatures that might be emitted by intelligent life forms, such as intentional or accidental radio signals directed our way.

Earth has been unabashedly broadcasting its presence into the galaxy for about a century. According to Howard Isaacson, an astronomer at the University of California, Berkeley in the United States, the most notable period was from 1900 to World War II when our radio transmissions were stronger. "They had to be more powerful because the radio receivers people were using were not as sensitive," he says.

Today, we continue to broadcast radio signals, from television shows to satellite communications, but in a less conspicuous manner. "Radio stations don't want to broadcast into space," says Thomas Bitti, an astronomer at the University of Wisconsin in the United States. "They want to broadcast to Earth." Other more modern forms of communication, like mobile phone signals, are unlikely to be detected.

However, not all our signals are so weak. Throughout the solar system, we have several spacecraft exploring different places like Mars, Jupiter, and even the outer reaches of the Sun. The farthest of them, NASA's Voyager 1 spacecraft, is 24 billion kilometers (15 billion miles) away from Earth, requiring a powerful network of dishes on Earth known as the Deep Space Network to communicate with it.

In April, Isaacson calculated whether some of these transmissions with power up to 20 kilowatts could reach other stars as they pass by distant spacecraft and continue their journey into space. He identified four nearby stars, and any accompanying planets would have already received the signals, with over 1,000 stars likely receiving them by 2300. "The signal would definitely be considered artificial," says Isaacson. By 2031, the closest star would have had enough time to receive the signals and potentially send back a response, which could be an interesting target for future exploration.

But what if extraterrestrial astronomers were more dedicated to their craft? They could try to observe our planet before receiving such signals. If they could see our planet transiting in front of our Sun (a so-called transit), they could observe the sunlight passing through our atmosphere and analyze its various gases.

In 2021, Faerdi found that within 300 light-years of Earth, there are about 2,000 stars that could potentially witness such a transit. "These are good candidate worlds," she says.

According to Paul Rimmer, an astrochemist at the University of Cambridge in the United Kingdom, the best indicators of life on Earth from such observations could be oxygen, nitrogen, and water vapor, which "would be indicative of a stable liquid ocean."

Nitrogen dioxide could also provide some clues that our planet is inhabited by intelligent life. This gas "is essentially a byproduct of combustion," says Hector Socas-Navarro, an astrophysicist at the Institute of Astrophysics of the Canary Islands in Spain. "So they could conclude that we are burning something here."

Chlorofluorocarbons from aerosols, refrigerants, and other sources could also be a sign of industrial activity on our planet. "We are almost certain that they can only be created with technologies," says Macy Houston, an astronomer at the University of California, Berkeley, in the USA. (Learn more about how we might detect extraterrestrial life through their pollution.)

By 2150, urbanization could have grown tenfold compared to current levels, and then we could shine like a beacon for modern telescopes. However, one of the most revealing technosignatures of Earth may not be our atmospheric pollutants or radio signals but the lights of our cities. In 2021, Bitti calculated that sodium emitted by such light sources could be detected in a planet's atmosphere. "It has very sharp spectral characteristics," says Bitti. "You wouldn't get that naturally."