How human activity can betray our existence to alien civilizations

But not all our signals are so faint. Across the solar system we have many spacecraft exploring places as diverse as Mars, Jupiter, and even the outer regions of the sun. The farthest, NASA’s Voyager 1 spacecraft, is 24 billion kilometers (15 billion miles) 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 capacities of up to 20 kilowatts, might be effective. Reach other stars They wash the distant spacecraft and continue their journey into space. He found four nearby stars and any accompanying planets that must have already received the transmission, and more than 1,000 stars are likely to hear the signals by 2300. “The signal will certainly appear to be artificial,” Isaacson says. By 2031, the closest stars will have enough time to receive signals and send their own messages, and this could be an interesting target for future study.

But what if space astronomers were more dedicated? They may try to observe our planet before receiving any such signals. If they could see our planet passing in front of our sun, Known as transitThey can see sunlight passing through our atmosphere and pick up its various gases.

In 2021, Faherty finds that there is Nearly 2000 stars It is 300 light-years from Earth that would likely be able to see such a transit. “It’s a good range of worlds,” she says.

The best indicator of life on Earth from these observations may be oxygen, nitrogen and water vapor, says Paul Rimmer, an astrochemist at the University of Cambridge in the United Kingdom, which may be “an indicator of a stable liquid ocean.”

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Nitrogen dioxide can also provide some clues That our planet has been blocked by an intelligent life form. The gas “is basically a byproduct of combustion,” says Hector Socas Navarro, an astrophysicist at the Institute of Astrophysics in the Canary Islands in Spain. “So they might conclude that we’re burning things down here.”

CFCs emitted from aerosols, refrigerants and other sources can also be a clear sign of industrial activity on our planet. “We are certain that they can only be produced by technology,” says Macy Houston, an astronomer at the University of California, Berkeley in the US. (Read more about how we too can detect alien life through its pollution.)