In theory, an advanced extraterrestrial civilization could potentially detect signs of life on Earth. Here are a few ways they might do so:
|Electromagnetic Signals||Since the late 19th century, humanity has been unintentionally broadcasting radio and television signals into space. These signals have expanded out from the Earth in a sphere that’s now more than 120 light-years in diameter. An alien civilization with advanced technology might be able to detect these signals.|
|Atmospheric Composition||When observing our planet from a distance, one could notice the unusual composition of our atmosphere. The combination of oxygen, ozone, methane, and other gases could be seen as a sign of life, because it’s unlikely these gases would exist together in such quantities in a lifeless world.|
|Light Pollution||A more direct sign of an industrialized civilization, such as ours, might be the artificial light we generate. A truly advanced civilization could potentially see the light pollution we emit on the dark side of our planet from a great distance.|
|Technosignatures||Apart from the unintentional signs we send out into the cosmos, there are the deliberate messages, such as the Arecibo message sent in 1974. Though this is a very limited effort, it does represent the idea that we could be “found” through our deliberate attempts to communicate.|
|Transit Method||When a planet passes in front of its star, it causes a small dip in that star’s brightness. This is one of the ways we discover exoplanets. An alien civilization using the same method might detect Earth as it transits the Sun and could potentially also detect changes in the Sun’s light caused by our atmosphere, which would give them clues about the presence of life here.|
It’s important to note that while these are plausible methods an advanced extraterrestrial civilization could use to detect us, it also assumes they have developed technology at least as advanced as our own and that they’re actively looking for signs of life in the universe. Additionally, the scale of the universe is incredibly vast. Even at the speed of light, signals from Earth have only reached a very small portion of our galaxy.