Could alien signals have already reached Earth undetected?

If advanced alien civilisations exist and have contacted Earth, they are more likely rare, scientists say

If advanced aliens lived on a planet within a few hundred to a thousand light years away from Earth, then vast numbers of their signals must already have crossed Earth without being noticed, a new study finds.

Astronomers have searched the skies for signs of extraterrestrial technology for decades. These searchers since 1960 have scanned the Milky Way for radio waves, optical flashes, infrared heat and other technological signatures indicative of advanced aliens.

However, no confirmed technosignatures like artificial radio transmissions, laser flashes, or excess heat from large-scale alien projects, have ever been detected.

For such signals to be detected on Earth two things must happen, researchers say: First, the signal must physically reach Earth, and second, our instruments must be sensitive enough to detect it, pointed in the right direction, and able to distinguish it from natural signals.

Some theories hint that such an alien signal may have already passed by Earth during the past six decades but have gone unnoticed due to our instruments being weak.

Now, a new study challenges this assumption.

The Allen Telescope Array at Hat Creek Radio Observatory (Luigi Cruz via Eurekalert)

In the new study, scientists asked a simple but intriguing question: What if alien signals have already reached Earth, but we just didn’t notice?

To explore this, they built a computer model of hypothetical alien civilisations in the Milky Way, which sent out signals that travelled at the speed of light.

Some of these signals last only days, while others continue for thousands of years, and a signal would only be detected on Earth if it came from close enough for our telescopes to pick it up.

Using this simulation, scientists then estimated how many alien signals might have passed Earth in the past, how long such signals typically last, and how far our current or near-future instruments can realistically detect them.

The results were surprising.

If humans were likely to detect alien technosignatures from within a few hundred, or even a few thousand light years today, then a very large number of such signals must already have passed by Earth unnoticed.

The number of such signals that should have already passed by Earth is so “implausibly large” it exceeds the number of potentially habitable planets within this region of the galaxy, according to the study published in the The Astronomical Journal.

This means the existence of an advance alien civilisation in a planet within a range of few hundred or even a few thousand light years from Earth is highly unlikely, but not strictly impossible, scientists say.

However, this probability changes when the search for alien life extends farther out.

So the decades-long silence means if advanced alien civilisations do exist and have ever contacted Earth, they are more likely rare and much farther distant.

“These findings suggest that, if undetected past contacts from the Milky Way have indeed occurred, the best prospects of detection may lie in searches extending over several thousand light years,” researchers concluded.