Just before midnight on August 15, 1977, a scientist spotted an unusual radio blast while scanning the stars for possible signs of extraterrestrial life as part of NASA's SETI Project. The 72-second blast, detected by Dr. Jerry Ehman through a radio telescope, seemed to be coming from Sagittarius but matched no celestial object. The signal was so strange that Ehman circled the signal on a readout and scribbled "Wow!" next to it.

The "Wow! signal," as it would come to be known, has since become one of the biggest mysteries in the scientific world and the best evidence so far of alien life.

At least, until now.

Professor Antonio Paris, an astronomer at St Petersburg College in Florida, believes that the signal wasn't from aliens – but was in fact two comets passing Earth on that very day.

After going back through the records of that night Paris found that comets 66P/Christensen and P/2008 Y2 (Gibbs) were both in the area where the Wow! signal was originally detected. He suggested that a cloud of hydrogen gas in the wake of the comets triggered the powerful signal.

So to prove his theory, he waited for the comets to fly past Earth again this year as part of their orbits around the Sun every 7 years or so. He saw that the comets produced the same kind of strong signal as the one detected by Ehman in 1977, putting an end to the mystery that perplexed scientists and conspiracy theorists for the past 40 years.

Paris also pointed out that the comets had only been discovered in 2006, and therefore were not accounted for during the signal emission. And even if it wasn't these exact comets, it's highly likely that another comet is the culprit.

The Wow! signal was detected at 23:16:01 on 15 August 1977 with a radio telescope known as Big Ears at the Ohio State University Radio Observatory in Delaware.