In August 1977, the Ohio State University Radio Observatory picked up a radio transmission from the Sagittarius constellation that was so strong it inspired the astronomer who discovered it to write "Wow!" in the margin of the data printout. Almost 40 years later, researchers from the Center for Planetary Science may have finally solved the mystery of the Wow! Signal's origin, and it's bad news for alien hopefuls: it was probably a comet.

At the time the signal was spotted, Ohio State's "Big Ear" observatory was specifically searching for transmissions that could be evidence of extraterrestrial civilizations. Based on the work of earlier astronomers, the team determined that a message of intelligent origin would most likely be transmitted at a frequency of 1,420 MHz – the electromagnetic frequency of hydrogen – and that the Big Ear would "hear" it for 72 seconds, since that's how long the observatory could focus on one specific point in space.

The Wow! signal was the first and only time exactly those criteria were met. Excited scientists tried training instruments on that region of space again, but the signal was never again recorded. Over the years, interference from Earth was ruled out, as were stellar bodies like planets, stars and asteroids. Aliens seemed unlikely, but tantalizingly, couldn't be ruled out.

Last year, a group of researchers from the Center of Planetary Science proposed a new hypothesis that argued a comet might be the culprit. The frequency could be caused by the hydrogen cloud they carry, and the fact that they move accounts for why it seemingly disappeared. Two comets, named 266/P Christensen and P/2008 Y2 (Gibbs), happened to be transiting through that region of space when the Wow! signal was detected, but they weren't discovered until after 2006.

To test the hypothesis, the team made 200 radio spectrum observations between November 2016 and February 2017. Sure enough, 266/P Christensen was found to emit radio waves at a frequency of 1,420 MHz, and to double check, the researchers moved their radio telescope by one degree. As expected, the signal vanished, and only returned when the telescope was trained back on the comet.

And 266/P Christensen wasn't an anomaly: the researchers tested three other comets, P/2013 EW90 (Tenagra), P/2016 J1-A (PANSTARRS), and 237P/LINEAR, and found they all emit signals at the same frequency.

Whether the Wow! signal was caused by 266/P Christensen or another comet, it seems that one of the most enduring astronomical mysteries has been put to rest.

The research was published in the Journal of the Washington Academy of Sciences.