UPDATE: The astronomers who detected WOW! in the first place have completely refuted the findings of this paper. An account of their rebuttal can be read here. 

For 40 years, the “Wow!” signal has fascinated and intrigued people worldwide. From an alien message to an unknown phenomenon, scientists and laypeople have been trying to find a solution to the mystery. Now, Antonio Paris seems to have found a solution. It was a comet all along.

The research, published in the Journal of the Washington Academy of Sciences, describes how comet 266/P Christensen has the right emission and was in the right position to produce the signal. Paris and the Center for Planetary Science conducted 200 observations between November 2016 and February 2017 in order to test the hypothesis they made last year.

The signal was detected on August 15, 1977, by the Big Ear radio telescope, which was the Ohio State University Radio Observatory at the time. The signal was more intense than anything recorded and it peaked at a frequency of 1,420 megahertz, which is a known hydrogen emission. It earned its name thanks to its discoverer, Jerry R. Ehman, who scribbled "Wow!" next to the computer print-out. This would typically be an intriguing but not puzzling discovery. The mystery started when researchers looked at the same region of the sky and didn’t find anything that could have produced it.

The signal was either a one-off emission or it was being constantly emitted and whatever emitted it had moved. There were two comets in the region in 1977, Comet 266P/Christensen and P/2008 Y2 (Gibbs), and Paris considered them both likely candidates. Comet 266P/Christensen was back in the region last January 25, so Paris took the chance to observe the object for a signal at the right wavelength. And he did see the signal.

"The importance of the discovery is primarily that we were able to track and detect a comets signature with a radio telescope. This is an important step in radio astronomy … because now we can trace a 1420 MHz signal to local celestial bodies," Paris told IFLScience.

During the observations, Paris made sure there were no other alternative radio sources within 15 degrees of the comet. Considering that the full Moon is about half of a degree, that’s a big chunk of the sky. The comet was detected emitting radio waves, and as a further test, Paris moved the 10-meter (33-foot) telescope 1 degree away from the comet and the signal vanished.

The researcher also picked three comets at random from the JPL Small Bodies catalog – P/2013 EW90 (Tenagra), P/2016 J1-A (PANSTARRS), and 237P/LINEAR) – and all three objects have an emission at 1,420 megahertz. The team will observe the Gibbs comet when it's in the region next year.