Almost 40 years ago, we received a mysterious signal from space. The signal lasted for 72 seconds and it was so unusual that it was dubbed the Wow! Signal, owing to the word scrawled by its discoverer. It has been puzzling scientists ever since. Several explanations from unusual phenomena, transient events, and even aliens have been put forward. Now a Florida professor might have found a different explanation.

Antonio Paris, a professor of astronomy at St Petersburg College in Florida, thinks the signal might have originated from one or two passing comets. The objects were not known at the time, but their orbit and position in 1977 was remarkably close to the location the Wow! Signal seemed to have originated from.

The Big Ear telescope in Ohio, which made the original discovery, had a fixed field of view, so it depended on Earth’s rotation to span the sky; it could only observe any given area for 72 seconds. On August 15, 1977 it was looking in the direction of the Chi Sagittari star group when it detected the signal. It was clearly of extraterrestrial origin and definitely worth follow-up observations.

The same region has been observed several times since, but the observation has never been replicated. For this reason, some researchers suggested that it was a one-off event, something passing in the area of the sky at the time. This made Paris think of the comets 266P/Christensen and P/2008 Y2 (Gibbs). “I came across the idea when I was in my car driving and wondered if a planetary body, moving fast enough, could be the source,” he told New Scientist.

The signal was observed at 1420 MHz. The telescope used that frequency to observe neutral hydrogen, which can emit at that wavelength. Paris claims that the two comets release a lot of water and the UV light from the Sun breaks the water, liberating the hydrogen.

Some researchers are more skeptical. Comets would have to release a significant amount of hydrogen to produce a signal as strong as the Wow! Signal. Talking to New Scientist, James Bauer of the Jet Propulsion Laboratory in Pasadena, California, said: “If comets were radio-bright at 21 centimetres [the wavelength the signal was observed at], I would be puzzled as to why they aren’t observed more often at those wavelengths.”

“The hypothesis must be tested before it is ruled out,” added Paris, and he’ll get his chance in the next few years. Comet 266P/Christensen will be back in that region on January 25, 2017, and P/2008 Y2 (Gibbs) on January 7, 2018. By studying their radio emission and how quickly they move in the sky, astronomers should be able to tell if it really was this that produced the Wow! Signal.

[H/T: New Scientist]