A powerful radio signal received from space in 1977 and never explained may have been the result of a then-unknown comet, not extraterrestrials, an astronomer proposes in a new paper.

The "Wow! signal" is named after what astronomer Jerry Ehman wrote ("Wow!") next to the paper readout of the radio telescope that recorded it. The signal was very powerful and emanated briefly from a single point in the sky, right at a wavelength many have suggested would be a natural one for extraterrestrial life to transmit in: one indicating the presence of water.

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But Antonio Paris at St. Petersburg College in Florida has found that two comets, yet to be identified in 1977, were in just the right position, and due to their halo of hydrogen might have given off energy in the right wavelength.

Neither skeptics nor hopefuls should get excited yet, though: Paris' proposal isn't that the comets are definitely the cause, but rather that they are a possibility that has never been ruled out. Little is known about 266P/Christensen and P/2008 Y2 (Gibbs), as the comets are (awkwardly) named, so Paris suggests we keep an eye on them.

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The best chances to do so are in January 2017 for one and a year later for the other — at which point we'll be able to rule them out or close the book on this long-lived cosmic mystery.