The Wow! Signal: Aliens or just comets?

A composite of the Wow! Signal record, and the signal's apparent origin points in space. Credits: Ohio State University Radio Observatory/North American AstroPhysical Observatory (NAAPO), and Benjamin Crowell/Wikimedia Commons

Tuesday, January 12, 2016, 5:30 PM - First comets take away our alien megastructures, and now they may lay claim to one of the most enduring mysteries in the search for extraterrestrial intelligence - the Wow! Signal.

On August 15, 1977, SETI astronomer Jerry R. Ehman was tending the Big Ear radio telescope, at Ohio Wesleyan University's Perkins Observatory, when the instruments recorded a curious spike in the data at 10:16 p.m. EST.

In the simple alphanumeric code used by the telescope to record the intensity of the signals it received, the spike shows up as the number-letter sequence "6EQUJ5" which represented a signal around 30 times louder than the background "noise" of space.

Not only was the intensity eyebrow-raising, but the frequency of the signal - between 1420.356 MHz and 1420.4556 MHz - is very close to the frequency of radio waves that would be emitted by a hydrogen atom that was changing from an exited state to a neutral state. This was significant because it had already been speculated, years before, that this would be the perfect kind of signal for an alien civilization to send out as a cosmic "hey, we're here and we're intelligent!" message, as it implies a certain level of knowledge about the universe.

Excited by this find, Ehman circled it and wrote "Wow!" next to it in red pen.

The original Wow! Signal printout, along with a more modern graphic representation. Credits: The Ohio State University Radio Observatory and the North American AstroPhysical Observatory (NAAPO), and Maxrossomachin/Wikimedia Commons

In the over 38 years since then, no explanation has ever accounted for this signal.

Earth sources have been ruled out. It was not due to any satellite in orbit. There was not even a definite source out in space at the location it apparently originated from, such as a star or other kind of radio-emitting object.

Ever cautious about jumping to conclusions, SETI astronomers have kept this signal in their case files, waiting for repeat occurrence, but have not gone so far as to officially say that it was aliens. The best guess as to its source, however, is that it could ... maybe ... have been a lone signal from some alien civilization out in space.

A new study may have the answer to this mystery, though.

According to a paper penned by astronomer Antonio Paris and space science writer Evan Davies, it wasn't aliens. It was comets.

A comet is essentially a giant lump of frozen water, carbon dioxide and other gases, that is typically coated in dust and includes (probably in most cases) a certain amount of rock as well. As these objects get close to the Sun, they produce tails and a wide coma of gases - composed mainly water vapour, carbon dioxide, carbon monoxide and oxygen.

Comet Lovejoy captured on November 19, 2013. Credit: NASA/MSFC/Jacobs Technology/ESSSA/Aaron Kingery

The water molecules, bombarded by solar radiation, emit electrons which split apart other water molecules into energized oxygen and hydrogen atoms, and the light hydrogen atoms spread out into a wide halo around the comet.

DID YOU KNOW? The hydrogen halos surrounding some comets are enormous! According to data collected from Comet Kohoutek in 1977, the comet's hydrogen halo was over 10 million kilometres wide - around 8 times bigger than the Sun!

Since these hydrogen atoms "prefer" to be in a neutral state, they release their excess energy in the form of radio waves, at a wavelength of 21 centimetres, which corresponds to a frequency of 1420.40575177 MHz.

Thus, according to Paris and Davies, the large hydrogen cloud from a comet could be responsible for the "hydrogen line" detection on August 15, 1977.

Were there any comets in that part of the sky on that date? Yes!

Although they weren't actually discovered until 2006 and 2008, respectively, comets 266P/Christensen and P/2008 Y2 (Gibbs) were both traced back to that area of space at that time. Therefore, the combined hydrogen clouds from these two comets could be responsible for this signal.

According to New Scientist, not everyone agrees about comets being a potential source of the hydrogen line signal:

Some researchers are sceptical, saying it isn't clear the comets would release enough hydrogen to generate something like the Wow! signal. James Bauer of the Jet Propulsion Laboratory in Pasadena, California, agrees that the hydrogen from comets can extend quite far, but still thinks the signal won't be strong enough. "If comets were radio-bright at 21 centimetres, I would be puzzled as to why they aren't observed more often at those wavelengths," he says.

Although it's been nearly four decades since the Wow! Signal was recorded, astronomers will only have to wait a few years more to test out this claim, as Comet 266P/Christensen will be passing through that section of space again on January 25, 2017 and Comet P/2008 Y2 (Gibbs) will fly by there on January 7, 2018.

"During this period," Paris and Davies wrote, "the astronomical community will have an opportunity to direct radio telescopes toward this phenomenon, analyze the hydrogen spectra of these two comets, and test the authors' hypothesis."

Sources: Washington Academy of Sciences (pdf) | ESA Rosetta | New Scientist

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