Return to ‘Wow!’

The famous Wow! signal, picked up on August 15, 1977 at the Big Ear radio telescope (Ohio State University) is back in the news, with a new theory suggesting a source for the signal right here in the Solar System. Antonio Paris (St. Petersburg College, FL) asks us to consider a cometary origin for the signal, generated as two comets released hydrogen as they passed near the Big Ear’s search field. The now-dismantled telescope had a fixed field of view, so a bright signature at 21 centimeters — the hydrogen line — would have appeared short-lived.

Specifically, 21-cm refers to the line in the spectrum of neutral hydrogen atoms, a wavelength corresponding to 1420 megahertz associated with the most common element in the universe. It was back in 1959 that both Philip Morrison and Frank Drake fixed on the hydrogen line as a rational place to look for interstellar beacons, the assumption being that any civilization trying to reach another would choose a wavelength associated with some sort of universal constant.

Project Ozma grew out of this as Drake studied the nearby stars Tau Ceti and Epsilon Eridani at this wavelength, while Morrison, working with Giuseppe Cocconi, wrote the most famous paper in the history of SETI, ”Searching for Interstellar Communications,” which appeared in Nature in 1959 and is a fascinating read to this day (available online). Hence the interest of Jerry Ehman at Ohio State’s Big Ear, and the enthusiasm with which he wrote “Wow!” on the printout of the signal detected that day in 1977. Had we found an interstellar beacon?

The Cometary Hypothesis

We do know that the ‘Wow!’ signal’s intensity rose and fell over the same 72-second interval that the Big Ear itself could keep an object in its view — with a fixed field of view, the Earth’s rotation governed this. Hence Ehman could assume the signal had an origin in space, and Antonio Paris makes the same assumption. Scheduled to appear in the Journal of the Washington Academy of Sciences, the paper notes that the size of a comet’s hydrogen cloud is determined by the size of the comet, extending for as much as 100 million kilometers in width. The cloud increases significantly as the comet approaches the Sun. From the paper:

Since the rate of hydrogen production from the comet’s nucleus and coma has been calculated at 5 x 1029 atoms of hydrogen every second, the hydrogen cloud is the largest part of the comet. Moreover, due to two closely spaced energy levels in the ground state of the hydrogen atom, the neutral hydrogen cloud enveloping the comet will release photons and emit electromagnetic radiation at a frequency along the hydrogen line (1420.40575177 MHz).

Two comets are of interest. Looking back to 1977, Paris found that from July 27 to August 15, the Jupiter-family comets 266P/Christensen and P/2008 Y2 (Gibbs) were transiting near the Chi Sagittarii star group, placing them close to the source of the “Wow!” signal. Back to the paper:

Of significance to this investigation, the purported source of the “Wow” signal was fixed between the right ascension and declination values… of comets 266P/Christensen and P/2008 Y2 (Gibbs). On their orbital plane, moreover, 266P/Christensen was 3.8055 AU from Earth and moving at a radial velocity of +13.379 km/s; and P/2008 Y2 (Gibbs) was 4.406 AU from Earth and moving at a radial velocity of +19.641 km/s…

If the cometary hypothesis is correct, this would explain why subsequent searches using the Very Large Array and the Ohio State University Radio Observatory between 1995 and 1999 found nothing, for neither comet would then have been near the right ascension and declination values of the original signal. Paris suggests that the period of 266P/Christensen (6.63 years) and P/2008 Y2 (Gibbs) (6.8 years) can explain why the signal was never again detected.

The idea that the Wow! signal was produced from clouds of neutral hydrogen emanating from the two comets seems quite a stretch, but usefully, Paris offers a way to falsify the hypothesis. We learn that comet 266P/Christensen will again pass through the neighborhood of the “Wow!” signal on January 25, 2017, while comet P/2008 Y2 (Gibbs) will be in the area on January 7 of 2018. So we will have the opportunity to test the notion and analyze the hydrogen spectra of the two comets. Shouldn’t the Big Ear have picked up the same cometary signature 24 hours later? We can’t be sure, but scanning the hydrogen signal from each comet sounds like a good idea.

The paper is Paris and Davies, “Hydrogen Clouds from Comets 266/P Christensen and P/2008 Y2 (Gibbs) are Candidates for the Source of the 1977 “WOW” Signal,” accepted at the Journal of the Washington Academy of Sciences (abstract).