It’s SETI’s most famous mystery.
The Wow! signal was a strong, narrow-band radio signal in the frequency range of 1420 MHz discovered in 1977 by radio astronomer Jerry Ehman after reviewing recorded data from Ohio State’s Big Ear Telescope. It has remained the best evidence to date for a signal that might have come from an extraterrestrial civilization. Now that interpretation is being called into question.
In a recent re-analysis of the Wow! signal, Antonio Paris from the Center for Planetary Science at St. Petersburg College in Florida provides further evidence that the signal was most likely caused by a natural source such as a comet.
Paris and co-author Evan Davies suggested in a paper published last year that the comet 266/P Christensen, which was discovered nine years after the Wow! signal, was in the celestial vicinity of the signal at the time it was detected, and might in fact have been the source. They reached that conclusion after extrapolating the comet’s trajectory back to 1977. For the new study, Paris conducted more than 200 observations between November 2016 and February 2017 with a 10-meter radio telescope. He found that various natural sources have a strong signal at 1420 MHz, and that the peak intensity of Comet 266/P Christensen is particularly strong. When the telescope was moved 1 degree away from the comet, no signal could be observed. But when he moved back to the target, the radio signal could again be detected at the same frequency. His conclusion: The Wow! signal came from a natural source rather than from an extraterrestrial civilization.
Advocates of an artificial source point to the extraordinary strength of the 1977 signal. Paris, however, thinks that could be because comet 266/P Christensen had much more mass 40 years ago, which it has subsequently lost. What is more difficult to explain—and what Paris does not discuss—is why Ehman and his colleagues could not find the Wow! signal again when they pointed the Big Ear Telescope at least 50 more times in the same direction. The comet and its signal should have been measurable for days, and perhaps weeks, after the initial detection. Also, the bandwidth of the 1977 signal was extremely narrow—about 10 kHz—which is difficult to explain with a natural source, but would be more consistent with a strong, narrow-band artificial transmitter. But the short duration of the detection, 72 seconds, was due to technical limitations of the Big Ear Telescope, and so is consistent with both explanations.
For most of us, an artificial source of the signal, such as the one William Bains suggested in 2015, would be much more exciting. Even though the new study makes a strong case for a natural source, this is probably not the last time we’ll hear about the Wow! signal. And perhaps the real lesson is that the 1420 MHz band emitted naturally by hydrogen might not be the best frequency to search for messages from extraterrestrial civilizations, given that there are many natural sources in that window.