Astronomers are always interested in enigmatic signals coming from the dark reaches of space, particularly if they can’t initially explain their origin. Some of these powerful signals come from the heart of galaxies, including our own, but some are emitted by exoplanets, worlds far from our humble Solar System.

So when a team of scientific stargazers from France detected a radio signal coming from a distant mini-Neptune, an ice giant-like world 26 times more massive than our own world, they were understandably intrigued. Although it was incredibly unlikely to be generated by techno-savvy aliens, a definite cause could not be attributed to it. A second attempt to locate the signal ended in failure after it mysteriously disappeared.

Now, as revealed in follow-up work by astronomers at the University of St. Andrews’ School of Physics and Astronomy, this energy reading was probably caused by a phenomenon rather familiar to us, but far grander in scale. Based on a series of mathematical calculations, the most likely culprit behind the somewhat weak radio signal was a planet-wide lightning storm.

“We assumed that this signal was real and was coming from the planet,” Gabriella Hodosán, a Ph.D. student at the university and lead author of the study, said in a statement. “Then we asked the question: could such a radio signal be produced by lightning in the planet's atmosphere, and if yes, how many lightning flashes would be needed for it?”

^{This exoplanet wouldn't be a pleasant place to live. HelenField/Shutterstock}

Lightning is inarguably energetic. There are 40 to 50 lightning strikes somewhere around the world every single second, which means there are around 1.6 billion every year, releasing a total of 16 quintillion joules of energy. That’s equivalent to 254,000 “Little Boy” atomic bombs.

The researchers, writing in the Monthly Notices of the Royal Astronomical Society, reasoned that a significant amount of lightning strikes on a far-flung world could generate enough energy to make it noticeable across the cosmos. This particular exoplanet, HAT-P-11b, is about 122 light-years from Earth, so in order to generate a detectable radio signal this far away, it would have to have been generated by a pretty humungous lightning storm.

Observations of the planet several years after the signal was detected show that plenty of hydrogen cyanide, something astronomers would expect to see in the event of a powerful thunderstorm, still lingers in its atmosphere. The team calculated that it would have taken 53 powerful flashes of lightning per square kilometer across at least half of the entire exoplanet to generate this much hydrogen cyanide.

This gargantuan storm would have been 530 times more energetic than typically dense storms found within the U.S., and several orders of magnitude more powerful than those observed on Saturn. The researchers conclude that a storm like this could easily have generated the radio signal seen coming from the exoplanet back in 2009.

“In the future, combined radio and infrared observations may lead to the first detection of lightning on an extrasolar planet,” Hodosán notes. “The importance of the study is not just this prediction, but it shows an original scenario for the explanation of radio emission observable on extrasolar planets.”