It turns out violent lightning storms that are millions of times more powerful than here on Earth may be guilty for the unexplained radio signals from planets.
French astronomers, in 2009, observed what they believed to be a weak radio signal coming from the exoplanet HAT-P-11b, a “mini-Neptune” which is about five times larger in size than Earth and 26 times more massive. The French team attempted to locate the signal again the following year; however they were unsuccessful, which left the phenomenon unexplained.
A team from the University of St Andrews set to solve this so far unexplained mystery. PhD student, Gabriella Hodosán from the Life, Electricity, Atmosphere, Planets (LEAP) Project, and who is leading the study, said in a statement:
“We assumed that this signal was real and was coming from the planet. 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?”
If we assume that the physics of lighting is the same for all solar system planets, the researchers found that 53 lightning flashes of Saturnian lightning-strength in a km² per hour would explain the observed radio signal on HAT-P-11b, according to the university. LEAP researcher and co-author of the paper, Dr. Paul Rimmer, said:
“Imagine the biggest lightning storm you’ve ever been caught in. Now imagine that this storm is happening everywhere over the surface of the planet. A storm like that would produce a radio signal approaching 1 percent the strength of the signal that was observed in 2009 on the exoplanet HAT-P-11b.”
“Such enormous thunderstorms are not unreasonable.
“Studies conducted by our group have also shown that exoplanets orbiting really close to their host star have very dynamic atmospheres, meaning that they change continuously, producing clouds of different sizes, even whole cloud systems, all over the planet’s surface.
“HAT-P-11b, being so close to the star, is likely to have such a dynamic, cloudy atmosphere, which would allow the formations of huge thunderclouds, focusing the lightning activity to a certain regime of the planetary surface, such as the face of the planet, which was observed in 2009.”
The team was hoping to be able to observe the lightning with optical telescopes; however the powerful light emissions from the star HAT-P-11b orbits made it difficult.
Because lightning discharges involve plasma processes at extremely high temperatures, which then release a huge amount of energy, it results in chemical reactions in the atmosphere that would not otherwise occur. These chemical reactions produce molecules that can then be used as lighting tracers.
The team considered whether such enormous thunderstorm clouds produce these tracer molecules, which then could be observed by Earth-telescopes, and suggested hydrogen cyanide (HCN) to be such a potential fingerprint of lightning. This molecule could be observable in the infrared spectral band; even years after the huge storm on HAT-P-11b would have occurred, the university wrote.
Hodosán added that:
“In the future, combined radio and infrared observations may lead to the first detection of lightning on an extrasolar planet.
“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.”
Dr Christiane Helling, the LEAP Project principal investigator, said:
“With all necessary caution, linking extraterrestrial lightning and radio emissions will open a new window to prove the presence of atmospheres and of clouds on extrasolar planets, both being essential for the existence of life as we know it.”
The study titled: “Lightning as a possible source of the radio emission on HAT-P-11b” was published in MNRAS.