ENORMOUS lightning storms millions of times more powerful than those on Earth could be behind radio signals from other planets, scientists claim.

Eight years ago French astronomers observed what they thought was a weak radio signal from exoplanet HAT-P-11b, a “mini-Neptune” five times bigger than Earth. Subsequent attempts to trace the signal failed, leaving the phenomenon unexplained.

Now a team from St Andrews University claims huge thunderstorms could have sent out an electrical signal detectable from Earth.

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 planet’s surface.

“A storm like that would produce a radio signal approaching one per cent the strength of the signal that was observed in 2009 on the exoplanet HAT-P-11b.”

The snappily-named planet lies around 124 light years – a mere 729 trillion miles – from Earth in the Cygnus constellation and in 2014 scientists detected water molecules on its surface. The signature of water vapour was discovered around the planet by Nasa telescopes.

Assuming the physics of lightning is the same on HAT-P-11b as on Earth, St Andrews researchers believe the radio signal would be produced in conditions of 53 “Saturn-strength” bolts in a km2 per hour. Such conditions are unimaginable on Earth, but lead researcher Gabriella Hodosán said there is sound scientific basis for the claim. However, the PhD student is clear that the signal is unlikely to emanate from extra-terrestrial attempts to contact humanity.

She said: “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 formation 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.”