Is lightning a possible source of the radio emission on HAT-P-11b?

TL;DR

This study evaluates whether lightning could produce the observed radio emission from exoplanet HAT-P-11b, concluding it is unlikely but suggesting future combined radio and infrared observations could detect extraterrestrial lightning.

Contribution

The paper models lightning-induced radio emission on HAT-P-11b and assesses its plausibility as the source of observed signals, introducing constraints based on lightning frequency and chemical signatures.

Findings

Lightning would need to be 3.8 million times more frequent than Earth's largest storms.

Optical emission from such storms would rival the brightness of the host star.

Lightning-produced HCN could be detected 2-3 years after storms in infrared bands.

Abstract

Lightning induced radio emission has been observed on Solar system planets. There have been many attempts to observe exoplanets in the radio wavelength, however, no unequivocal detection has been reported. Lecavelier des Etangs et al. carried out radio transit observations of the exoplanet HAT-P-11b, and suggested that a small part of the radio flux can be attributed to the planet. Here, we assume that this signal is real, and study if this radio emission could be caused by lightning with similar energetic properties like in the Solar system. We find that a lightning storm with 3.8 x $10^6$ times larger flash densities than the Earth-storms with the largest lightning activity is needed to produce the observed signal from HAT-P-11b. The optical emission of such thunderstorm would be comparable to that of the host star. We show that HCN produced by lightning chemistry is observable 2-3 yr after the storm, which produces signatures in the L (3.0-4.0 \mu m) and N (7.5-14.5 \mu m) infrared bands. We conclude that it is unlikely that the observed radio signal was produced by lightning, however, future, combined radio and infrared observations may lead to lightning detection on planets outside the Solar system.