  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.
