Extraterrestrial lightning and its past and future investigation

All extraterrestrial lightning detected so far on the planets Jupiter, Saturn, Uranus, Neptune, and potentially at Venus is attributed to intracloud lightning. On Mars other types of discharges might exist within dust storms. In this commentary we will focus on the historical development of planetary lightning investigations at Jupiter, Saturn, and Venus. Special attention will be devoted to the controversies around the existence of lightning on Venus. Thereby we will highlight the importance of combined optical and radio observations for the epistemological degree of confidence in lightning detection from space as well as for its detailed investigation. Future spacecraft missions to various planets should take this into account. The main future extraterrestrial lightning investigations will be outlined. We also plead for radio observations with a time resolution of the order of microseconds and for more modelling and laboratory work in this field.