Earth is not the only planet where lightning occurs, lightning has been detected on Venus, Jupiter, Saturn, Uranus, and Neptune
http://www.nasaimages.org/luna/servlet/detail/NVA2~4~4~7236~107762:Lightning-on-Jupiter?qvq=q:Sun%2BOR%2BMercury%2BOR%2BVenus%2BOR%2BMars%2BOR%2BJupiter%2BOR%2BSaturn%2BOr%2BUranus%2BOR%2BNeptune%2BOR%2BPluto;lc:NVA2~25~25,NVA2~57~57,NVA2~31~31,NVA2~60~60,NVA2~33~33,NVA2~26~26,NVA2~36~36,NVA2~62~62,NVA2~56~56,NVA2~55~55,NVA2~54~54,NVA2~45~45,NVA2~35~35,NVA2~53~53,NVA2~29~29,NVA2~27~27,NVA2~17~17,NVA2~46~46,NVA2~30~30,NVA2~44~44,NVA2~16~16,NVA2~47~47,NVA2~48~48,NVA2~61~61,NVA2~19~19,NVA2~52~52,NVA2~4~4,NVA2~1~1,nasaNAS~22~22,NVA2~20~20,nasaNAS~8~8,nasaNAS~10~10,NVA2~15~15,nasaNAS~13~13,nasaNAS~5~5,NVA2~18~18,NVA2~23~23,NVA2~8~8,nasaNAS~16~16,nasaNAS~2~2,NVA2~34~34,NVA2~14~14,nasaNAS~7~7,nasaNAS~6~6,NVA2~24~24,NVA2~13~13,nasaNAS~9~9,nasaNAS~4~4,NVA2~58~58,NSVS~3~3,NVA2~9~9,nasaNAS~20~20,nasaNAS~12~12,NVA2~21~21,NVA2~22~22,NVA2~49~49,NVA2~50~50,NVA2~51~51,NVA2~28~28,NVA2~43~43,NVA2~38~38,NVA2~32~32,NVA2~37~37,NVA2~39~39,NVA2~41~41,NVA2~42~42,NVA2~59~59&mi=19&trs=27920