Spacecraft observations show that the presence of lightning activity is not confined to the terrestrial atmosphere, but is also found in the atmospheres of Venus, Jupiter and Saturn. Lightning activity may also occur in Titan's thick atmosphere. Calculations show that lightning produces a significant fraction of the nitric oxide that reacts with the ozone and chlorine compounds in the terrestrial stratosphere. In the atmosphere of the primordial Earth, lightning could have been the major source of many of the molecules required for the formation of life. To determine the effects of lightning activity in the atmospheres of other planets from spacecraft images requires a knowledge of the optical properties of the lightning discharge. Here we report the first simulations of lightning in planetary atmospheres by laser-induced plasmas. These simulations show that the fraction of the energy in lightning discharge channels that is radiated in the visible spectrum is similar for Earth, Venus and Titan, but quite different for Jupiter. One implication of our results is that the amount of trace gases produced by lightning in the jovian atmosphere must be larger than previously estimated.