Large sprite discharges at high atmospheric altitudes have been found to be physically similar to small streamer discharges in air at sea level density. Based on this understanding, we investigate possible sprite discharges on Venus or Jupiter-like planets through laboratory experiments on streamers in appropriate CO2-N2 and H2-He mixtures. First, the scaling laws are experimentally confirmed by varying the density of the planetary gasses. Then, streamer diameters, velocities, and overall morphology are investigated for sprites on Venus and Jupiter; they are quite similar to those on Earth, but light emissions in the visible range are fainter by 2 orders of magnitude. The discharge spectra are measured; they are dominated by the minority speciesN2 on Venus and by themajority species H2 on Jupiter-like planets. The spectrum of a fully developed spark on Venus is measured. We show that this spectrum is significantly different from the expected sprite spectrum.