We point out that dark matter (DM) can illuminate planetary skies via ultraviolet airglow. Dark matter annihilation products can excite molecular hydrogen, which then deexcites to produce ultraviolet emission in the Lyman and Werner bands. We search for this new effect by analyzing nightside ultraviolet radiation data from Voyager and New Horizons flybys of Neptune, Uranus, Saturn, and Jupiter. We set new constraints on the dark matter-nucleon scattering cross section for DM above the electron mass; these extend down to ${10}^{-40}{\mathrm{cm}}^2$ for DM masses around 1 GeV when most of the energy is deposited in the atmosphere. We highlight that future ultraviolet airglow measurements of Solar System planets or other worlds provide a new dark matter discovery avenue.

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• Dark matter
• Particle astrophysics
• Particle dark matter
• Planetary atmosphere
• Planets & planetary systems