Significance Planets and stars contain matter at extreme pressures and temperatures hidden deep beneath their opaque surfaces. Unable to see these states of matter directly, we instead produce them in laboratory experiments. Here a novel method of studying extreme states in a tabletop experiment is described and applied to common planet- and star-forming materials, the noble gases. Helium, neon, argon, and xenon transform in the experiments from transparent electrical insulators to opaque electrical conductors. In Saturn, rain composed of noble gas becomes conductive as it falls and can form a protective layer around the planetary core that prevents the core from dissolving into surrounding metallic hydrogen. White dwarf stars have unexpectedly opaque helium atmospheres, causing them to age slower than anticipated.