Most accreted xenon on Earth is missing from its atmosphere, relatively to lighter noble gases, and half of the argon budget might be missing too. These observations point to their early escape from the atmosphere, or retention at depth. Although chemists have been exploring xenon reactivity since the 1970s, recognition of its potential occurrence in planetary interiors is only just beginning. This talk will review the reactivity of heavy noble gases under the extreme pressures found in planetary interiors, how we can probe it, and why this matters to understand planetary processes leading to Earth’s habitability.