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In just 25 years comma we went from not knowing if the solar system is a fluke of Nature to realising that it is totally normal for stars to have planets. More remarkably comma it is now clear that planet formation is a robust process comma as rich multi-planet systems are found around stars more massive and less massive than the Sun. More recently comma planetary systems have been identified in increasingly complex architectures comma including circumbinary planets comma wide binaries with planets orbiting one or both stellar components comma and planets in triple stellar systems. We have also learned that many planetary systems will survive the evolution of their host stars into the white dwarf phase. Small bodies are scattered by unseen planets into the gravitational field of the white dwarfs comma tidally disrupt comma form dust discs comma and eventually accrete onto the white dwarf comma where they can be spectroscopically detected. HST-COS has played a critical role in the study these evolved planetary systems comma demonstrating that overall the bulk composition of the debris is rocky and resembles in composition the inner the solar system comma including evidence for water-rich planetesimals. Past observations of planetary systems at white dwarfs were limited to biased and incomplete samples. Here we propose a legacy HST survey of all white dwarfs within 100pc identified with Gaia to answer the following questionsdoublePoint * How efficient is planet formation around 2-10Msun starsquestionMark * What are the metallicities of the progenitors of debris-accreting white dwarfsquestionMark * What is the fate of circumbinary planetsquestionMark * Can star-planet interactions generate magnetic fields in the white dwarf hostquestionMark