Around dying stars, smashed asteroids stop at a kilometre — they do not vanish into dust

When an asteroid wanders too close to a white dwarf — the dense ember of a dead star — gravity shreds it. But new modelling from the Planetary Science Institute, published in The Astrophysical Journal Letters, finds the shredding has a floor: most of the debris stays in chunks between a tenth of a kilometre and a kilometre across, rather than grinding straight to dust. That 'kilometre-size barrier' changes how astronomers read the dusty rings around dead stars — which are, in effect, autopsies of planetary systems, showing what other worlds were made of.