A planetesimal orbiting within the debris disc around a white dwarf star

Many white dwarfstars show signs of having accreted smaller bodies, implying that they may host planetary systems. A small number of these systems contain gaseous debris discs, visible through emission lines. We report a stable 123.4-minute periodic variation in the strength and shape of the Ca ii emission line profiles originating from the debris disc around the white dwarfSDSS J122859.93+104032.9. We interpret this short-period signal as the signature of a solid-body planetesimalheld together by its internal strength.