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The nucleosynthetic site of the rapid ($r$) neutron-capture process iscurrently being debated. The direct detection of the neutron star mergerGW170817, through gravitational waves and electromagnetic radiation, hasconfirmed such events as important sources of the $r$-process elements.However, chemical evolution models are not able to reproduce the observedchemical abundances in the Milky Way when neutron star mergers are assumed tobe the only $r$-process site and realistic time distributions of such eventsare taken into account. Now for the first time, we combine all the availableobservational evidence of the Milky Way and its dwarf galaxy satellites to showthat the data can only be explained if there are (at least) two distinct$r$-process sites: a quick source with timescales comparable to core-collapsesupernovae, $t_{quick}\lesssim10^8$ yr, and a delayed source withcharacteristic timescales $t_{delayed}\gtrsim4$ Gyr. The delayed $r$-processsource most probably originates in neutron star mergers, as the timescale fitswell with that estimated for GW170817. Given the short timescales of the quicksource, it is likely associated with massive stars, though a specificfast-track channel for compact object mergers cannot be excluded at this point.Our approach demonstrates that only by looking at all the available data willwe be able to solve the puzzle that is the $r$-process.

Evidence for $\gtrsim{4}$ Gyr timescales of neutron star mergers from Galactic archaeology