Observations of planet-hosting stars reveal that multi-planetary systems are not rare, hosting over 1600 confirmed planets (panel a). The period ratio distribution of neighbouring planets is shown in panel b. One can observe an overall broad distribution as well as a number of peaks slightly wide of resonant ratios, especially the 2:1 and 3:2 commensurabilities. Data was obtained from the Nasa Exoplanet Archive (https://exoplanetarchive.ipac.caltech.edu/). Panel a: histogram of multi-planetary system by number of planets in each system. Panel b: distribution of period ratios of neighbouring planets in exoplanetary systems.