Comparison of the distributions of period ratios of adjacent pairs of planets for a) observation and b)−d) simulations. The distribution of period ratios is presented as a histogram (see the left y-axis) and as a cumulative distribution (see the right y-axis). Panels b)−e) show the results of Models 1, 2, 3, and 4, respectively. The dashed lines in panels b)−d) represent the cumulative distribution of observed planets shown in red in panel a). The vertical lines indicate locations of mean motion resonances.