Viscous stirring for equal-mass planetesimals. The figure shows root mean square eccentricity (top panel) and inclination (bottom panel) of a ring of equal-mass planetesimals with mass 1 × 10²⁴ g. The ring is centred at 1 au and the surface density is 10 g cm⁻². Shown are the results of our model: the close-encounter approach (orange), a full N-body simulation with mercury (grey), and the semi-analytic toy model of Ohtsuki et al. (2002; black dashed).