Impulsive excitation of the orbits by an encounter with the 2:1 mean motion resonance. This figure depicts the results of two numerical experiments where a Jupiter-mass planet and a Saturn-mass planet encounter a 2:1 mean motion resonance. Panel A) shows the evolution of the eccentricities while panel B) shows the evolution of the difference in the perihelia. In one experiment (labeled JS), Jupiter is placed on the inner orbit. In the other numerical experiment (labeled SJ), Saturn resides closer to the Sun. Although the extent of eccentricity excitation differs between the two runs considerably, the post-encounter apsidal alignment is clearly evident in both cases.