If the net momentum isn't 0, the system will drift off of the screen. Notice in the first example, that the first particle has 10x more mass than the second planet, so it has a 10x slower velocity in the opposite direction.
Disregarding a few exceptions, two-body systems are always stable.
Disregarding a few exceptions, n-body systems (n > 2) are always unstable and will degrade into a two-body system.
