> They're really not stable in two dimensions? Stable is not really the right word. "Well behaved" is better. In 2D all orbits are stable in the sense that e

> They're really not stable in two dimensions? Stable is not really the right word. "Well behaved" is better. In 2D all orbits are stable in the sense that escape velocity is infinite, but a consequence of that is that you don't naturally get nice (nearly) circular orbits that keep planets in habitable zones. That only happens in 3D.