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CIRCULAR ORBITS: THE PLANETARY NORM ?
The occurrence of circular orbits may require special initial conditions. More common initial conditions may lead to gravitational perturbations of planes by other planets or by the protoplanetary disk, leading to orbital ellipticities or ejection. Perhaps our Solar System, with its coplanar, nearly circular orbits, represents a fortuitous unperturbed, low-entropy state for a planetary system.
The circular orbit of Jupiter in our Solar System promotes the stability of circular orbits among the other 8 planets. If our Jupiter were in an eccentric orbit, the Earth and Mars would likely be gravitationally scattered, perhaps out of the Solar System. Thus an anthropic argument can be made for Jupiter's circular orbit, if it affects the onset or the evolution of biology on Earth. It remains a question of molecular and evolutionary biology regarding the necessity of circular orbits and the resulting nearly uniform temperatures for life.
Eccentric orbits may occur relatively commonly in extrasolar planetary systems. The second law of thermodynamics suggests that orbits, once scrambled, will remain so. While an eccentric giant planet would certainly induce dynamical dominoes for terrestrial planets, the supposed demise of life may be a circular argument.
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