Hamilton, D.P. 1998. Circular Problems. Nature 396, 413.


Probably the most striking regularity observed in our Solar System is that the planets all follow nearly circular, nearly coplanar orbits about the Sun. This feature lends strong support to the theory that the planets accreted out of the solar nebula - a flattened disk of gas and dust that surrounded the young Sun. But do flattened disks of dust and gas inevitably lead to a few massive widely-spaced planets on circular uninclined orbits? In the case of the inner Solar System, results from recent numerical simulations* suggest that this may not be the case.
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