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All of the observed planetary orbits are ellipsoid. Is there any possibility that there will be a perfect circular orbit in this universe? Also what are the requirements for a circular orbit?

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    $\begingroup$ Perfect within what errors? $\endgroup$ – Qmechanic Feb 24 '17 at 10:09
  • $\begingroup$ Perfect within the quantum mechanical limitations. $\endgroup$ – ASCII Feb 24 '17 at 10:11
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    $\begingroup$ Obviously that's possible, it's just that the probability is very small (zero, in fact, in the continuum approximation). $\endgroup$ – Pirx Feb 24 '17 at 12:50
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If the word "perfect" should be taken in mathematical sense then the answer is no because of "statistical" reasons. A probability that a planet ends up on a perfectly circular orbit is zero, because perfectly circular orbit is one among an infinity of other orbits (roughly speaking).

If the question is: Does the physic laws allow for a perfectly circular orbit? The answer is yes, Newtonian mechanic and also general relativity allow for perfectly circular orbits. But planets gain their orbits in a complex process of star-system formation and it is improbable (impossible in mathematical sense) that a planet ends up on an exactly circular orbit.

Actually there is a mechanism that actively puts planets on circular orbits. Tidal forces change progressively planetary orbits and those orbits become more and more circular, finilly becoming very close to a circle, see "Tidal locking":

https://en.wikipedia.org/wiki/Tidal_locking

So in this sense all planets (which remain on some orbit) have their orbits progressively changed becoming more and more circular... arbitrary close to a circle when waiting long enough.

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