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I just saw How does orbital eccentricity affect positions of Lagrange points $L_4$ and $L_5$? and it questions the difference between circular and elliptical orbits.

We know the Moon does not have a circular orbit. Do we know of any natural bodies that are actually in a circular orbit? Is there any possible way for a circular orbit to occur naturally?

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    $\begingroup$ How do you define circular? Perfectly 100% circle according to x squared plus y squared equals value? Then I think that the answer will be "No". How much variation will you accept? $\endgroup$ – Hennes Nov 23 '13 at 15:22
  • $\begingroup$ Similar: physics.stackexchange.com/questions/69997/… $\endgroup$ – Everyone Nov 23 '13 at 17:27
  • $\begingroup$ Having conveyed that, I wonder whether A*.SE is possibly a better fit for the question $\endgroup$ – Everyone Nov 23 '13 at 17:28
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If my understanding in correct, the orbit is perfectly circular if the dimensionless orbital parameter of eccentricity (e) is zero. However, I'm not sure how the inclination works with this, and that comes down to the question "circular in what plane?" Anyway, I present to you:

Asteroid 113474 (2002 ST57).

http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=113474

Element     Value   Uncertainty (1-sigma)    Units 
e   9.338379075815981E-5    1.3966e-07

orbit diagram

Sadly, however, the uncertainty is sufficiently lower than the value itself to show that it does, in fact, have some eccentricity. If you are satisfied with any object that could theoretically be circular with our current understanding then that's easy. There are lots of objects with an uncertainty enough such that a perfectly circular orbit is possible. Actually, this turns up a huge number of candidates due to the simple fact that we don't know the the orbital parameters in the first place!

I argue that this above orbit is pretty darn circular. If you want a theoretically perfect circle, then the answer is probably "no". If you want further navel-gazing, I will question whether the concept of a circle is valid in non-Euclidean geometry. Since we have the spacetime distortion of Jupiter thrown in there, perhaps circles don't exist?

In case it wasn't obvious, my example was possible because I used a large sample set and just searched for a candidate that fit my criteria

Nonetheless, there is a very good reason that things are circular-ish. Our solar system's plane exists for very good physical reasons, which is that a collapsing gas cloud has an angular momentum value that it can't get rid of, and can't be held in a single body by gravity. Then, the planets are relatively circular, because if they were not, the solar system would not be as stable. Orbital resonances (which is what the long term dynamics depend upon) apply to relatively circular orbits, and like to keep things relatively circular. There are certain to be some exoplanet systems that more strongly favored circularizing the orbits. Because of that, I'm sure that one day we'll discover some exquisitely circular orbit. But that still doesn't make it a literal circle by the theoretical standard.

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  • $\begingroup$ A little late, but I just read Deimos also has a close to circular orbit with an eccentricity of 0.00033 $\endgroup$ – Everyone Oct 10 '14 at 16:24

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