Why aren't planet orbits circular? Please forgive the simplicity of my question.  I have only had basic science courses.  Common sense seems to indicate that planetary orbits should be very close to circular, any perturbations having been eliminated over the ages.  Could it be that the composition of the planet be influencing the pull of gravity to produce a different gravitational attraction due to its mass?  I mean that if a planet were composed all of one substance, would the planetary orbit be perfectly circular?  I think about earth having constantly shifting water and gasses and even solids that affect its own rotation.  Might these shifts also affect the orbit of the planet around the sun, or does the total mass of the earth and moon together determine the orbit?
They are close to circular, although we might quibble about the meaning of "close". Except for Mercury, all planetary orbits have eccentricities below 0.1. That's close enough to a circle to actually pass for a circle at a superficial examination.


Mercury 0.2056
Venus   0.0068
Earth   0.0167
Mars    0.0934
Jupiter 0.0484
Saturn  0.0542
Uranus  0.0472
Neptune 0.0086


Check out these ellipses. The first one has an ecc. of 0.1. Looks like a circle, doesn't it? Even the second one (ecc = 0.2) kind of looks like a circle, too, if you don't examine it too long.



As for why they are not exactly circular, down to the last digit? There will always be perturbations that will squish perfect circles a little. Planets interact with each other, and pull each other off perfect circular orbits. It's very unlikely that in a complex system like ours you'll find highly circular planetary orbits.

But, again, the differences between the current planetary orbits and perfect circles are for the most part quite tiny, and you should think of them as "basically circles", unless you're an astronomer or a NASA engineer.

Comets, OTOH, those do tend to move on narrow elliptic orbits.