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Are the orbits of inner-solar system planets more circular than outer planets? Or is it the other way around? What's the reason for this? We were taught in our high school Physics class that outer planets had more circular orbits, but some sources online and even on SE state otherwise.

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The degree to which an orbit deviates from a perfect circle is measured by its orbital eccentricity. An eccentricity of $0$ is a perfect circle; an ellipse has an eccentricity between $0$ and $1$ - the higher the eccentricity, the more "elliptical" the ellipse becomes; an eccentricity of $1$ is an open parabolic orbit and an eccentricity greater than $1$ is an open hyperbolic orbit.

According to Wikipedia the current orbital eccentricities of the planets of the solar system are:

  • Mercury $0.2056$
  • Venus $0.0068$
  • Earth $0.0167$
  • Mars $0.0934$
  • Jupiter $0.0484$
  • Saturn $0.0541$
  • Uranus $0.0472$
  • Neptune $0.0086$

so in order of increasing orbital eccentricity the planets are Venus, Neptune, Earth, Uranus, Jupiter, Saturn, Mars, Mercury. There is no obvious correlation between orbital eccentricity and distance from the Sun.

Note that these values are current values - we know that the orbital eccentricities of the planets do vary slightly over time scales of tens of thousands of years. In $30,000$ years' time the Earth's orbit will be less eccentric than that of Venus.

Some Trans Neptunian Objects such as Pluto ($0.2488$), Eris ($0.4407$) and Sedna ($0.8549$) have higher eccentricities than any of the planets. Many comets have orbital eccentricities between $0.9$ and $1$, and interstellar objects such as 'Oumuamua have orbital eccentricities greater than $1$ because they are not gravitationally bound to the Sun.

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The eccentricity of a planet's orbit is a measure of how 'circular' it is.

According to this website (14th line down) - there is no clear pattern in the eccentricities of the planets.

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