The orbits of planets are often illustrated as perfect circles, a simple model that makes it easy to visualize the solar system. In reality, the path a planet takes around the Sun is an ellipse, which is essentially a slightly flattened or oval shape. This means a planet’s distance from the Sun constantly changes throughout its year, raising the question of which planet has the most circular path. To find the answer, astronomers use a precise measurement that quantifies how much any given orbit deviates from a perfect circle.
Quantifying Orbital Shape
The scientific metric used to define the shape of an orbit is called orbital eccentricity, represented by the letter \(e\). This value is a dimensionless parameter that determines the extent to which an orbit deviates from being perfectly circular. Eccentricity provides a simple scale for measuring orbital shape, where a value of \(e=0\) represents a perfect circle. As the value moves above zero, the orbit becomes increasingly elliptical or elongated. Bound orbits, such as those of planets around the Sun, have eccentricity values between 0 and 1.
The elliptical path means a planet has both a closest and a farthest point from the Sun during its revolution. The point of closest approach is called perihelion, and the point of farthest distance is called aphelion. The difference in distance between perihelion and aphelion increases as the orbital eccentricity rises.
The Most Circular Orbit in Our Solar System
The planet with the most circular orbit in our solar system is Venus. Its orbit is the closest of all the major planets to the ideal shape of a perfect circle, which would have an eccentricity of 0. The measured orbital eccentricity for Venus is approximately 0.0068, which is the lowest value among the eight planets.
For comparison, Earth’s orbit is also very circular, but its eccentricity is about 0.017, making it slightly more elliptical than Venus’s path. The contrast is greater when compared to Mercury, which has the most elliptical orbit of all the planets with an eccentricity of about 0.206. Because of its minimal eccentricity, the distance between Venus’s perihelion and aphelion varies by only about 1.46 million kilometers.
While Venus holds the title for the most circular orbit, no planet has a perfectly circular path with an eccentricity of exactly zero. The present eccentricity of Venus is actually at a periodic low point, oscillating over millions of years.
Why Planetary Orbits Are Not Perfect Circles
The reason no planetary orbit is a perfect circle is due to the complex gravitational environment of a multi-body system like the solar system. An orbit will only be a perfect ellipse in a theoretical two-body system, such as a single planet orbiting a single star. Once other massive objects are introduced, the path immediately deviates from a perfect ellipse.
The primary reason for these deviations is gravitational perturbations, which are the slight gravitational tugs exerted by other planets. These forces constantly act on a planet, subtly modifying its velocity and trajectory over time. The giant planets, especially Jupiter, have a significant influence on the orbits of the inner planets, slowly changing their eccentricity over long periods.
The initial conditions of the solar system’s formation also played a role in establishing non-circular orbits. Planets formed from a disk of gas and dust through a process of chaotic accretion and collisions. The combination of a planet’s initial speed and direction, coupled with the Sun’s gravity, rarely results in the precise balance needed for a perfectly circular trajectory.