It's tricky to definitively say which planet has the *least* circular orbit, as "least circular" can be interpreted in different ways.

Here's why it's complicated and some factors to consider:

* Eccentricity: This is the standard way to measure how elliptical an orbit is. A perfect circle has an eccentricity of 0, while a highly elongated ellipse has an eccentricity closer to 1.

* Planetary Orbits Change: The orbits of planets are not fixed. Gravitational interactions between planets, the Sun, and other objects in the solar system can slightly change their shapes over time.

* Different Definitions of "Least Circular": You could consider the planet with the highest eccentricity, or the one with the greatest difference between its perihelion (closest point to the Sun) and aphelion (farthest point from the Sun).

Considering these factors, here are a couple of strong contenders:

* Mercury: Mercury has the highest eccentricity of all the planets in our solar system (e = 0.206). This means its orbit is significantly elliptical.

* Mars: Mars has the second-highest eccentricity (e = 0.093), making it another strong contender.

Important Note: Even with a high eccentricity, these orbits are still relatively close to circular compared to some comets or asteroids, which can have highly elongated orbits.

Let me know if you'd like to explore a specific aspect of this topic further!