Here's why:
* Orbital Distance: Mercury has the smallest average distance from the Sun, with an orbital semi-major axis of about 57.9 million kilometers (35.9 million miles).
* Elliptical Orbits: All planets have elliptical orbits, meaning their distance from the Sun varies throughout their orbit. While Mercury does have a point in its orbit where it's furthest from the Sun, its average distance is still the smallest.
While other objects like comets and asteroids can sometimes get closer to the Sun than Mercury at certain points in their orbits, Mercury's average distance from the Sun makes it the closest planet.
