Why does Pluto sometimes become closer to the Sun than Neptune?

Pluto sometimes becomes closer to the Sun than Neptune because its orbit is highly eccentric, or elongated, compared to the nearly circular orbits of the planets. While Neptune maintains a relatively consistent distance from the Sun, Pluto’s path varies significantly, ranging from 29.7 astronomical units at its closest point (perihelion) to 49.5 AU at its farthest (aphelion).12

Because of this extreme elongation, Pluto crosses inside Neptune’s orbital path for a small portion of its 247.69-year revolution. Despite this, the two bodies never collide. Pluto is locked in a stable 3:2 gravitational resonance with Neptune, meaning it completes two orbits for every three of Neptune's, which ensures they never pass closer than about 17 AU.23