At the current time, until 1999, what planet is farthest from the sun?

For a long time, Pluto was considered the ninth and most distant planet in our solar system. However, for a brief period, it swapped places with its neighbor. Due to its highly elliptical orbit, Pluto spends 20 years of its 248-year journey around the sun inside Neptune's orbit. This last happened between February 1979 and February 1999. During these two decades, Neptune was actually the planet farthest from the sun, while Pluto was the eighth.

This unusual orbital dance might make it seem like the two bodies are at risk of a cosmic collision. However, their orbits are not on the same flat plane. Pluto's path is tilted, and it is also in a stable 2:3 orbital resonance with Neptune. This means that for every two orbits Pluto completes around the sun, Neptune completes three. This gravitational lockstep ensures they never get too close to one another, preventing any potential impact.

After 1999, Pluto moved back outside of Neptune's orbit to once again become the most distant world in our solar system, a title it held until its reclassification as a dwarf planet in 2006. Because of this reclassification, Neptune is now permanently considered the farthest planet from the sun. It will be more than 200 years before Pluto's unique orbit brings it closer than Neptune again.