Pluto slips inside Neptune's orbit

On February 7, 1979, Pluto crossed inside Neptune’s orbit and became, temporarily, the eighth planet from the Sun. The cause is Pluto’s unusually elongated and tilted path: where most planets trace near-circular circuits, Pluto’s eccentric orbit swings it both far beyond Neptune and, for one stretch each lap, well within.

That stretch lasts about 20 years out of a 248-year orbit. Having dipped inside in 1979, Pluto made its closest solar approach in 1989 before crossing back out on February 11, 1999, restoring the familiar order.

For two decades, the solar system’s ninth planet was running ahead of its eighth.

The two worlds never risk collision. Their orbits are steeply inclined to each other, and a stable 3:2 resonance keeps them choreographed so they are never close when Pluto is at its innermost. The next inside-Neptune passage will not begin until the 2220s.