On this day, and for the first time since they were discovered, Pluto slipped inside the orbit of Neptune. It stayed closer to the sun until 11 February 1999. But you might wonder, given that Pluto is such a puny object compared to Neptune (or even the Earth), why the larger planet hasn’t “cleared” it (basically: crashed into it). Well, there are a few reasons for that.
Right now, it would be impossible for Pluto to crash into Neptune because the dwarf planet orbits in a plane that is at quite an angle from that of the other planets — including Neptune. And when it is at the same axial distance to the sun as Neptune, it is way off the plane. And I do mean way off the plane: along the axis of the solar system, it gets as far as 8 times the distance from the Earth to the Sun (AU).
But that wouldn’t save Pluto forever. Orbits precess (this explains part of the Earth’s ice age cycle). So there have been and will be times again when it will be on the plane at the right time. But again, Pluto is not in danger. Obviously, if it were in danger, Neptune would have long ago swallowed it up and we never would have had to have this argument about whether Pluto is a planet or not.
The two planets are in a 2-to-3 orbital resonance. This means that for every two times Neptune goes around the sun, Pluto goes around it three times. And they end up right back where they started. This sounds amazing, but there are lots of examples of this kind of thing in our solar system. Thus, they will not run into each other, because their precessions are locked together.
But Pluto Is Doomed Anyway!
Pluto is so small and so far away from the sun that it is chaotic. I mean that in a strict mathematical sense. Models of its future are highly constrained because very minor perturbations can have huge nonlinear effects on its orbit. But thus far, it really is the little planet that could. It has defied the odds and maintained its independence — to a large extent due to its having a powerful friend in Neptune. But it’s okay that Pluto is doom because so is everything. And it is going to last a whole lot longer than humanity.