What would happen if Neptune and Pluto collide?

That resonance is the result of Neptune's gravity acting on Pluto. That resonance has the effect of making sure Neptune and Pluto never get very close together. read more

From 1979 to 1999, Pluto was the eighth planet from the sun. In 1999, it slipped beyond Neptune to become the ninth. But Pluto's 248-year orbit around the sun takes it 17 degrees above and below the plane in which Neptune and the other planets travel. So their paths don't actually cross as they swap positions. read more

It is true that the orbits of Neptune and Pluto cross, with the two objects swapping positions in the Solar System. Despite this, however, there is no chance of them colliding. read more

Diagrams of the Solar System give the impression that the orbits of Neptune and Pluto cut right across each other, and textbooks even state that Pluto crossed the orbit of Neptune in February 1999. Yet in reality the two planets can never get close to colliding, for two reasons. read more