Asked by: Josh Davidson, Gloucester
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.
Firstly, the apparent crossing-points are optical illusions, caused by the fact that the two orbits are actually steeply inclined to one another. The second reason that Neptune and Pluto can’t collide is because Neptune’s orbital period of 164.8 years means it makes three orbits for every two made by Pluto, with its 248.8-year orbital period. That puts them in a so-called gravitational resonance, where each planet speeds up or slows down as the other approaches, which alters their paths and prevents them coming closer than around 2600 million km to each other.