The most distant planet in the Solar System is Neptune, orbiting at a distance of 4.5 billion km from the Sun. But there used to be 9 planets in the Solar System, including Pluto. And for most of the time since its discovery, Pluto was considered the most distant planet from the Sun.
Pluto and Neptune couldn’t be more different. Pluto is a tiny Kuiper Belt Object; a ball of rock and ice measuring only 2,390 km across. This is a tiny fraction of the diameter of Neptune, which is 49,500 km across. You could fit 20 Plutos side by side to match the diameter of Neptune.
Neptune and Pluto have a very interesting orbital dynamic between them. Neptune has a roughly circular orbit; however, Pluto’s orbit is highly eccentric, varying its distance to the Sun a tremendous amount over the course of its orbit. Because of this, Pluto can actually get closer to the Sun than Neptune. The last time this happened started in 1979 and went until 1999. During that period, Neptune was actually the most distant planet from the Sun, and Pluto was actually closer. But then Pluto was demoted, from planet to dwarf planet, so Neptune is now the most distant planet; and it doesn’t matter what Pluto does. Pluto spends 20 years out of its entire 248 year orbit within Neptune’s orbit.
Since Pluto and Neptune cross orbits, is it possible that the two planets will collide? No, they actually can’t collide because Pluto’s orbit takes it much higher above the Sun’s orbital plane. When Pluto is at the same point as Neptune’s orbit, it actually much higher up than Neptune. So the two planets will never be at the same place at the same time.
You can read some interesting information about the orbital patterns of Pluto here.
We have written many articles about Pluto and Neptune on Universe Today. Here’s why Pluto is no longer considered a planet, and here’s an article about how there could be liquid oceans inside Neptune.
We have recorded an entire episode of Astronomy Cast just about Neptune. You can listen to it here, Episode 63: Neptune.