Pluto's orbit isn't as circular as the other planets. Because of this, sometimes Pluto is only the 8th planet from the Sun, (when it is inside Neptunes orbit) and other times it is the 9th planet from the Sun, (when it is outside Neptunes orbit)
As shown in the picture below. (The image is not drawn to scale, it is just to show you what Pluto's orbit is like!)


Back to The Planets