Q Why is Pluto considered a dwarf planet?
— Peyton Saar, Omaha, Nebraska
A Sanjay Limaye, planetary scientist at the University of Wisconsin-Madison Space Science and Engineering Center:
Before Pluto was discovered, it was predicted. Astronomers had observed that massive objects can affect the orbits of their neighbors and, after seeing deviations in the orbits of Uranus and Neptune, assumed something substantial existed beyond their orbits.
When Pluto was spotted, it was thought to be the predicted object and was identified as a ninth planet.
A few decades later, astronomers started discovering more and more objects around other stars and didn’t know whether to call them planets or not. There appeared to be a need to define what a planet means, and that led to what some people consider Pluto’s demotion to a dwarf planet.
The International Astronomical Union decided that full-size planets must orbit the sun, have a round shape and have cleared their orbits of other objects. Pluto fulfills the first two criteria, but not the third.
It still goes around the sun, it’s round enough, it’s got moons, and it behaves like a planet. But the idea is that Pluto did not form the same way as the rest of the planets. Pluto’s orbit is both eccentric and inclined more than the rest of the planets by about 17 degrees. That suggests something is different about this object.