Ask a Teacher

Why is Pluto not accepted as a planet?

• It needs to be in orbit around the Sun – Yes, so maybe Pluto is a planet.
  
• It needs to have enough gravity to pull itself into a spherical shape – Pluto…check
  
• It needs to have “cleared the neighborhood” of its orbit – Here’s the rule breaker. According to this, Pluto is not a planet.

What does “cleared its neighborhood” mean? As planets form, they become the dominant gravitational body in their orbit in the Solar System. As they interact with other, smaller objects, they either consume them, or sling them away with their gravity. Pluto is only 0.07 times the mass of the other objects in its orbit. The Earth, in comparison, has 1.7 million times the mass of the other objects in its orbit.

Any object that doesn’t meet this 3rd criteria is considered a dwarf planet. And so, Pluto is a dwarf planet. There are still many objects with similar size and mass to Pluto jostling around in its orbit. And until Pluto crashes into many of them and gains mass, it will remain a dwarf planet.