The International Astronomical Union (IAU)’s decision to downgrade Pluto from being a planet to a dwarf planet in 2006 surprised many people, including some scientists. Even years later, some astronomers want to: Review planet definition To clarify the parameters that distinguish planets from other celestial bodies.

So why is Pluto no longer considered a planet? It begins with the description of a planet, or lack thereof. Before 2006, there were no strict criteria for a planet. Planets instead loosely accepted As objects larger than asteroids orbiting the Sun. For example, in the mid-1800s, more than a dozen objects we now consider asteroids were thought to be planets.

When did Pluto become a planet?