Once upon a time, in 1930, Pluto was celebrated as the ninth planet in our solar system. But after more than seventy-five years, scientists decided it wasn’t a planet anymore. Instead, they called it a “dwarf planet.” Let’s find out why this change happened and what makes a planet different from a dwarf planet.
The story of Pluto’s change began in 1992 when scientists discovered a special area beyond Neptune called the Kuiper Belt. This area is filled with icy objects, many of which are similar to Pluto. This made scientists wonder what really makes something a planet.
In 2005, scientists found another object in the Kuiper Belt named Eris. Eris seemed to be even bigger than Pluto! This discovery made people question if Eris should be the tenth planet. Some scientists thought that if there were many objects like Pluto and Eris, maybe Pluto shouldn’t be called a planet anymore.
To solve this puzzle, a group called the International Astronomical Union (IAU) came up with a new way to decide what is a planet. They said a planet must:
Pluto could do the first two things, but it couldn’t clear its path of other objects. So, it was reclassified as a dwarf planet.
“Dwarf planet” is a term used for objects that orbit the Sun and are mostly round but haven’t cleared their orbits of other stuff. There are five known dwarf planets in our solar system:
As scientists continue to explore space, they might find even more dwarf planets!
The change from calling Pluto a planet to a dwarf planet shows how our understanding of space is always growing. The rules made by the IAU help us tell the difference between planets and other space objects, showing just how amazing and complex our universe is!
Create Your Own Solar System Model: Gather some craft materials like clay, foam balls, or paper to create a model of the solar system. Include the eight planets and the five known dwarf planets. As you build, think about why Pluto is a dwarf planet and not a regular planet. Discuss with a friend or family member what makes each celestial body unique.
Planet or Dwarf Planet? Sorting Game: Create cards with names and pictures of different celestial bodies, including planets, dwarf planets, and other objects from the Kuiper Belt. Sort them into two groups: planets and dwarf planets. Use the IAU’s definition to decide where each card belongs. After sorting, explain your choices to someone else.
Observation Journal: Start a journal where you record observations about the night sky. Try to spot planets or constellations with the help of a star map or a mobile app. Write down what you see and any questions you have about space. Think about how scientists use observations to learn more about celestial bodies and how they classify them.