A little reason to celebrate
Pluto Demoted Day
Fun · History · August 24
Pluto Demoted Day recalls when astronomers adopted a formal definition of a planet and Pluto was reclassified as a dwarf planet.
Pluto Demoted Day falls on Tuesday, August 24, 2027. That is 301 days away.
Pluto did not change. The definition did. Astronomers settled on a formal meaning of the word planet, and under it Pluto moved into a new category called dwarf planet.
The rule has three parts. A planet orbits the Sun. It has enough mass for its own gravity to pull it into a roughly round shape. And it has cleared its orbital neighborhood, meaning it is the dominant body along its path and has swept up or flung away most of the smaller material nearby. Pluto passes the first two tests easily. It fails the third, because it shares its region of space with many other objects of similar kind.
The decision came from a vote of the International Astronomical Union, the professional body that names and classifies objects in space. Plenty of people took it personally. Classrooms had taught nine planets for generations, and the memory tricks for listing them suddenly had one word too many. Pluto was not alone in the new group, either. Ceres, the largest object in the asteroid belt between Mars and Jupiter, is also classed as a dwarf planet.
Marking the change tends to be lighthearted and a little nerdy. People revisit the old planet mnemonics, argue over whether the rule is fair, or simply read up on what makes something a planet at all. It is also a nice way into a bigger idea: science sorts things into categories, and those categories can be revised when the evidence or the thinking improves.
The orbit-clearing test is the part most people never learned in school, and it is the whole reason the count went from nine to eight.
Ways to celebrate
- Write a new mnemonic for the eight planets and share it.
- Explain the three planet tests to a kid or a friend.
- Look up the other recognized dwarf planets and read about one.
- Hold a friendly debate on whether the orbit-clearing rule makes sense.
- Sketch the solar system to scale on a long strip of paper.
Dates
| Year | Date |
|---|---|
| 2026 | Monday, August 24 |
| 2027 | Tuesday, August 24 |
| 2028 | Thursday, August 24 |
| 2029 | Friday, August 24 |
| 2030 | Saturday, August 24 |
All national days on August 24
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