Pluto Demotion Day

Pluto Demotion Day

Discovery Place Nature

This day in space science history, Pluto was demoted to “dwarf planet” decades after its discovery and designation as the ninth planet in our solar system. On August 24, 2006, the International Astronomical Union (IAU) made it official when they stripped Pluto of its planet status. Here is a closer look at how they arrived at this controversial decision.

The word planet derives from the Greek word for wanderer, and originally objects that moved in the night sky—as opposed to remaining fixed, like stars—were designated as planets. Over time, as astronomers gained more knowledge about the universe, it became clear that it was time to reevaluate what constitutes a planet. After some serious debate, the IAU determined that to be considered a planet, a celestial body must meet the following criteria:

  1. The object orbits the Sun.
  2. The object has a nearly round shape or has enough mass to assume hydrostatic equilibrium (HE). Note: an astronomical body is in a state of HE when its self-gravitational force is balanced by its internal pressure—meaning it can pull itself into a spherical shape.
  3. The object has cleared the neighborhood (in other words, is the dominant gravitational body) around its orbit.

Pluto got check marks for the first two criteria but lost out in the third because, simply put, Pluto isn’t big enough to properly “clear its neighborhood.” Thus, 76 years after its discovery, Pluto became a dwarf planet.

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  • Discovery Place Nature