Kuiper stares at the camera in disbelief. A space shuttle plush toy lies on the ground as if dropped by him.

Why Pluto got a demotion

This was Kuiper’s face yesterday when he asked me what happened to Pluto. I explained to him that when I was his age, it was considered a planet, but not anymore. He was a little disappointed about the “downgrade” until he learned that Pluto is still considered a “Kuiper belt object.”

In 2006, the International Astronomical Union decided that it was time for us to come to a consensus on what makes a planet a planet (since it’s a pretty arbitrary label.) This decision was accelerated by the discovery of an planet bigger than Pluto on the outskirts of our solar system (we named it Sedna.) Astronomers assumed (correctly) that we would continue discovering more objects like Sedna (like Eris, and Haumea, and Makemake, and Orcus, and Varuna…)

In an effort to avoid making the mnemonic “My Very Educated Mother Just Served Us Nine Pizzas” wayyyyyyy longer, the IAU astronomers agreed that a planet:

☄️Must be in orbit around the Sun
☄️Must be dense enough that it’s mostly round
☄️Must have shown that it’s a good celestial citizen by clearing the neighborhood (this means that other than its own satellites, there’s nothing in its orbital path)

Dwarf planets are objects which fulfill criteria 1 & 2 but not 3 AND are not a satellite (which is why our moon is not considered a dwarf planet.) Since Pluto makes up a small fraction of the mass of objects in its orbit, it’s a dwarf planet by the new definition.

So, the bad news is that we lost our nine pizzas, but the good news is that mom served us nachos. And as Kuiper’s dad just told me, “That’s a very educated mother. Nachos are great!” Don’t you agree?

Original post: https://www.instagram.com/p/BdelNObAzLg/

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