When I need to remember the order of the planets I recite the phrase I learned in school: My Very Excellent Mother Just Served Us Nine... and then I stumble just as I get to “Pizzas.”
Because “Pizzas” stands for “Pluto,” and Pluto is no longer a planet. For 76 years people knew it as the smallest, most distant member of the nine-object club, but then, in 2006, the International Astronomical Union (IAU) changed all that. An astronomer from the California Institute of Technology named Mike Brown had discovered a few odd objects out beyond all the known planets. One, Eris, appeared to be larger than Pluto (although we now know they’re almost exactly the same size).
The astronomers who make up the IAU faced a hard choice: label all the new objects and hundreds of future objects as planets, or pick a narrow definition that would save the deeper meaning of the title. They picked the second option. A couple hundred scientists voted to demote Pluto and named it the first of a new group of worlds: the dwarf planets.
Why didn’t Pluto make the cut? When the IAU officially defined the word “planet” for the first time, Pluto simply didn’t fit. To keep its planetary status along with Earth, Saturn, and the rest, it needed to pass three tests:
Pluto passes the first test with flying colors, making one loop around the sun every 248 years. Things that fail this test include objects that circle other bodies, like how the Moon orbits Earth.
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