Kids cried. Adults grumbled. People wrote irate Letters to the Editor. (Umm...really?)
All because poor little Pluto, so small and far away, and so incapable of defending itself, was kicked out of the planet family!
It was demoted to—gasp! the horror!—dwarf planet!!!
Back on August 24, 2006, when the International Astronomical Union voted to relabel Pluto based on many new findings in astronomy, I nodded my head. New data often requires us to re-sort, re-categorize, re-label. It's something to cheer, because it means we humans have learned more; we're closer to achieving an accurate picture of the universe.
But some people got very upset! Some children sent hate mail to astronomer Neil deGrasse Tyson. Legislatures of two states voted to refuse to recognize the IAU's decision. People “yelled” at each other on the internet (and you know how rarely THAT happens!).
One of the reasons that some people got upset, apparently, is because Pluto getting kicked out of planethood messes up the mnemonics that kids learn to remember the order of the planets in the solar system. One of the most common of these mnemonics is: “My Very Energetic Mother Just Served Us Nine Pizzas.” (Mercury – Venus – Earth – Mars – Jupiter – Saturn – Uranus – Neptune – Pluto.)
But this is silly. For one thing, Pluto sometimes dips inside Neptune's orbit, and making the sentence end with the words “served us pizzas nine” sounds pretty goofy. (If Pluto were a planet, it would have been the eighth, not ninth, planet from 1979 to 1999.) Also, surely we can easily come up with a new mnemonic? How about “My Very Energetic Mother Just Served Us Nachos,” or even “My Very Easygoing Mother Just Served Us Nothing”?
I gather that it is Americans, and American school children, that got especially upset about Pluto's demotion. The guy who discovered Pluto, Clyde Tombaugh, was an American astronomer, and Walt Disney named Mickey Mouse's dog “Pluto” after the planet to celebrate that discovery. Maybe a lot of people were upset by Pluto being kicked out of the planet club because it is the smallest planet (by far—it's really tiny, even compared to the very small Mercury), and people want to stick up for the little guy, and root for the underdog.
To which some people have replied, “Why are you getting so upset about this? You don't hear Pluto complaining, do you?”
And some people have pointed out that, it's not as if Pluto were blown up, or ejected out of the solar system. It's just where it always has been. We just have added the word "dwarf" to its category.
Hmmm...so what is a planet?
The IAU was struggling with the question of whether or not to call some newly discovered bodies planets. Eris, Makemake, and Haumea all orbit far from the Sun, like Pluto, they're spherical, like Pluto, and some of them are roughly the size of Pluto. If Pluto is a planet, shouldn't they be planets, too?
So, does the solar system have 10 planets? Or 12? Or more and more as we discover more and more?
(If so, we're gonna need a much bigger mnemonic!)
Some scientists reasoned that Pluto and these other Kuiper Belt Objects could be considered a new sort of thing in the solar system. Instead of being the last and littlest planet, Pluto could be thought of as the first and one of the largest Plutoids (KBOs that are large enough to be roughly spherical in shape).
See, it's not a DEmotion, it's a PROmotion!
The definition of a planet is:
- an object that circles a star (the IAU definition said “that circles the sun,” but that was a mistake that would leave out all of the planets we have discovered circling other stars)
- an object large enough to be spherical
- an object that has “cleared the neighborhood” around its orbit. That would exclude the largest asteroids in the asteroid belt as well as the largest Plutoids in the Kuiper belt.
Also on this date: