The
news shocked the world. (Apparently.)
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.
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