This cache is part of a 1:133,700,000 scale
model of the solar system. The dome on the Monroe County Courthouse in downtown
Bloomington, Indiana serves as the model of the Sun, and all the
planets, dwarf planets, large moons, and asteroids are all
constructed to the proper size and placed at the proper distance
based on that scale.
You might be thinking that Mooresville is a bit far away for a
solar system model centered in Bloomington, but Pluto is pretty far
out there. (It's not the furthest, though.) Pluto ranges
from about 4.44 to 7.38 billion miles from the sun, which
corresponds with 20.6 to 34.3 miles at this scale. Sometimes Pluto
is even closer to the sun than Neptune. This model is roughly 31.8
miles from the courthouse. To give you a sense of perspective, the
entire inner solar system (including Mercury, Venus, Earth, Mars, and the asteroid belt) all fit within the city
limits of Bloomington and all the gas giants are within 20
miles.
When I tell people that I'm making a scale model of the solar
system, the first question is almost always "Are you going to
include Pluto?", which is a silly question because I never told
anyone that I was only going to include the planets in my
model. My rule is that I'll include anything with a name that's at
least a few millimeters across (and a few things smaller than that,
if I feel like it). That includes all the planets, a whole bunch of
moons, several asteroids, and a handfull of other objects
(trans-Neptunian objects, Kuiper belt objects, cubewanos, etc.)
What they really want to ask me is whether or not I think Pluto
counts as a planet. I think I'm going to have to go with the IAU on
this one and say no, but you're welcome to call it whatever you
want at home.
Back in 2006, when Pluto-sized objects seemed to be popping up
everywhere, the International
Astronomical Union decided that they needed to actually decide
what a "planet" is, instead of their usual "we'll know it when we
see it" technique. Everyone was pretty sure that a planet had to be
big enough to be roughly spherical, but it was also clear that this
wasn't enough. Even if you ruled out moons, there were still quite
a few asteroids and a seemingly endless supply of rocky ice balls
way out in the Kuiper belt that
were all big enough and round enough. If we included all those, "my
very educated mother" would have to do a lot more than just serve
us nine pizzas. So they decided that as long as it was big enough
to be round and wasn't orbiting something else, it would count as a
"dwarf planet". But to count as a proper planet, it had to also be
big enough to have cleared out its own orbit. That eliminated all
the asteroids and also all the Kuiper Belt objects. Unfortunately,
Pluto also lost its status as a proper planet, and this ticked a
lot of people off for some reason.
It was kind of a fluke that it was considered a planet in the
first place. Using the same technique that had led to the discovery
of Neptune from perturbations in the orbit of Uranus, astronomers
measured tiny little wobbles in Neptune's orbit and hypothesized
that there must be a ninth planet out there whose gravity was
jerking Neptune around. Little did they know that those wobbles
were actually just the result of a miscalculation of Neptune's mass
and didn't need any sort of explaining at all. But by sheer
coincidence, when they started looking to where this mysterious
"Planet X"
would've been, they found a little faint dot moving slowly across
the sky.
At first (in 1931), they thought that it was about the same size
as Earth, but then they realized that it was a lot whiter than the
other planets, and they had to revise their initial size estimates
down a few notches. Then they realized that it wasn't actually one
object at all, but rather a pair of objects orbiting each other,
and they had to further downgrade their size estimates. By 1948,
Pluto was thought to be only a tenth the mass of Earth. In 1976,
the estimate was down to a hundredth of Earth's mass, and by 1978,
the estimates finally stabilized at about two thousandths
of Earth's mass.
Pluto is about 2,300 kilometers across, and its moon Charon is about
1,200 kilometers in diameter, so this model has them at about 18
and 9 millimeters, respectively. The two objects are about 19,600
kilometers from each other, orbiting around a point between them.
In this model, they are about 6 inches apart. Pluto actually has
two other moons that we know of, Nix and Hydra, but they're
both too small (less than a millimeter across) to show up at this
scale.
Between 1985 and 1990, the Earth was so perfectly lined up with
the plane of the orbit of the Pluto-Charon system that we could
watch one of them eclipse the other every three or four days.
Eclipses like this are very useful to astronomers. The timing on
this was very lucky for us as the Earth only lines up with Pluto
and Charon like this twice every 248 years. Even though we couldn't
make out any surface details on Pluto at the time, we could watch
how the brightness of Pluto changed as Charon passed in front of
it, giving us a pretty good idea of where the bright and dark spots
on Pluto are.
We've known for a while that Pluto is pretty a pretty bright
object, as far as planets (dwarf or otherwise) go, but more
recently, we've learned that it's very splotchy with white, black,
and dark orange spots. It's like a calico planetoid. Charon, on the
other hand is a more uniform gray color.