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 31.82 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) fits entirely 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?" This 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, a number of asteroids, and a handfull of other objects (trans-Neptunian objects, Kuiper belt objects, cubewanos, etc.) Of course, 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 relying upon their usual "we'll know it when we see it" strategy. 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 at least a "dwarf planet". But to count as a proper "planet", it had to also be big enough to have cleared out its own orbit of any smaller pieces of debris. 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. Way back in the 1890s and 1900s, 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. Of course, we now know that those wobbles were the result of a miscalculation of Neptune's mass and don't need another planet to explain them. But by sheer coincidence, when they started looking for 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 so 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, and considerably smaller than Earth's moon.
Pluto is about 1,200 kilometers across, and its moon Charon is about 600 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 four other moons that we know of: Styx, Nix, , and Hydra, but they're all 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 good guess of where the bright and dark spots on Pluto are. We knew for a long time that Pluto was pretty a pretty bright object, as far as planets (dwarf or otherwise) go, but these kinds of transitions taught us that it was a fairly splotchy pinkish brown.
When I painted an older version of the models of Pluto and Charon, those guesses were all I had to go on. But since then, the New Horizons space probe finally made its pass by Pluto, and we got some good pictures of what the dwarf planet and its moons look like. So the latest version is much more accurate. See if you can find like the heart-shaped Tombaugh Regio on Pluto (essentially a big nitrogen-snow-capped mesa) and the big brown spot on Charon (which is informally named Mordor).
The illuminated sculpture that this cache is hidden near was built to celebrate the bicentennial anniversary of the state of Indiana.