4) Pluto is shrinking!
OK, not really. Pluto
itself isn't shrinking. But our estimates of its size and mass are. Or did.
One of the easiest things to figure out about a new object is its distance. As I mentioned earlier, the orbit can be found by observing the object over the course of a few nights or weeks, and the distance to that object naturally comes out of the equations. Pluto is a long way off, between roughly 4.4 to 7.5 billion km (2.5 and 5 billion miles) away (its orbit is elliptical).
If you know its distance, you can guess at its size. After all, a bigger object reflects more sunlight and would look brighter, and a smaller one reflects less and would look fainter. This depends on how reflective the object is as well; a mirror reflects almost all light that hits it, while a chunk of charcoal reflects almost none. At the same distance, a small mirror looks just as bright to you as a huge piece of coal.
Astronomers could guess how reflective Pluto was and use that to estimate its size. At first it was thought to be near the size of Earth. This jibed with estimates of its mass, determined from what astronomers thought was its gravitational influence on Neptune (which turned out to be totally wrong, in fact).
But as time went on, more observations and more calculations were made. And every time astronomers used the new data to figure out Pluto's size, it got smaller... prompting one famous astronomer
to predict that Pluto would disappear in 1980. Eventually, observations indicated that Pluto was icy, and exceptionally shiny. It reflected about 50% of the sunlight hitting it. That means it was smaller than previously thought, leading to the modern size measurements.
We've finally settled on a mass for Pluto of about 10 quintillion tons (0.2% of the Earth's mass), and a diameter of about 2300 km (1400 miles), or smaller than the Earth's moon. When you think of it that way, it's remarkable we can see it at all! Most objects in the deep solar system are very dark, reflecting less than a tenth the light Pluto does. That's one reason it was found so long before any other deep solar system objects: it's simply brighter!