Long before modern telescopesm rocket ships, and planetary probes, astronomers had predicted that a ninth planet hung out in the far reaches of our solar system but it wasn't until the 1930's that they were able to get their had on concrete proof. With the help of photographic plates and years of observations, astronomer Clyde W. Tombaugh was finally able to prove that there was something out there. This body was later named "Pluto" after the Roman god of the underworld.
From that point on, poor little Pluto has been considered the oddball of our solar system family.
Scientists bickered for decades about how big Pluto really was but could never truly come up with a number that everyone could agree upon. With each revision and clarification, Pluto became smaller and smaller until scientists wondered if it was even a planet.
It wasn't until they discovered Pluto's largest moon, Charon, in 1978 that they could finally calculate out a more accurate estimate of the tiny planet's mass. Once they knew this, they could determine its size in distance terms you and I could understand – approximately 2,400 km or 1,500 miles across. Though Pluto is pretty small compared to our world (one of our Earth continents could barely fit on it), it is still considered one of the largest found in the Kuiper Belt located past Neptune's orbit.
As time passed and scientists began to study Pluto more, they came to discover that its orbit didn't look at all like the rest of the planets in our solar system. While Earth and Jupiter and Venus all make circular orbits around the sun along a pretty common plane, Pluto followed an elliptical one cocked out at a 17% inclide, swinging in as close as 30 astronomical units to the sun and out as far as 39 AU's. This orbit brings is precariously close to its neighbor, Neptune, and sometimes even switches places with it!
It wasn't until the 1990's that scientists begin to discover other "oddballs" out there. Pluto and its little moon are now known as only an example of what you can find in the Kuiper Belt – a region of rock and ice-filled space that extends almost 55 AU from Neptune's orbit. Scientists have found over 70,000 objects out there, all with the same composition as Pluto and some almost as big.
One such discovery – an object estimated to be 25% more massive than Pluto (later named Eris) – sent the scientific community into an uproar. Would this be our solar system's 10th planet? What happened if they discovered five of these larger planets? Would we then end up with 15 planets in our solar system? Something had to be done!
That is when the IAU decided to step in and re-examine the scientific community's basic classification of the universe. What should they call these icy, faraway bodies? Planets? Dwarf planets? Planetoids? Asteroids?
After much discussion and debate, the IAU met in Prague in 2006 to vote in a new definition of a "planet". To be called a planet, an object must:
orbit around a star (sun)
have enough gravity to pull itself into a spherical shape
have "cleared the neighborhood" – sweeping up asteroids, comets, and other space debris in its orbit
The IAU added that anything that did not fit the last criteria could be classified as a "dwarf planet" but never a true "planet".
When you compare what we know about Pluto to this definition, you can clearly see that it is not a planet – while it is spherical and orbits around the sun, it has not "clear the neighborhood" of the rocks, asteroids and chunks of ice in its orbit.
And that is the story of how Pluto was demoted from a high and mighty planet to a dwarf planet in one single blow of scientific democracy.
Debate still rages on even six years after this ruling. Some scientists claim that with this definition in place, none of the current planets can be called a "planet", since they don't always clear out stray asteroids or space trash that wanders into their orbit. Also, scientists have found large planet-like objects in other solar systems that do not revolve around the sun – would they call these dwarf planets even though they are bigger than Earth?
While the scientists bicker about names and classifications, the public is left to explain to their kids why the old books they find at the library still has Pluto listed as a planet when their science textbook states that it's not. An interesting lesson in history and politics for the whole family!