Most of you have heard the news that the International Astronomical Union (IAU) made a very important decision in August 2006.  It ruled there are only eight planets; not nine.  Poor little Pluto has been demoted from official planet status and no longer qualifies to play with the big boys.
 

An artist's conception of Pluto and its moon Charon. Image credit: NASA.

So, why would they do that?  Just what makes a world a planet anyway, as opposed to a sun or a moon or a comet or asteroid?  One of the reasons the IAU's decision is important is that there has actually never been a definition of what constitutes a planet until now.

We humans have known about five of the planets since we lived in caves; but your ancestors didn’t call them planets; they called them gods.  The stars appear fixed in place to human eyes.  Their patterns don’t change in a human lifetime or many human lifetimes.  But your ancestors knew there were five stars that weren’t fixed.  They wandered among the others along the same path in the sky.  The Sun and Moon also moved along that same path.

The ancient Greeks called the stars ‘planan’ which means wanderer.  Today we use the Roman names for these five stars.  The Romans named them for their gods: Mercury, Venus, Mars, Jupiter and Saturn and we call them planets.  Galileo’s observations of the heavens through his telescope began the slow but steady demotion of Earth from its position as the center of the universe, and by the late 1600’s we realized that there were six planets, including the spaceship we ride every day of our lives.

William Herschel discovered the planet Uranus by accident in 1781 bringing the planetary total to seven.  His discovery ignited a frenzy of searching for additional planets and on the first day of the new century, 1 January 1801, the Italian astronomer Giuseppe Piazzi discovered the solar system’s 8th planet.  In 1802 the 9th planet was discovered; the 10th in 1804 and the 11th in 1807.  We drove merrily along with an eleven-planet solar system for 38 years.  Telescopes steadily improved during that time, and the discovery of the 12th planet in 1845 was followed by a flurry of new planet discoveries.  By the 1850’s astronomers realized these very small rocks could NOT be planets and reclassified them as asteroids.

Astronomers realized shortly after its discovery that Uranus was being tugged from a circular orbit by a large body beyond it.  The mathematicians of the time worked their magic and told the astronomers to point their telescopes at a certain place in the sky.  They did so and in 1846, Urbain Le Verrier discovered Neptune.  We were up to eight planets again.  The mathematicians then said there was something large perturbing the orbits of both Uranus and Neptune and astronomers searched for this mystery planet for 80 years.  In 1930, Clyde Tombaugh thought he found it.  It was named Pluto.

Pluto doesn’t have a nearly circular orbit like the other planets and it’s very small, less than half the size of Luna, our moon.  It is usually about 4 billion miles away from us here on Earth, and trying to see things that far away and that small is like hovering over Ritidian Point and trying to spot a black marble floating in the water off Cocos Island.  But in the 1990’s, as telescopes improved, astronomers began to find more and more black marbles in the infinite deep past Neptune.

Sound familiar?  The member astronomers of the IAU thought so too and they demoted Pluto from its planet status and made it the King of the Kuiper Belt, the asteroid belt that occupies the outer solar system beyond Neptune, thus saving us from the embarrassment of the multiple-planet solar system of the early 1800’s.  There are hundreds of thousands of asteroids in the Asteroid Belt between Mars and Jupiter and probably at least that many in the Kuiper Belt.  Just how many ‘planet names’ do you want to learn?

All you visitors to the UOG Planetarium who had Isa and DB teach you how to remember the names of nine planets can all chant “My Very Educated Mother Just Served Us Nine Pizzas”.  So what do you do now?  Just remember “My Very Educated Mother Just Served Us Noodles”!

What happened to the large world beyond Neptune that was pulling on both Uranus and Neptune?  In the 1970’s, astronomers put all those horrendous math calculations through a supercomputer and discovered that in the 1870’s the mathematicians did the math wrong.  There is NO large body in the outer solar system; just thousands and thousands of small ones.

So don't mourn for Pluto.  Instead of being the odd man out in a small family, it's now the King of the Kuiper Belt!