NASA, ESA and G. Bacon (STScl)/nasa.gov
An artist's concept of Pluto and its moon Charon hovering over the surface of another possible moon.
There were only two people around when the rest of the solar system was discovered. It happened in 1992 in a dingy telescope control room 14,000 feet above sea level. On that night David Jewitt and Janet Luu found the tell-tale trace of a asteroid-sized body tumbling through the far edges of our planetary 'hood. They had found the first Kuiper Belt Object and Pluto's descent from the pantheon of planets had begun.
As a professional astronomer I am used to questions about black holes, about what lies at the edge of Space and about the existence of life in the Universe. After all these years, I still have those questions myself. So its always a pleasure to talk about the answers we think we have, or want to have. But the Pluto question always throws me for a loop.
"Why did you do it?" people ask, (especially kids with big pleading eyes). "Why did you whack Pluto?" There is such real pathos in the way the question gets asked, as if the entire cadre of professional astronomers had just choked humanity's favorite puppy. My first instinct is to reply "Sheesh! It’s a rock. Get over it!" I have learned with time, however, that my Jersey-tuned sensibilities in this regard aren't going to help anyone.
After pondering the problem for some time, I believe our collective grief over Pluto's demise as a planet is not because of its link to a dopey Disney dog but because of something deeper: a hunger for order and simplicity.
Pluto was tossed from the family of planets for a very good reason. Beginning in 1992 with Jewitt and Luu's discovery of the prosaically named 1992 QB1, astronomers began to get a handle on exactly how much more of the solar system there was than just the planets (and asteroids and comets).
In 1952 astronomer Gerard Kuiper suggested that a belt of material left over the formation of the solar system existed beyond the orbit of Neptune. (Neptune has an orbital radius of 30 "astronomical units," with an astronomical unit being the distance from the Earth to Sun, or 93 million miles). Kuiper was not the first or the last to make this suggestion but, for reasons that are beyond explanation, his name stuck. Jewitt and Luu soon found other trans-Neptunian bodies and it became clear that space beyond the last giant planet was full of construction debris.
There are an estimated 70,000 KPOs (Kuiper Belt Objects) out there larger than 100 meters. More importantly, at least 3 KPOs are large enough for gravity to work its symmetric magic and pull rock and ice into a sphere. These are the newest class of solar system's inhabitants the Dwarf Planets: Huamea, Makemake and, yes, Pluto.
With the discovery of the KPOs and, in particular, the KPO Dwarf Planets, Pluto lost any claim to being special. It was just one cinder-block in a field of cinder-blocks left over from building our solar system. It wasn't even the biggest cinder block. In 2005 the dwarf planet Eris was found orbiting the Sun at distances beyond the Kuiper Belt in yet another new region of the solar system that astronomers call the Scattered Disk.
So what does all this tell us about our love for the pitiful little Pluto? Well, in the last 20 years something remarkable has happened in our understanding of solar systems and, in some deep recess of our collective imaginations, we just don't like it.
We have come of age. We have grown up. Instead of the tidy vision we were taught as children, with 9 planets moving along their color coded orbits, we now know that solar systems can be very messy places.
From studies of other solar systems (discovered only since 1995), we know that giant Jupiter-sized planets can live right up against their stars in orbits so close it would make Mercury blush. We know that rather than the stately circles our planets move along, some of these systems have giant planets winging back and forth on wildly cigar-shaped orbits (ellipses) that can play hell with smaller Earth-sized worlds tossing them into the frozen depths of space just (perhaps) as life was getting going. And in our own corner of the galaxy, this solar system that once seemed so orderly and compact (even with that untidy asteroid belt) is now populated by all manner of malformed worldlets.
These discoveries have all taken us beyond the cosmos of our ancestors (with their little mechanical models of rotating planets). They have forced us to see something we might have known by just looking around at our own lives. It’s all a mess. The Universe is often chaotic, often disordered and it's never, never finished.
For centuries we looked to the planets as a model of clockwork regularity and economy. Now we know better. Perhaps our longing for Pluto is really just pining for a simpler time that never really was.