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Astronomer Adam Frank addresses the kerfuffle about removing Pluto from the pantheon of planets:

"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.

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 0743 pluto 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.

(Artist's reconstruction of Pluto (seen at center from one of its moons) courtesy of NASA, ESA and G. Bacon (STScI))

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