By Peter Sachs
Q: Pluto isn't a planet anymore? What's next-the sun no longer the center of the solar system? -Jennifer, Idaho
A: It's a shift of cosmic proportions, in the truest sense of the term. Pluto, that icy outpost that takes 248 years to tumble around the sun just once, is no longer a full-fledged planet.
The International Astronomical Union came up with a new definition Thursday of what makes a planet a planet after a week of galactic wrangling and light saber battles. OK, not quite, but by all accounts the debate got pretty frenzied, complete with factions of scientists hurling their best Fast Fourier Transforms at each other.
After all that, Pluto has been downgraded to the status of "dwarf planet," along with several very large asteroid-like objects that orbit at the fringes of our solar system. A new term has been bestowed on another lesser class of objects: "small solar system bodies," as if "asteroid" wasn't enough.
The planet definition has three main parts now: it must orbit the sun, it must have a round shape, and it must able to sweep up other debris in its orbit path.
The really interesting part of this, apart from the need for rapid reform of mnemonics taught to first graders so they can remember the order of the planets, is that the astronomers doing all this extraterrestrial fiddling almost went in the opposite direction with their decision.
Yes, for a time at their meeting in Prague, there was thought of broadening the definition of a planet, which would have reclassified a number of moons (though not our own) and dozens of asteroids orbiting in the faraway Kuiper Belt, as planets themselves. Imagine tripling the Solar System, and then having to come up with a mnemonic that includes objects like 6765 Fibonacci or 1997 Leverrier.
Ultimately, perhaps the decision to demote the littlest planet was the sensible one to make. Pluto was discovered 76 years ago. But more recently, astronomers found a larger object than Pluto with a similar orbit, but it was left relegated to the category of "large things that don't fit into our solar system paradigm."
As a grade-schooler myself once, I was perturbed that Pluto's orbit crossed paths with Neptune, the next-farthest planet. The source of the concern was that the two planets would someday hit each other and then we'd be down to seven planets, but it took me a while to come to terms with that.
Just look on the bright side: Uranus can still be the butt of endless jokes.