Pluto not a major planet? Oh say it isn't so!"

Jack Kramer

The week of August 21, 2006, was one of turmoil in the astronomical community as the International Astronomical Union first considered increasing the number of planets in our Solar System to twelve, then reversed direction and stripped Pluto of its present planetary status altogether. So it ends up that we are now down to eight planets. The following is from an Associated Press summary of the IAU's thinking.

"Much-maligned Pluto doesn't make the grade under the new rules for a planet: a celestial body that is in orbit around the sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a ... nearly round shape, and has cleared the neighborhood around its orbit. Pluto is automatically disqualified because its oblong orbit overlaps with Neptune's. Instead, it will be reclassified in a new category of 'dwarf planets', similar to what long have been termed 'minor planets'. The definition also lays out a third class of lesser objects that orbit the sun 'small solar system bodies', a term that will apply to numerous asteroids, comets and other natural satellites."

Reports indicate the IAU meeting in Prague included some contentious debate. Of the 2500 IAU members present, only 300 saw fit to vote on the issue. That tells us something about the nature of the decision. But since the IAU is the world governing body for all things astronomical, we should probably assume that the decision will stand. Note that this action is not unprecedented. In the 1800's, the asteroid Ceres was considered a planet, but eventually was downgraded to minor planet status.

In one of the local newspaper columns, the writer wondered whether astronomers don't have anything better to do with their time. Of course, this misses the point. With our improved capability of detecting objects in the far reaches of the Solar System, the issue has lately assumed more importance. What is and is not a planet?

From a purely observational standpoint, Pluto is the only planet that doesn't look like a planet. It appears as a mere speck of light, while the others are clearly disks. Fifteen years ago when Pluto was in that part of its orbit that brings it closer to Earth, I set out to observe it in my 10-inch scope. Using finder charts, I located the field where it was supposed to be, but naturally, everything in the field looked like a star. At the time, I was observing with a group of LCAS members at the Von Bergen farm and Kevin Bonges had a scope equipped with digital setting circles. He too was looking for Pluto and his DSCs told him he was in the right field of view. I compared the view in his scope with what I saw in mine and we were both in the same star field. Now which of those points of light was Pluto? In my case, I re-observed the field a couple of nights later and was able to identify one point of light that had moved since the first observation. I had found Pluto! In Kevin's case, he photographed the field that night, then a few nights later and was thus able to see which object had moved - basically what Clyde Tombaugh had done in 1930 when he discovered Pluto. We both had a personal sense of satisfaction knowing we had viewed all nine planets in our Solar System. Well, okay ... eight, plus one "dwarf planet". Maybe Pluto - that undistinguished spot of light - doesn't deserve to be counted with the rest, after all.

But I can't help feeling sorry about Pluto's demotion. After all, every one of us grew up in the hobby of astronomy believing in nine planets. Tradition is strong. And I think many of the 2200 IAU conferees who abstained in the voting were similarly conflicted.

When making presentations to school groups and in other public programs, it was always fun talking about Pluto's highly elliptical orbit. Because its orbital period is almost 248 years, Pluto has yet to complete an entire orbit of the Sun since its discovery. That orbit sometimes brought it within Neptune's orbit so that Pluto then became the eighth planet and Neptune became the ninth, in terms of distance from the Sun. This surprised most people. In the end, that interesting elliptical orbit became Pluto's undoing.

While I can understand the reasoning behind the IAU's decision, why couldn't they have sort of "grandfathered" Pluto? They could have bowed to a sense of history and the legacy of Clyde Tombaugh and left things well enough alone. But nostalgia doesn't count for much when faced with hard science. I will always remember the time when I saw Pluto in my own telescope, as unimpressive as it was. I saw it when it was a full-fledged planet, and will always remember it that way.

Published in the October 2006 issue of the NightTimes