What’s in a name? That which we call a rose
By any other word would smell as sweet.
—From Romeo and Juliet (II, ii, 1-2)
As we contemplate the fact that Pluto is now to be classified as a “dwarf planet,” becoming one of a class of similar objects beyond Neptune, some people may feel a bit sad. The reality, of course, is that Pluto has not gone anywhere, and nothing has changed in the shape of our solar system. The astronomers are simply changing and clarifying how we classify the objects that make it up.
Consider the story of Ceres. It was discovered on January 1, 1801 by Giuseppe Piazzi. With a diameter of about 590 miles, Ceres is by far the largest and most massive body in the asteroid belt: it contains approximately a third of the belt’s total mass.
Initially, Ceres was classified as a planet. In fact, it held that classification for nearly fifty years. Eventually, however, as other objects were discovered littering the space between Mars and Jupiter, it was reclassified, joining this mass of rocks as simply the largest of the debris that fills that region of space. Ceres did not suddenly vanish in a puff of pink smoke. It simply was reclassified, to make better sense of it, to put it into a category in which it better fit.
Likewise, Pluto has not suddenly vanished. The New Horizons space ship on it way to Pluto has not been recalled because of its new designation as a “dwarf planet.” But it is a reasonable reclassification which helps us better understand the structure of our solar system. As more and more observations of the edge of our solar system have been made over the last decade, astronomers have found a large number of objects out there, all of which are similar to Pluto, and many of which are nearly as large—or, in the case of UB313, nicknamed Xena, actually larger (though both are considerably smaller than Earth’s own moon). These objects are called Kuiper Belt objects (KBOs).
The first astronomers to suggest the existence of this belt were Frederick C. Leonard in 1930 and Kenneth E. Edgeworth in 1943. In 1951 Gerard Kuiper suggested that the belt was the source of short period comets (those having an orbital period of less than 200 years). More detailed conjectures about objects in the belt were done by Al G. W. Cameron in 1962, Fred L. Whipple in 1964, and Julio Fernandez in 1980. The belt and the objects in it were named after Kuiper after the discovery of (15760) 1992 QB1. Although no known object in the Kuiper belt is a possible candidate to become a comet, the name Kuiper for the region has stuck.
And like Pluto, these KBOs orbit the sun in highly eccentric orbits between 30 and 50 AU from the sun (AU is “astronomical unit,” the distance from the sun to the Earth, about 93 million miles). Pluto has a highly elliptical orbit that sometimes brings it closer to the sun than Neptune. It is canted to the ecliptic—the level at which the eight planets orbit the sun—at a 17 degree angle. UB313, which also has a highly elliptical orbit, is canted at a 44 degree angle, and is sometimes further from the sun than Pluto, and sometimes is closer than Neptune. Other major objects out on the edge of the solar system include Sedna, Ixion, Varuna and Quaorar, all larger than Ceres. All together, more than 800 of these Kuiper belt objects have been discovered; astronomers anticipate finding thousands more. Given their size and relationships, thinking of these objects as “planets” like the other eight seemed unreasonable to the astronomers gathered in Prague in August, and so Pluto, as being an obvious member of this group—simply the first one of them to be discovered—needed to be redesignated.
Our solar system is simply a far more interesting, wonderful and complex place than people first thought in the early part of the 20th century and the classification—or reclassification—of the objects within it, is simply a way of helping us more easily comprehend and organize it in our minds. Designating Pluto, and the other distant bodies like it, as “dwarf planets” makes it much easier than trying to memorize a planetary list which otherwise would have grown to the hundreds, if not thousands. Chances are, in the future, we’ll have to make more adjustments in our thinking, too, especially as the details of other solar systems around neighboring stars become clearer.