Just what is Pluto? If you were to have asked an average person that very question in July of 2006, you would likely have received one of two answers: Pluto was either an old Disney cartoon character or the smallest planet in the solar system.
Since the International Astronomical Union’s 2006 definition of “planet,” Pluto’s status as one of the nine planets that this generation has grown up with has been suddenly changed, instead designating the tiny world as a “dwarf planet.” The public outcry against this apparently sudden and cruel exclusion was enormous, leading to what has become popularly known as “the Great Pluto War.” People have rebelled against the change without understanding the reasons behind that change. The story of Pluto’s planethood is not so simple a tale as most people believe it to be.
Pluto’s tale begins not with its discovery by Clyde Tombaugh in 1930, but more than a century before that, in 1828. A glance in textbooks of the period would tell a time-traveling visitor that the solar system did not have nine planets. Rather, there were eleven planets to a student of astronomy in 1828: Mercury, Venus, Earth, Mars, Jupiter, Saturn and Uranus would all be familiar to a modern observer. Neptune and Pluto had yet to be discovered. The remaining four planets -Ceres, Pallas, Juno, and Vestawould take almost anybody by surprise today, as they have been classified as minor planets for nearly 150 years.
Those four tiny worlds were reclassified to put an end to a growing problem in the nineteenth century, as astronomers continued to find new objects and label them as planets. In 1851, the solar system contained as many as 22 planets, most of which we now call minor planets or asteroids. By 1855, astronomers had begun to reclassify the planets in the solar system until they were left with the familiar eight “major” worlds, from Mercury to Neptune.
Discovered in 1930 and given a name by the 11 year-old Venetia Burney in 1931, Pluto was originally believed to be much larger than it actually is, and was hence immediately given full planet status. Despite the subsequent realization of its diminutive size, the classification stuck until 2006, when the IAU created a firm set of characteristics for planets.
What exactly qualifies a world for planethood? Must it be a certain size? Must it have moons? Must it have an atmosphere? None of these qualities are truly relevant to planet status, nor would they distinguish Pluto in particular; there are larger Kuiper belt objects than Pluto, some of which have moons and atmospheres as well. It would seem that Pluto is not particularly special after all.
Or is it? It is true that Pluto fails to meet all of the IAU’s new standards for planethood; while it is in orbit of the sun and it has sufficient mass to maintain hydrostatic equilibrium and hence a reasonably round shape, Pluto lacks sufficient gravity to clear the neighborhood of its orbit as the eight major planets all have and is therefore not considered to be a full planet. It is equally true, however, that Pluto’s real importance may not lie in whether or not it is considered to be a planet.
Pluto’s story is important because it illustrates how the process of ongoing discovery continually changes our understanding of the universe. It is a testament to the dynamic nature of knowledge, and proof positive that you need not be big to make a difference.