Pluto
Pluto

As children, everyone is taught the traditional order of the nine planets in the solar system. However, as we learn more about our neighboring planets— and space in general— this may have to be changed. Between and beyond Neptune and Pluto, thousands of small, somewhat planet-like objects have been found. These have been shown to occupy three distinct areas: the Kuiper belt, the scattered disc, and the Oort cloud. The discoveries have so far had two major implications:

First, there are several possible ‘tenth planet’ contenders out there – objects roughly the same size as Pluto and not unreasonably far away. The major examples are Quaoar, Sedna, and “Xena”, all discovered since 2002. Several smaller, unnamed ones have been found also, and surely will not be the last.

Second— perhaps conversely— the categorization of Pluto as a planet has come to be disputed. In fact, many astronomers argue that if Pluto had been discovered only in the last few years, it would not be labelled a planet at all.

The definition of “planet” is contentious and, considering how many things are possible in space, may be somewhat arbitrary in the first place. However, there are several signs that Pluto may be better considered a “Kuiper belt object” than a full-fledged planet. Among the nine current planets, Pluto proves to be the single anomaly in a number of ways— most obviously in size, or maybe lack thereof— but it is strikingly similar to some of its new-found neighboring objects.

Clyde W. Tombaugh (1906-1997)
Clyde W. Tombaugh (1906-1997)

History is partially to blame for the controversy. When it was first found in 1930 by young American astronomer Clyde Tombaugh, Pluto was believed to be larger than Earth – in fact, 400 times what we now know its size to be. Fueled by a national wave of patriotism at the discovery, Pluto entered the history books as the first planet discovered by an American. Through the 20th century, any attempts at labelling Pluto as anything less significant was met with outrage by proud astronomers and scientists in the U.S. When British astronomer Brian Marsden suggested at a 1980 conference that Pluto be deemed a mere asteroid, the Americans reportedly almost threw him into a swimming pool.

Pluto did, though, turn out to be problematically tiny. Its equatorial diameter of roughly 1485 miles (2390 km) makes it smaller even than its new-found neighbor “Xena”. It is also smaller even than some of the moons in the solar system, Earth’s included. This would be less of an issue if Pluto were one of the roughly similarly-sized inner planets – but it is out beyond the ‘gas giants’ and in a region of space that is rapidly becoming known for its relatively high concentrations of small objects.

Pluto's orbit, tilted about 17º relative to the others'
Pluto's orbit, tilted about 17º relative to the others'

Also, while the orbits of the first eight current planets lie in a plane, Pluto’s orbit is tilted by 17º relative to them. Its orbit also much more elliptical than the others’, so much so that it actually overlaps with Neptune’s orbit. During certain periods of Pluto’s orbit, it is nearer to the sun than Neptune is. Most recently this happened between 1979 and 1999, and will occur again starting in the year 2227.

Then there’s the issue of Pluto’s relationship with its three moons. Two— Nix and Hydra— are so tiny that they weren’t found until 2005. The largest, Charon, is strikingly large for a moon: its diameter is almost half of Pluto’s.

Because of this, Charon has an unusual amount of gravitational influence over Pluto; Charon does not actually orbit Pluto at all. Instead, Pluto and all three moons orbit the common point between Pluto and Charon – their center of mass or barycenter. No other objects in the solar system do this. It is unclear, then, whether Charon can be strictly labelled a moon – or, for that matter, Pluto a planet. For a while, some astronomers took to referring to Pluto and Charon as ‘double planets’ or ‘sister planets’. Neither of the terms ever caught on, though— and the recent discovery of Nix and Hydra does nothing to help clear up the situation. (Does that make them quadruplets now?)

It is looking increasingly likely, then, that Pluto is probably better categorized as a mere Kuiper belt object than a planet. The decision may come in September, when astronomers, historians, scientists, and teachers are hoping to formalize a definition for ‘planet’ once and for all. Could Pluto be demoted? It has been a long time since Tombaugh first identified it, and there is very little left of the fierce American pride that kept Pluto very securely a planet for the first few decades after its discovery.

New Horizons
New Horizons

And such a reclassification has actually been done before. Ceres, the largest asteroid and the first to be discovered, was considered a planet from its accidental discovery in 1801 until about 1850. What caused astronomers to change their minds was simply the discovery of more asteroids— a lot more. With the recent similar discoveries of the many trans-Neptunian objects, Pluto’s claim to planethood is looking more and more dubious. Whatever it is, Pluto will be studied in detail for the first time in 2015, with the arrival of NASA’s New Horizons, launched in January 2006.

Distinguishing between a ‘planet’ and a ‘Kuiper belt object’ may be nothing more than pickiness at the moment, but the need for a firm definition for ‘planet’ may become more pronounced as we look farther beyond our solar system. As of June 2006, over 200 extrasolar planets have been found, and we have few ways of knowing what sorts of astronomical objects are still out there. Agreed-upon classifications, therefore, will only become more crucial as we gradually explore and catalogue the universe around us.