On August 24, 2006, the International Astronomical Union defined the term "planet" in such a way that former planet Pluto and newly discovered SDO Eris would be excluded from the planetary club. While some people take this as a deliberate stab against Pluto, and it may indeed be just that, it is not unprecedented. Many celestial bodies were considered planets at one time, and subsequently downgraded. These include the Sun, the Moon, the asteroids Ceres, Pallas, Juno, and Vesta, the Galilean moons of Jupiter, and some of the largest moons of Saturn. In fact, most newly discovered bodies were termed planets until 1851, when the planet count grew to 23, and astronomers began to term the small objects "asteroids". The difference with Pluto is that it has been considered a planet for over seventy years in contemporary time. Its closest competitor, Ceres, was considered a planet for the first half of the 19th century.
The decision has been controversial, but was long overdue. Recent years have seen the discovery of many celestial oddities. More and more large objects are discovered in the outer reaches of our own solar system. Planet-mass objects have been observed orbiting pulsars. We see Brown Dwarfs, barely more than planets themselves, happily orbiting larger stellar companions. Even rogue planets, appearing to orbit nothing at all, have been found. How, then, do we define a planet?
Under the IAU ruling, a planet is defined as a celestial body that:
- is in orbit around the sun. The first criterion limits the definition to our own solar system. With the discovery of new extrasolar planets by the day, it should be noted that such objects are covered under a separate agreement signed in 2003.
- has sufficient mass so that it assumes a hydrostatic equilibrium (nearly round) shape. This is where the guidelines get fuzzy. How round is round enough? The Earth itself is not a perfect sphere (although it varies by only 0.33%). Naturally, all the major planets and Pluto, Eris, Ceres, and several others meet this guideline, but hard numbers should be established for borderline objects.
- has "cleared the neighborhood" around its orbit. This is arguably the most controversial aspect of the new definition. However, even this can be seen as perfectly logical. While there are many asteroids which occupy the same orbit as Earth (and, indeed, all the other planets, too), their combined mass is tiny when compared with Earth's itself. The ratio µ of Earth's mass to that of all other objects in a similar orbit is about 1.7 million. The planet with the smallest µ is Mars at 5,100. Compare this with Pluto, with a µ of merely 0.077 (sharing an orbital zone with Neptune doesn't help things). Dwarf planets Ceres and Eris weigh in at 0.33 and 0.10, respectively. Likewise with "roundness", a clear distinction should be made for borderline objects, perhaps µ=1,000 (an object thereby accounting for 99.9% of the mass in its orbital zone). However, Pluto and the others are clearly well below any arguable threshold.
Naturally, further discussion and elaboration of the definition is needed. This will likely be a prominent topic at the next IAU General Assembly in 2009. However, for the time being, Pluto and the others remain "dwarf planets".
Please don't misunderstand me. I love Pluto as much as the next guy. I'm eagerly awaiting the New Horizons flyby in 2015. But the more I think about it, the more I agree with the IAU ruling. Pluto has always been an oddity, but no more. Instead of being the tiny one with a crooked orbit, it's now King of the Dwarf Planets. For the naysayers, consider this: if Pluto were discovered today, would we call it a planet?