Most of us grew up with the conventional definition of a planet as a body that orbits a star, shines by reflecting the star's light and is larger than an asteroid. Although the definition may not have been very precise, it clearly categorized the bodies we knew at the time. In the 1990s, however, a remarkable series of discoveries made it untenable. Beyond the orbit of Neptune, astronomers found hundreds of icy worlds, some quite large, occupying a doughnut-shaped region called the Kuiper belt. Around scores of other stars, they found other planets, many of whose orbits look nothing like those in our solar system. They discovered brown dwarfs, which blur the distinction between planet and star. And they found planetlike objects drifting alone in the darkness of interstellar space.

These findings ignited a debate about what a planet really is and led to the decision last August by the International Astronomical Union (IAU), astronomers' main professional society, to define a planet as an object that orbits a star, is large enough to have settled into a round shape and, crucially, "has cleared the neighborhood around its orbit." Controversially, the new definition removes Pluto from the list of planets. Some astronomers said they would refuse to use it and organized a protest petition.

1 Comments

1. 1. laurele 11:51 PM 10/2/08

   You are clearly misrepresenting the work of Stern and Levison. Yes, they distinguished between dynamically dominant "uber-planets" and non-dominant "unter-planets," but they never say that the smaller, "unter-planets" are not planets at all. That is what the IAU definition, which was adopted by only four percent of its members, most of whom are not planetary scientists, dictates--a statement that makes no linguistic sense. Lumping dwarf planets in with the asteroids and non-round KBOs blurs the very important distinction between round and non-round objects. The round ones are in hydrostatic equilibrium, meaning they have enough self-gravity to have pulled themselves into a round shape. When this happens, differentiation and geological processes akin to those on the larger planets occur. These processes do not occur on shapeless, inert asteroids. Yes, in some cases of very small objects, it may be difficult to tell if they have achieved hydrostatic equilibrium, at least with the observation technology we have now. There will always be objects on the fuzzy edges of any definition. However, Pluto, Ceres, Eris, Makemake, and Haumea are not at these edges. Their spherical shape and resulting geological processes are well known. It is not a useful scientific classification scheme to ignore this and lump these as just more objects in belts. That defines the objects solely by where they are while ignoring what they are. If we use only dynamics, Earth would not be a planet if it were placed in Pluto's orbit. The best way to fix this is to amend the IAU resolution and include dwarf planets as a subclass of the broader umbrella term "planet." The number that results in does not matter, as memorization is far less important than is understanding the different types of planets and their characteristics. As for the round moons of planets, these were in the past referred to as "secondary planets," reflecting their composition being that of a planet but their orbital path being around another planet rather than around a star. We can still colloquially refer to them as moons or satellites.

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