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Across the Universe: Planetary Prejudice — 2 Comments

  1. Br. Guy:

    This is one time that I do have to disagree with you! The question about how we classify Pluto and other objects in the Solar System and beyond is more than just wanting Pluto to be a planet. It is more about how we do science and how scientists and humans classify the things around us. A scientist does not classify things based on just one characteristic. He or she creates a list of characteristics and then determines if a given thing has some or all of those characteristics.

    The IAU definition of a planet is completely mathematical (it comes down to having enough gravity to “clear your space,” “roundness” does not count for much) and is dependent on the object’s distance from the Sun (the farther from the Sun, the larger the area it must “clear” and the slower it moves in its orbit). It completely leaves out the geology of the object. If we assume that a planet has to orbit a star (the IAU definition REQUIRES that it must orbit the Sun so forget about 3,000 or 4,000 exo-planets), is Pluto any less of a planet than Mercury? Pluto has a more dynamic surface, Pluto has a thicker atmosphere and seasons, Pluto has more moons, etc. You mention that an object such as Vesta was big enough to have been defined as a planet (along with Pluto and the asteroid Ceres), but the IAU definition of a planet has evolved, too. I am far from an expert in this area, but the original definition would have meant that an Earth-sized body at the distance of Pluto would not cut it as a planet! With the “revised” definition, an Earth-sized body at 100 AU would still qualify, hence the recent discussions about the “discovery” of a ninth planet.

    I personally do not have a problem with there being more than one class of planets: classical planets (8), dwarf planets (at least 5), and planet-like moons. It should be noted that we orbit a dwarf star and the last time I taught Intro Astronomy, the Sun was still a star.
    If one does an extensive search on the definition of a planet (I Googled it), one comes up with a nice, self-consistent planet classification scheme that takes into account for a range of possible physical properties:

    Larry Lebofsky

  2. A few years back Steven Dick, who was chief historian for NASA, wrote a whole book on the question of classification”

    Discovery and Classification in Astronomy: Controversy and Consensus

    “Astronomical discovery involves more than detecting something previously unseen. The reclassification of Pluto as a dwarf planet in 2006, and the controversy it generated, shows that discovery is a complex and ongoing process – one comprising various stages of research, interpretation, and understanding….”

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