Whatever the probes find, it probably won’t help untangle the tortuous reasoning that led to Pluto and Ceres being labelled as dwarf planets in the first place. That happened in 2006, a few months after New Horizons launched and about a year before Dawn did, at a meeting of the International Astronomical Union, the organization that is in charge of classifying and naming celestial objects. The I.A.U. defines a dwarf planet according to four criteria: it must orbit the sun, it must be spherical, it must not be a satellite of another planet, and it must not have “cleared the neighborhood” of other objects of comparable size. Ceres has a diameter of fewer than six hundred miles, Pluto of about fourteen hundred miles. By comparison, Mercury, now the smallest official planet in our solar system, is more than three thousand miles across. So it’s not unreasonable, Stern says, to call Pluto both a planet and a dwarf, provided that one doesn’t cancel out the other. “I’m the one who originally coined the term ‘dwarf planet,’ back in the nineteen-nineties,” he told me. “I’m fine with it. But saying a dwarf planet isn’t a planet is like saying a pygmy hippopotamus isn’t a hippopotamus. It’s scientifically indefensible.”
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Ceres, Pluto, and the War Over Dwarf Planets - The New Yorker - 1 views
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