The International Astronomical Union published an article, “Definition of a Planet in the Solar System: Resolutions 5 and 6,” ten years ago today. (The link is to a PDF file.)

In popular culture, what the IAU did ten years ago today was demote Pluto (seen above) from one of the classic nine planets of the solar system, the nine whose names we all grew up memorizing and reciting, to one of several or many or thousands of “dwarf planets,” leaving us with eight planets. Schoolchildren everywhere possibly find it odd that this elicited as much controversy as it did.

The article employed in an official way a term that had been around since the early 1990s: dwarf planet. A dwarf planet is an object that is massive enough to be a ball (in scientific terms, it is a spheroid that is in hydrostatic equilibrium) and orbits the Sun, but it is not massive enough to have cleared its lane through space.

The decision made sense and simply outlined the fact that as astronomers observed (and observe) more and more objects in our solar system, they needed (and need) new terms to describe what they observe. Pluto is large enough to have been seen from Earth (by Percival Lowell in 1930), so it was deemed a planet, which made sense in 1930. But there are other objects out beyond Neptune that are even larger than Pluto. Should they be deemed planets as well? The term would lose some of its meaning each time that the definition had descriptions added to it or codicils created to force the definition into shape. For a time, the debate over which of the newly found objects would be a planet centered on size, but several are larger than Pluto.

Pluto also has an irregular orbit: the midpoints of each of the eight planet-sized planets pretty closely line up. The Sun and the eight planets form a giant platter, but Pluto comes in from above the platter and shoots below and also comes in closer to the Sun than Neptune’s orbit. Many of the other dwarf planets also have wildly variable orbits that bring them from very far away to incredibly far away.

Five objects are officially designated as dwarf planets: Ceres (the largest object in the asteroid belt), Pluto, Haumea, Makemake, and Eris. There are several dozen other objects being studied, each one farther away than Pluto, that may be added to the list.

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Pope Innocent III annulled the Magna Carta on this date in 1215. Innocent supported King John, who had pledged support for the latest crusade, so he viewed John as something of a victim of bullying barons who had, in his words, forced the king, “by such violence and fear as might affect the most courageous of men,” to sign the document declaring different powers between the king and the barons. The papal bull, welcomed by the king, was largely ignored by the barons.

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Microsoft released Windows 95 on this date in 1995. “Start Me Up”:

 
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Louis Prima died on this date in 1978. He had fallen into a coma during brain surgery in 1975 and did not recover.

 
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Jorge Luis Borges was born on this date in 1899. Howard Zinn was born on this date in 1922. The late Kenny Baker would be 82 today. (He died earlier this month.)

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Dame A. S. Byatt is 80. Mason Williams is 78. Williams performing his composition, “Classical Gas” in 1988:

 
Senator Max Cleland is 74. Vince McMahon is 71. Paulo Coelho is 69. Orson Scott Card is 65. Stephen Fry is 59. Steve Guttenberg is 58. Cal Ripken Jr. is 56. Dana Gould is 52. Marlee Matlin is 51. Dave Chappelle is 43. John Green is 39.

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