Once upon a time, our local solar system had nine planets. The smallest and most remote of these was an unassuming if eccentric little ball of rock and ice called Pluto. Unfortunately, Pluto wasn’t like the other planets in our neighborhood.
First, Pluto’s orbit sometimes brought it closer to the sun than its nearest planetary neighbor, Neptune. Pluto’s largest satellite, Charon, is more than half Pluto’s mass. Perhaps most damning, Pluto is smaller than several other local celestial objects that aren’t considered planets, notably the scattered disk object Eris and Earth’s own moon.
Finally, in 2006, the International Astronomical Union (IAU) decided that Pluto was just too different to be called a planet, redesignating it as a new category of celestial object: a dwarf planet. (Ceres and Eris also earned the dwarf planet title, as did the trans-Neptunian object Haumea and the Kuiper belt object Makemake.)
The so-called “demotion” of Pluto upset a number of observers and scientists, who felt that despite its abnormality, Pluto should have retained its planetary status. While many of these arguments were made on astronomic grounds, a large number of them boiled down to a simple desire to “grandfather” Pluto in simply because most contemporary astronomers (and laypeople) grew up considering Pluto a planet.
That’s a pretty lousy argument — not least because Pluto is far from the first local solar system object to get its planethood revoked.
WHAT IS THE TOTAL NUMBER OF LOCAL SOLAR SYSTEM OBJECTS THAT HAVE BEEN CALLED PLANETS?
Today we recognize eight full-fledged planets — Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. If every local celestial object that was ever called a planet was grandfathered in — as some Pluto enthusiasts want to do for that recently demoted dwarf planet — our solar system would sport a whopping 36 planets.
Besides the eight planets we recognize today, objects previously considered local planets include:
- One star: The sun.
- Three dwarf planets: Ceres, Eris, and Pluto.
- 10 moons: Jupiter’s moons Callisto, Europa, Ganymede, and Io; Saturn’s moons Dione, Iapetus, Rhea, Tethys, and Titan; and Earth’s moon.
- 14 asteroids: Astrea, Egeria, Flora, Eumonia, Hebe, Hygeia, Irene, Iris, Juno, Metis, Pallas, Parthenope, Vesta, and Victoria.
Now, no one would seriously argue that the sun should be considered a planet. It’s clearly a star. Most wouldn’t argue that moons should be considered planets, as they orbit other planets more directly than they orbit the sun. Where the distinction gets fuzzy is the border between asteroids and planets. You can’t go by size alone; eight non-planetary objects orbiting our sun are larger than Pluto, and two of them — Neptune’s moon Triton and the trans-Neptunian object Xena — have never been classified as planets.
That’s why the IAU had to define a planet with objective criteria, which to date requires 1) that the object directly orbit the sun, 2) that the object be massive enough to have collapsed into a generally spheroid shape, and 3) that the object has “cleared the neighborhood” by absorbing and deflecting any other major objects in its orbital path.
Pluto fails point #3, but meets the other two. Thus, it’s a dwarf planet, and no amount of historical precedent can make it otherwise.
That’s more than just an otherworldly orbital reorganization; it’s an empirically interplanetary instance of Geek Trivia.