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?