Planetary Demotions

When I was growing up we remembered the names of the planets in order with the mnemonic My Very Excellent Mother Just Sent Us Nine Pizzas. However, on August 24, 2006, the International Astronomical Union voted on a stricter definition of what it means to be a planet. Pluto had become a problem.

It was discovered by Clyde Tombaugh at Flagstaff’s Lowell Observatory in 1930, and nearly a dozen more distant worlds were discovered within our solar system in the early 2000s. Our planetary family was expanding to include at least nine more, or we needed a better way to classify different types of worlds.

It was decided that a planet must be in orbit around the sun, large enough that its own gravity creates an essentially spherical shape, and that as the gravitationally dominant body in its orbit it has successfully cleared its neighborhood of other large bodies, excepting natural satellites or moons. Pluto did not meet this definition, nor did the recently discovered Eris, Haumea, Makemake, Gonggong, Sedna, or Orcus. As such, planetary scientist Alan Stern is credited as coining the term ‘dwarf planet’ to classify these worlds that border on the definition. Pluto-lovers worldwide were disheartened as we amended the mnemonic to to My Very Excellent Mother Just Sent Us Nachos.

This wasn’t the first time that a planet had been demoted. On January 1, 1801, a Catholic priest at the Academy of Palermo in Sicily, searching for a dim star, found one that moved from night to night. First assuming it to be a comet, 24 consecutive observations led Giuseppe Piazzi to conclude, “since its movement is so slow and rather uniform, it has occurred to me several times that it might be something better than a comet.”

Mathematical predictions placed this new  object’s orbit as coming back into view on the other side of the sun, and astronomers worldwide looked skyward for its return. On December 31 of that year the orbit of Ceres was confirmed, and astronomers celebrated the addition of a new planet. Located between the orbits of Mars and Jupiter, Ceres is in what we now call the asteroid belt, so named because of its high concentration of solid bodies, some large and spherical and many small and irregular.

Just 15 months after the discovery of Ceres, Heinrich Olbers found Pallas, and by 1807, Vesta and Juno joined the planetary family. William Herschel, who discovered Uranus in 1781, suggested the term ‘asteroid’ for these new worlds, meaning ‘starlike.’ By 1868 over 100 asteroids had been discovered, and by 1921, primarily due to the introduction of astrophotography in 1891, the number of asteroids discovered tallied over 1,000. ‘Asteroid’ stuck, and we now count around a million within our solar system.

Ceres is not visible to the naked eye, but a good pair of binoculars or small telescope should allow you to find this dim world, which fluctuates in apparent magnitude from 6.7 to 9.3 through its orbit. At 2.97AU and just 595 miles in diameter, it represents about 25% of the asteroid belt’s total mass. Look for it this month as it approaches Denebola, the bright star marking the tail of Leo.