|A comparison of sizes of Pluto and Earth.|
How did Pluto lose its planetary status? Discovered by Clyde Tombaugh in 1930 while working for the Lowell Observatory in Arizona, Pluto captured the imagination of the country. Pluto was named by Venetia Burney, an eleven year old girl from Oxford, England. She thought naming the new planet after the Roman god of the underworld was appropriate, considering the new world would be so cold and dark. The Lowell Observatory agreed unanimously. Pluto-mania followed: Disney would name its new side-kick for Mickey Mouse after Pluto. In 1941, Glenn Seaborg named the newly-formed element Plutonium after Pluto as well.
Pluto’s status as a planet was questioned by astronomers over the years due to its small size, but the final blow was administered by Caltech’s Mike Brown. In 2005, Brown discovered a body in the Kuiper Belt—a region on the outskirts of our solar system littered with icy objects—that was larger than Pluto. Brown named the body Eris and initially considered it to be the 10th planet. But because it was so far from the sun—roughly three times farther than Pluto—members of the International Astronomical Union (IAU) began to worry. What if more planets were out there waiting to be discovered? Where would it end? This prompted them to do something that hadn’t been done since the time of the ancient Greeks—to actually define what it means to be a planet.
The pivotal criterion in the IAU definition was that a planet must clear the neighborhood around its orbit of matter, which Pluto does not do. So, after 76 years, Pluto was given the boot along with Eris and any other yet-undiscovered body. They were reclassified as “dwarf planets” instead. At least now the school-kids mnemonic for remembering the order of the planets, “Martha Visits Every Monday and Just Stays Until Noon, Period.” can lose the Period.
- Pluto is smaller than our Moon.
- Pluto’s moon, Charon, is more than half the size of Pluto.
- Because Pluto is so far away, the Sun would look like a bright star from Pluto’s surface.
- Pluto’s has an irregular orbit, and starting in the year 2227, Pluto will be closer to the Sun than Neptune.