If you memorized the names of our solar system's planets via some variation of the mnemonic sentence: "My Very Educated Mother Just Served Us Nine Pizzas," then perhaps the issue of Pluto's celestial status resonates with you, one way or the other.
You learned via a smart mom and an absurd number of pizzas that there are nine planets. Nine. And that the smallest of these is Pluto. (Pluto the planet, of course; not to be confused with the lanky, spicy-mustard-colored, whip-tailed canine of the Disney persuasion).
Pluto has held the exalted position of smallest, most-distant-from-the-sun planet since it was first discovered by Clyde Tombaugh on February 18, 1930, and named by eleven-year-old English school girl Venetia Burney. School children all over the US happily recited that Mom-and-Pizza mnemonic as they dutifully learned the names and placements of all nine planets in our solar system.
Did I mention there are nine planets? Well. There were. Until 2006, when They went and screwed it all up. "They" is the International Astronomical Union. Due in part to an increasingly heated debate in the scientific community over the years as to whether Pluto is, in fact, a bonafide, card-carrying member of Club Planet, the IAU decided to come up with an official set of membership criteria. To be a planet, a celestial body must meet all of the following:
- Orbit the sun.
- Be large enough to have a near spherical shape due to the force of its own gravity.
- Must have swept its orbital neighborhood clean of large objects.
It is item #3 that did Pluto in: at least one object in its orbital neighborhood (the largest of its three moons, Charon) is nearly half Pluto's mass.
And so, with a few strokes of a pen (or rather, a few characters typed on a computer screen, most likely) the far-out, frozen, Planet Formerly Known as Pluto has been relegated to mere dwarf planet status. And dwarf planet, of course, has a definition, too. A dwarf planet:
- Orbits the sun.
- Is large enough to have a near spherical shape due to the force of its own gravity.
- Has not swept its orbital neighborhood clean of large objects.
- Is not a satellite.
Lest you consider the matter closed, here's a new wrinkle: there are some planetary folks who feel Pluto is really in a binary system with its largest moon, Charon. But since the IAU hasn't yet officially defined "binary dwarf planet," Pluto is still just a dwarf planet. For now.
In the meantime, while Pluto lives through a very controversial celestial identity crisis, the IAU decided to spare Pluto's largest moon a similar fate: Charon still gets to keep its "moon" status until the whole binary dwarf system thing is resolved.
To add insult to injury, Pluto's fall from grace has spawned a new verb: "to pluto," meaning, "to demote or devalue someone or something," as in, "Dude, you just got plutoed!" And it doesn't even get the dignity of capitalization.
So, from celebrated new planet to so much space flotsam and jetsam in a span of only 80 years?
Hold on! Before you lament that life just isn't fair, know that there are some consolation prizes:
- First, in 2008, the IAU announced the newly created category of plutoids: a term which describes Pluto, the newly discovered Eris, and other similar celestial bodies which have an orbital semimajor axis greater than that of Neptune, and which have enough mass to be of near-spherical shape.
- Second, "plutoed" received the dubious honor of being chosen as the American Dialect Society's 2006 Word of the Year.
- Third, the good people in both the state of Illinois and the state of New Mexico have declared - officially - that Pluto is indeed a bonafide planet every time it passes through the skies over their respective states. And, they went even further, declaring that March 13th* be henceforth known as "Pluto Day" (in Illinois) or "Pluto Planet Day" (in New Mexico). No matter what the IAU says.
That's something, I guess.
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*Note: March 13th is the birthday of Percival Lowell, an astronomer who first called the scientific community's attention to the existence of a "Planet X." It was during the search for Planet X that Clyde Tombaugh discovered Pluto.
In the interest of fairness, these articles contain compelling discussions supporting the notion that Pluto is not a true planet. (I may disagree, but I love a good discussion):
The discovery of 2003 UB313 Eris, the 10th planet largest known dwarf planet - Article by Mike Brown, Professor of Planetary Astronomy, California Institute of Technology