The recap: Pluto, deemed a Planet since its discovery in 1930, got 'demoted' to 'dwarf planet' by the International Astronomical Union (IAU) in 2006. That decision was controversial. Alan Stern, principal investigator of the New Horizons mission to Pluto, has stated he still considers Pluto a planet. The matter got only more messy when, in 2008, the IAU decided it wasn't a dwarf planet either, but a Plutoid.
Not to be outdone, Illinois voted to keep Pluto as a planet. So now Pluto is not a planet, except in Illinois or when New Horizons does research?
The latest resurgence of the Pluto debate is covered over at space.com. But first, another example of nomenclature (drawing from some of my earlier blogwork).
Spectroscopic binary star systems of a certain type are called 'Algol-type binaries', named after the first discovered-- Algol. That's how nomenclature often works. But, since later observations revealed Algol itself is a triple star system, that means Algol is not necessarily an Algol-type system.
So it is with Pluto. It may be a planet that is not necessarily a planet. It depends on the definition, and who is defining. The IAU has made their case, via voting. But are they correct? The extremely entertaining Alan Stern (himself a Pluto-is-a-planet person) has this on the IAU planet definition. "By the IAU's definition," writes Stern, "when a cowboy herds his cattle he becomes a cow by association." Is Pluto a cow then? Read on!
Whatever name we call it, Pluto doesn't changed. Here's an example I often run for students. All you readers out there, if you have dark hair-- black or brown hair-- raise your hands. Don't worry about your officemate starting, just raise those hands. Got them up?
Okay, I want everyone who has dark hair-- just black hair-- to raise your hands, but anyone else with their hand up, lower it. Yes, dark hair is now defined as 'black'. Brown hair is no longer what I'm calling dark.
Okay, those of you who first raised your hands for 'dark hair' but then lowered them when I changed the definition-- did your hair color change? Did my changing the term 'dark hair' change you in any way? No? Good!
The New Horizons mission didn't change course when the labeling of Pluto shifted. "Abort, abort, Pluto just turned into a not-planet, turn away before we're doomed!" The name is not the thing.
I've heard others describe Pluto as a 'binary planet' since its moon, Charon, is massive enough that their shared center of mass is above Pluto's surface. And I've heard Pluto called a big comet. Pluto gets called a lot of things.
So does the Earth. Did you know the Earth is not just a planet, but a 'water planet' and an 'earthlike planet', and we're looking for similar ones around other stars? It may sound obvious, but each planet is unique.
In actual astronomical use, the term 'planet' is fairly meaningless, akin to saying it's an "orbiting around a star thing, that isn't a star itself". But then there are brown dwarfs... neither star nor planet. So that definition breaks down. Everything is a special case.
When the IAU voted to state that The "dwarf planet" Pluto is recognised as an important proto-type of a new class of trans-Neptunian objects, that did not change what Pluto is. Nor does it fundamentally change how Pluto should be considered. Pluto is still a fascinating object, rich in history, worth studying, becoming more intriging the more we learn about it.
Pluto is a planet, a trans-neptunian object, a comet, a binary, a Kuiper Belt object, and an animated dog. If you want a precision, try this oft-used definition: Pluto is the tenth-largest body observed directly orbiting the Sun.
Many so-called scientific controversies are rooted in semantics. Evolution is just a theory. Pluto is not a planet. Cold weather means there is no Global Warming. Nature is stronger than nurture. Men and women are the same. Man is no longer evolving.
Each of these statements are good fodder for cocktail party discussion, but they aren't science. You can argue either side just by which precepts and which specific cases you are interested in. As broad statements, though, they hold no content. They are all both true and not true.
The world is a complicated place. Science is a tool to discover and explain the complexity, but as a result, scientific stances are themselves complicated.
In the end, what matters about science is what you do with it. Knowledge is a tool, application is what affects our lives. So for Pluto, the important detail is not its name, but what it is made up, how it formed, what it tells us about our solar system, what technology we've gained from the engineering needed to study it, whether we can go there, whether there are resources we can use, and whether it provides clues to the existence of other solar systems past our Sun's.
The science of Pluto is, in the end, far more interesting than the naming of Pluto.
Alex, the Daytime Astronomer
- The Daytime Astronomer on Pluto the Planet
- Buy New Textbooks Again? Pluto May Go Back To Planet Status
- Pluto- When Is A Dwarf Planet Not A Planet?
- IAU Planet Definition Has "Use Before Date"- Within Decades- Let's Call Pluto, Ceres & Our Moon Planets Right Now!
- Can There Be A Planet Or Dwarf Planet With The Same Shape As Ultima Thule's Snowman?