When I first heard the talk (which I believe happened many years ago) about Pluto not being a planet, I had two main thoughts:
- They’re right. It’s not a planet! (though this was not immediate… I had to think about it for a bit)
- I’m okay with them calling it a planet for tradition’s sake.
Many other thoughts came later, such as:
The scientific community isn’t much for tradition
Isn’t every natural (as opposed to man made) object orbiting the sun considered a planet?
Hmm… I guess they’re called “planetoids.” Or are they? Crap, I can’t remember.
Well, after some thought on the matter, I decided that Pluto was not a planet, and here’s why:
First off, all the outer planets were gas planets. Pluto was a solid. On top of that, it was tiny. Smaller than our own moon in fact. The fact that it had a moon itself helped the “it’s a planet” case a little, but not enough for me.
Second, it’s orbit is really weird. Here’s a layout that shows it’s distance from the Sun compared to other planets:
Ignore the red line. That’s the projected path of the New Horizons spacecraft.
Note that Pluto’s orbit is much farther “off center” than the others. So far off, in fact, that for part of its orbit it is actually closer to the Sun than Neptune.
If you were to rotate the above image 90° on the x- or y-axis (not around like a clock (z-axis), but “back”; like the way a clock’s hands would move if looking at it sideways), you would see that the 8 “traditional” planets orbit the Sun on the same plane. All the planets’ orbits vary “vertically” slightly, but do so very little. Which brings me to point #2. Pluto is the only “planet” that doesn’t orbit the Sun on that plane:
These two things I’ve know for most of my life. However, with the “Is Pluto a planet?” debate heating up, I decided to study up on the little ice ball a bit more. And that’s when I discovered what would be a bit of a clincher for me.
Turns out that Charon is not actually a moon of Pluto. I had always thought that Charon orbited around Pluto. After all, that’s what a moon is, right? An object that orbits around a planet. Well, turns out that Charon does NOT orbit around Pluto, nor does Pluto orbit around Charon. The truth is they orbit around the empty space between them. Some refer to this phenomenon as “dual planets”, a definition that suits me just fine. This occurs because the two objects are tidally locked (sometimes called gravitationally locked) to each other. By contrast, a moon would be tidally locked while the body it orbits around is not. Not all moons are tidally locked however.
So why have we called Pluto a planet for so long? I believe much of it comes from how Pluto was discovered. Way back in the 1920′s, astronomers miscalculated the mass of Neptune. These miscalculations made it appear as if there were some unseen gravitational effect on Neptune, causing unexplainable variations in its orbit. The most obvious explanation was that there was another planet (it had to be large enough to cause the variations on an object as huge as Neptune), so everybody was looking for this so-called “planet x.”
Clyde Tombaugh (whose story is rather fascinating in itself) was an amateur astronomer that, while looking for planet x, stumbled across Pluto quite by accident. Though Pluto’s status as a planet was debated early on, it was eventually decided that this was the fabled planet x, thus it was determined (though it be falsely so) to be a planet.
Decades later, thanks to a flyby of Neptune by Voyager 2, Neptune’s true mass was discovered and we all found out that there were no unexplained variations in its orbit, and therefore no outside gravitational effects acting on it.
Furthermore, Pluto’s orbit doesn’t ever bring it anywhere near Neptune, nor does Pluto (even combined with Charon) have anywhere near enough mass to affect Neptune’s orbit regardless of how close it ever got to Neptune.
There is some new data that makes Pluto an even more fascinating object. Turns out that there are two other “moons”: Nix and Hydra. Does that mean they both orbit around the “dual planets”, orbiting around both Pluto AND Charon? That would be pretty interesting. I have as of yet to find any information regarding the orbital behavior of these two newly discovered moons known as Nix and Hydra:
Anyway, to summarize: Pluto is NOT a planet! Thank you, and good day to you all.