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The solar system is a much more interesting, complicated place if we throw out the classical "Solar System has 9 planets" model we learned in grade school. And Pluto is just as special.

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In science, the term "planet" is no longer useful. It's too vague and general.

The term goes back thousands of years to a time when the visible objects in our own solar system were lumped in with the stars. They only difference between "planets" and stars, as far as our ancestors could tell, is planets didn't orbit around the Earth once a day in a fixed position relative to one another; instead, they wandered throughout the constellations in patterns, that with time, were predictable, but weird. "Planet" just means "wanderer."

Eventually we figured out that the worlds we called "planets" were going around the sun, just like Earth, but even at that point, there were only six official planets. Then there were seven (Uranus). Then there were eight (Neptune).

Pluto was discovered in 1930. For decades, Pluto was the oddball of the solar system. A tiny rock floating out beyond the orbit of four gas giants--Jupiter, Saturn, Uranus, and Nepture. All the other "terrestrial worlds" (meaning balls of rock) were in the inner solar system: Mercury, Venus, Earth, and Mars. One other rock ball that was once beyond Mars was crushed to smithereens by Jupiter's gravity and became the asteroid belt.

Plus, Pluto's not even in "the plane of the ecliptic." If you think of the solar system in three dimensions (x, y and z), most of the classical planets have a different distance from sun and rotation (x and y) from each other, but they each share the same z axis. Look at the solar system on its side, and their orbits line up in a straight line. Pluto rotates at an extreme angle relative to the plane of the ecliptic.

None of these facts are in and of themselves reasons to kick Pluto out of the planet club, however. And it is partly Pluto's small, oddball status that made it so dear to many people's hearts. It was the underdog of the solar system. Children especially love it.

But the classical model of the solar system Pluto emerged in is outdated and oversimplistic. If we define "planet" as any body of a given size that orbits the sun, then there are at least 13 planets in the solar system, and Pluto is not number 9 in distance (so put down those hands, 9-finger Pluto saluters). It's number 10. The others, besides the ones we learned about in primary school, are Ceres, Eris, MakeMake, and Haumea, the so-called "Dwarf Planets" (let's throw that term out, too).

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Where are these other worlds? A variety of places. Ceres is in the asteroid belt between Mars and Jupiter. The others are in one of two regions of the solar system outside the orbit of Neptune, the Edgeworth-Kuiper Belt and the Scattered Disc, that together make up a donut-shape full of icy rocks:

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Once we understand Pluto is a world in the Kuiper Belt, it's no longer an odd ball. It's quite at home, thank you very much, surrounded by thousands of similar worlds.

So why not just say all the Kuiper Belt and Scattered Disc objects are planets, too? Well, if we did, the number of known "planets" balloons to close to 3,000 plus. And if eliminate all the non-spheres to get the number down, we eliminate equally massive worlds like Hauma, simply because it isn't spherical (it's an oval). To hang on to the term "planet", we have to draw the line somewhere. But to do that, we'd have to pick some arbitrary amount of sphericity (no world is completely spherical) and an arbitrary size cut-off.

Judging a world by its size is a little unfair, when you consider Pluto is smaller than many moons in our solar system, including our own:

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The term "planet" has very little scientific utility. The most useful way to use the term is as a colloquial synonym for "world", in which case even our moon is a planet.

It is time to throw out the simplistic model of a solar system, ANY solar system, that defines it as "a certain number of 'planets' revolving around one or more stars." Solar systems, we are discovering from our own and other systems, have objects of all sizes and compositions: asteroids, comets, rings, dust, solar wind, and systems-within-systems, such as the dozens of moons that orbit Jupiter, or Pluto and its five moons.

So why can't we just say a planet is a body that has moons? Because even asteroids can have moons. One flew by Earth recently. And Venus and Mercury have no moons.

A much more interesting, accurate model of our solar system has at least five "regions". The terrestrial spheres, the asteroid belt, the gas giants, the Edgeworth-Kuiper Belt, and the Scattered Disc (and possibly six, if you count the theoretical Oort Cloud, a giant donut orbiting our solar system at one light year's distance, allegedly the origin point of long-period comets). In this model, Pluto is rightly classified as the King Kuiper Belt Object, the biggest known KBO. It has a close second in Eris, which was thought for quite a while to be bigger than Pluto because it's so much more dense (more rock than Pluto, which is 30% ice).

In this model, Pluto is special again*.

* Unless you're one of those people who resist this suggestion because their objection to Pluto's "demotion" is based strictly on anxiety caused by "Elementary school LIED TO ME!" They just want there to be nine planets. They don't actually give a hoot about The Plute.



( 8 comments — Leave a comment )
Jul. 16th, 2015 03:53 pm (UTC)
Very well put together. And you thought you weren't good at teaching. ;o) We can forgive our grade school teachers for not knowing how small Pluto is in comparison to the eight currently official planets, if the best astronomers had no idea.

I might add on to your lesson, some other things you probably know. Pluto's orbit is very different from 'the eight.' Not so long ago (during the time Buffy was first on the air as a matter of trivia) Pluto was closer to the Sun than Neptune. Since Pluto's trip around the sun takes over 247 years that time period was kind of special astronomically speaking, at least when we were talking about Pluto as a full fledged planet. All orbits are technically ellipses. The eights' orbits are very close to circles centered on the sun so they don't come anywhere near to crossing inside one another. Pluto's orbit is also at a considerable angle to the pretty much flat plain of the eight. Since it was discovered we knew Pluto was different. I suppose now little science-minded kids won't be as interested in Pluto, but as you demonstrate, it's still special.
Jul. 16th, 2015 03:58 pm (UTC)
It crosses inside the orbit of Neptune from time to time.
Jul. 16th, 2015 05:17 pm (UTC)
I'll bet those kids are very interested, right now. And maybe they'll be lucky enough to get teachers as good as Masq!
Jul. 16th, 2015 05:14 pm (UTC)
Wonderful! I didn't know all that! But seeing your diagrams and pictures makes me want even more to use Latin names for our own local objects: Terra, Luna, and Sol. That makes them seem more on a par with the rest of the system, to me at least.

But more important--this is so great and so clear. Can I link to it on FB? With your real name or without?

Edited at 2015-07-16 05:16 pm (UTC)
Jul. 16th, 2015 05:22 pm (UTC)
I am going to post it to my other blog, which will cross post to my facebook page, and then you can share that.
Jul. 16th, 2015 05:31 pm (UTC)
Jul. 18th, 2015 12:04 pm (UTC)
I always found it curious (and somewhat amusing) how emotional people got over Pluto's no longer being considered a planet. They often acted like they had some kind of personal stake in it, which I never understood.

Very interesting post though; I learned a lot.
Jul. 18th, 2015 04:13 pm (UTC)
I've informally polled people just by this topic coming up in casual conversation off and online and seen a variety of reasons for people caring about this topic. A lot of people, kids especially, identify with Pluto because it's the little guy out there hovering behind the big guys. It's like the child of the planetary family, if you consider that family to be "nine planets." Others identify with Pluto because it's been the mysterious one up to now. These sort of folks like to identify with whatever is the "weirdest" thing.

Then there are people who just are wigging out because science "changed its mind." They know nothing about how science actually works, putting forth models, and then refining those models based on new evidence. We discovered all this stuff out on the periphery of the solar system that made us see Pluto in a new light; get over it.
( 8 comments — Leave a comment )