ELi5 what makes the sun a star and not a planet? And why is the moon not considered a planet?
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Classically, the sun and the moon are planets too – the word ‘planet’ simply means ‘wanderer’, as in, lights in the sky that move around rather than staying static like stars do.
Of course, then we discovered that most of the stars in the sky are gigantic nuclear furnaces like the sun is, while the other planets were balls of rock and gas orbiting the sun. So we decided the gigantic nuclear furnaces are called ‘stars’, and the things that orbit them are ‘planets’.
And then we decided to name the things that orbit planets after the thing that orbits us.
If you look up at the sky in ancient times, you’d probably notice three distinct groups of things:
* The really big, bright thing that’s out in the daytime — the sun.
* The really big, sort of bright thing that is sometimes out at night, sometimes at day — the moon.
* Lots of little twinkling dots — stars.
That’s a pretty clean-cut way of classifying things. Though, if you watched the stars a lot, you’ll actually see a few that behave very strangely. Five of them, if you’re really perceptive. These starts don’t twinkle, and they *move around* over time relative to all the other stars, which seem to stay fixed in place. Since they look like they’re wandering about, they were called “wanderers”. Or in Greek, *πλανήτης* (roughly “planetes”). Planets.
For a long time in many civilizations, that was pretty much were things stood. You had the sun, the moon, the stars, and five funny starts that wandered around called planets.
As mathematics and telescopes advanced, we learned a few things that started to screw with this system:
* The “wanderers” were actually really close to us. This made astronomers conclude that all of the planets, Earth, the Sun, and Moon were all in one system, while the stars were either very far away or on some static backdrop. For a time it was assumed Earth was at the center. This made the Sun and Moon into “planets”, because what else would they be? So “planets” stopped referring to funny wandering stars and became “things that orbit Earth”.
* Earth is *not* at the center. The Sun is. That was a pretty hard pill to swallow. This changed the definition of “planet” into “things that orbit the Sun”. Now Earth is a planet and the Sun wasn’t. The Moon was a weird exception, because it was *very* clear that it still orbited Earth, not the Sun. So it also wasn’t a planet anymore. It was apparently its own kind of thing.
* Earth is *not* special for having the Moon orbit it. Other planets were discovered to have their own little subordinates orbiting them instead of the Sun. This was the point where *the* Moon became simply *a* moon, as the word “moon” was co-opted to refer to any small body orbiting something that isn’t the sun. (The fancy-pantsy word for it is technically “satellite”, but “moon” tends to be acceptable in common parlance.)
* The stars are actually the same kind of objects as the Sun. The only difference is that the Sun is obviously very close, while the other stars were very, *very* far away. This made the Sun a star.
* There are actually more planets we didn’t know about. Some big nerd noticed some strange movements of stars in a bunch of old photos and found Uranus. Then, it happened again when another big nerd found Neptune. And once again when a third big nerd discovered Pluto.
Over time, more and more things in the solar system were found. Since our definition at this point is “if it orbits the Sun, it’s a planet”, they all became planets. Ceres, Pallas, Juno, Vesta… Except, wait, why are there so many of these damn things?! Also, it seemed that a lot of the newcomers were *tiny*. Much, *much* smaller than Mercury, the smallest known classical planet. As we continued to discover more and more of these things, possibly thousands, it became evident that calling them “planets” wasn’t very helpful. We thus came up with the word “asteroid” for them. And it turned out to be a pretty good call, because these days we think there may be billions of asteroids out there in our solar system.
Pluto was safe from the asteroid reclassification, as it wasn’t located in the same place where all the asteroids were found, but very recently, the same exact thing started to happen in Pluto’s area. We started to find more “planets”. We even managed to find one bigger than Pluto was! So, after much deliberation, Pluto and these objects became reclassified into an asteroid-like category called Kuiper Belt Objects (KBOs). It’s currently estimated that the number of KBOs out there is in the *trillions*.
Except, something still bothered some astronomers. Because clearly objects like Pluto and even Ceres were quite special. Most of their asteroid or KBO cousins were way, *way* tinier than them. And unlike most of them, objects like Pluto and Ceres were big enough to be crushed into spherical shapes, just like the other planets, while the billions and trillions of asteroids and KBOs are lumpy. But they clearly weren’t big *enough* to be part of the “planet” classification. Where do we draw the line?
In 2006, it was agreed that a new classification, “dwarf planet”, would be created for these objects. Dwarf planets orbit the sun, and are spherical. Same for regular planets. But what sets the two apart is the third rule, that planets have to have “cleared out their orbital path of all other debris”. That’s something that dwarf planets like Pluto and Ceres haven’t done. So Pluto was officially demoted from planet status, and Ceres was official promoted from asteroid status.
All this to say, the art of classifying things you see in the world can be difficult to do objectively. Especially when your systems are expected to stand the test of time. Because it’s almost inevitable that you will eventually come across something you need to classify that blurs the lines, suggesting you need to rethink your system, or you learn something new about a thing you’ve already classified that makes you reconsider where it should really belong.
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