An Argument The Moon is a PLANET!

In 2006, the International Astronomical Union passed its now-infamous resolution that Pluto is not a planet. In fact, the resolution didn't just demote Pluto, but laid out a list of three criteria you must pass to be considered a planet. Whether or not you agree with the IAU’s definition (and I don't), it's possible to argue that – according to the criteria in their definition, our Moon also qualifies as a planet! Here’s that argument. We’ll start with criteria A: “A planet is a celestial body that: a) is in orbit around the Sun” – And while we often think of the moon as orbiting the Earth – I mean, from the Earth’s perspective, it is – from the Sun’s perspective, the moon and earth together orbit the Sun, nudging each other in and out as they go. In fact, as I’ve explored in a previous video, the gravitational force from the Sun on the Moon is twice as much as the force from the Earth and so the Moon’s orbit always curves inwards towards the Sun, never outwards towards the Earth. So. It’s possible to argue that the Moon meets criteria A: it’s a celestial body in orbit around the Sun. And now, criteria B: “A planet is a celestial body that: b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (or nearly round) shape. ” It’s fairly evident that the moon is, like the earth, a round sphere and not an irregularly-shaped object like many smaller asteroids, comets, and other moons that don't have enough gravity to pull themselves into rounded shapes. The moon not only meets this second criteria, it meets it by a long shot – the moon is many times more massive than other gravitationally rounded objects in our solar system like Pluto, the asteroid Ceres, etc. In short, the Moon meets criteria B: it has sufficient mass to be gravitationally rounded. So we come to criteria C: “A planet is a celestial body that: c) has cleared the neighbourhood around its orbit. ” This criteria is asking for a planet to have enough mass relative to its orbital radius that it gravitationally dominates everything nearby: a planet should either kick other things out of its orbit or control their motion within it, like, by capturing them as a moon, or as a trojan asteroid, or pulling them to crash into the planet itself... This third criteria is the one that disqualifies Pluto from the IAU’s blessing as a planet. Here’s a plot of the masses of various objects in our solar system and the distances of their orbits from the sun – anything above the diagonal line is big enough to “clear its orbit”. As you can see, Pluto is too far from the sun – given its size – to dominate other objects, and is itself actually gravitationally dominated by Neptune. Interestingly, if Pluto were as close to the sun as Mercury, it would be able to clear its orbit, and if the earth moved into the outer Kuiper belt, it wouldn’t be able to clear its orbit, but more importantly, look here – the moon is well and clearly positioned in the “big enough to clear its orbit” zone! If the Earth suddenly disappeared, the Moon would have no issue being considered a planet. Except there would be no one left to care. Anyway, the moon ticks all three of the IAU’s planet boxes. Why then, you might ask, why doesn’t the IAU celebrate the moon as a planet? Look closer at their definition of a planet and you’ll get your answer: there are two more hidden criteria! Just before the three primary criteria, we see that the IAU definition applies to “planets and other bodies, except satellites”. I’m calling this “criteria X” because the IAU didn't label it: Criteria X: the object must not be a satellite of another object – that is, not a moon. Ah. Well I guess that explains it. The moon is, after all, a moon. But there’s one final nail in the moon’s IAU-planethood coffin – a footnote! Really? Down here at the bottom it says: “The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. ” Too bad, the moon isn’t on the list. But… what was the point of a definition if they were just going to list what they count as planets anyway? Why not just publish the list and be done with it? This footnote totally reveals the underlying story behind the IAU’s definition of planethood: make a list in advance of what objects they thought should be called planets, and after the fact try to find scientific-sounding justifications for the list. Which to me is fairly non-scientific! Instead of being descriptive (that is, looking at what exists and seeing how it naturally breaks into categories), the IAU definition is prescriptive: it just says "these things on our list are planets. " If they were at least honest about that, I wouldn’t have as much of a problem. Ultimately, "planet" is a human concept that serves human needs. It wouldn't be that ridiculous to just write down a list and say "these are the objects in our solar system that we all agree to call planets because human culture and history values them as planets. " And if we wanted, Pluto could be on that list, or not. And the moon, asteroid Ceres, or not. But I’m not a fan of pretending that an arbitrary list has scientific justification behind it; and as we’ve seen here, the supposedly scientific definition is pretty poorly thought out, and finally, I personally don’t think we should just use an arbitrary list: science pushes us to re-evaluate our preconceptions and to try to use labels whose meaning has a basis in the natural world rather than human whims. And according to the best, most comprehensive modern planetary science: the IAU’s list of planets is wrong. According to planetary science, the moon is also a planet. The asteroid Ceres is also a planet! The big round moons of Jupiter, Saturn, Uranus and Neptune are planets! And of course, Pluto is a planet. But all that is a story for another day. Ps there’s another hilarious problem with the IAU’s definition of planet, which we’ll get to in a second, but first… Creating these videos is possible thanks to everyone who supports MinutePhysics on Patreon: your help allows me the freedom to create videos without worrying as much about sponsorships and so on. As a year-end thank you, I’m sending every active patreon supporter a print of an illustration from the most viewed MinutePhysics video this year: “The Magnetic Shadow Effect” If you’ve enjoyed watching my videos, please consider supporting me on patreon - thank you so much! Ok

so another problem with the IAU’s definition for a planet is that if a small red dwarf star passed by the solar system and got captured and went into orbit around the sun… according to the IAU’s definition, that star would also be a planet. Even though it’s a star! And people know this is a problem because definitions for exoplanets often say they have to be substellar