PenPaladin

Ganymede, the biggest moon in the solar system, is 0.008% the mass of Jupiter. The Moon is a bit over 1.2% of Earth's mass and a solid 27% of its radius. There's no other planet* in the solar system with a satellite anywhere close to the kind of similarity in size that the Earth-Moon system has.

You might also say "Fine but it's literally called 'The Moon' so that's a bit silly". To which I say that I've been calling it that to be more easily understood but it would be extremely easy to switch to calling it "Luna" which is what most people do when they encounter situations where saying "The Moon" creates ambiguity - like when writing sci-fi or a nontrivial amount of astronomical research.

In conclusion, lumping Luna in with the satellites of other bodies is unhelpful because it is geophysically distinct from most of them, and orbitally distinct from all of them. Luna is a planet and it's rad that we can see one so clearly in the night sky.

Hi, I'm an astrophysicist, I am well aware of the IAU (International Astronomical Union) definition of a planet. It is a definition that is relatively controversial and doesn't really make a lot of sense - it was mostly written to prevent the list of planets from getting too long as we discovered more dwarfs. This definition does not include anything to do with barycentres - I'll get back to that.

The IAU definition of a planet has 3 parts:

1. Object in orbit around our Sun

2. Object has reached hydrostatic equilibrium (i.e. it's a sphere, not a potato)

3. Object has cleared its neighborhood around its orbit

Both Earth and Luna satisfy 2. I will accept that 1 is debatable, but the crux of my argument is that Luna is the size of a terrestrial planet (yes, ~1% of Earth's mass, but still huge and of roughly the same order of magnitude as Mercury, which is a mere ~5% of Earth's mass) and exceptionally large relative to its orbital partner compared to any other "satellite", a factor the IAU does not account for.

It's important to note that 1 literally does not permit binary planets - even if both bodies are identical, one must be the "moon". This is major criticism of the IAU definition. It also doesn't account for exoplanets or rogue planets, but that's another story.

3 is where this definition falls apart. You could easily argue that most of the planets in our solar system fail depending on how you interpret it, because it's so fuzzy. The Earth fails, because there's a planet-sized body chilling out in its orbit (Luna), Jupiter fails because it has huge quantities of asteroids trapped in its Lagrange points, etc.

In the latter case, we say that because Jupiter is determining the motion of these bodies, it still counts. For the Earth we simply ignore Luna and say that the rule is more about stray bodies than orbital partners. But by the same logic we can say that for Luna we ignore the Earth, and Luna passes.

So I would argue that the IAU definition is bad, but if you fixed it so that it allowed binary systems to exist, it would readily define Luna as a planet in my view (if you deleted the Earth from the solar system and left Luna, it would unambiguously meet the criteria).

Now let's talk about barycentres.

The barycentre of two bodies in orbit is just their overall centre of mass. People often point to the external barycentre of the Pluto-Charon system to indicate that they are binary dwarfs. However, this is a poor metric in my view, because it's highly dependent on orbital distance.

The centre of mass of the Earth-Luna system is inside the Earth at present, but if Luna simply orbited further away, the barycentre would be in the empty space between them, with nothing about the two bodies individually, or the qualitative nature of their orbits, having changed. In fact, given enough time and pretending the Sun won't consume both bodies in a few billion years, Luna would actually drift far enough away due to tidal interactions for this to happen - it would be silly for it to suddenly be a planet one day when it wasn't the day before.

It's really frustrating to me when people lash out and say "You're wrong because there's a definition, for the love of god look up the definition" about a topic where I think it's clear my point is that I KNOW the definition and I disagree with it. And that's allowed! Definitions are made up! "Planets" are made up, and I think we should make them up differently!

All the things in our solar system are just different kinds of rocks dancing to each other's gravity- they're all affected by each one's gravity, even in tiny ways. They do not fall neatly into subcategories "planet" and "not a planet" - we made this distinction up because we wanted it. We noticed that some of the bodies in our solar system seemed much more important and dominant than others and we wanted a name for that. But the planets don't know that- they don't have an inherent major distinction between them, nor are they obligated to. When we wanted to come up with a way to clearly decide which bodies were planets, we had to make something up.

What we made up is a little bit vague, and even if it were extremely clear cut, we could still debate whether it was a reasonable or intuitive or useful definition.

Some of my favourites include:

Zoo Theory: What if there are loads of aliens out there, but they're not talking to us because of the Prime Directive from Star Trek? (Or because they're doing experiments on us???)

Dark Forest Theory: What if there are loads of aliens out there, but they all hate us and each other so they're all just waiting with a shotgun pointed at the door, ready to open fire on anything that moves?

Planetarium Theory: What if there's at least one alien with mastery over light and matter that's just making it seem to us that the universe is empty to us as, like, a joke?

Berserker Theory: What if there were loads of aliens, but one of them made infinite killer robots that murdered everyone and are coming for us next?!!

Like, the universe is at least 13,700,000,000 years old and 46,000,000,000 light years big. We have had the ability to transmit and receive signals for, what, 100 years, and our signals have so far travelled 200 light years?

The fact is biological life almost certainly has, does, or will develop elsewhere in the universe, and it's not impossible that a tiny amount of it has, does, or will develop in a way that we would understand as "intelligent". But, like, we're realistically never going to know because of the scale of the things involved.

So I'm proposing my own hypothesis. I call it the "Fool in a Field" hypothesis. It goes like this:

And it pissed me off so badly that I spent the ENTIRE WORK DAY today compiling evidence from every project my team has ever done for her, pulling past emails she’d sent us, putting together an entire case proving that she had been the problem all along. That she got projects mixed up, that she’d made requests that were nonsensical, literally everything you could possibly imagine. Screenshots of emails, reports we’d submitted, EVERYTHING.

This woman in particular has been terrorizing my team for years, her name is almost a slur in my office, I had simply had ENOUGH of her.

I put all of this evidence together and sent it to all of my bosses at 4:30pm. Then I took a long break to eat a sweet treat and drink some tea.

After my break, my bosses all called in an emergency meeting with me and they said they read my report and fucking loved it. And I sat on a teams call with my boss’ boss as she wrote my nemesis the scathing email I had always fantasized about sending, using the evidence I’d compiled, and hit send.

mushroom: can you feel your heart burning? can you feel the struggle within? the fear within me is beyond anything your soul can make. you cannot kill me in a way that matters

If someone were, as a random example, say hospitalized for over two weeks with no explanation, I think that should automatically trigger a special election to replace them.

If you're still able to do your job, then prove it. And if you're not, then you're actively obstructing democracy by not stepping down.

Which is to say, that if a public official were to pass away or into a coma, and their handlers choose to obfuscate that fact, this should be seen as intentionally obstructing democracy.