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Comment by naikrovek | Hacker News Reader

I still think he did it because he wanted to have his name on something significant. He’s a science communicator, not a researcher, and he’s not going to be making any discoveries. So he’s gotta change something that already exists to have his name on something that everyone knows. He had the power to change its status, so he did. I think that’s all it was. I hope I’m wrong but I’ve never heard a really GOOD reason to undo something that was so commonly known and taught. The definition for “planet” could change and Pluto could have been left alone, grandfathered in, in a way. There’s a reason it was discovered first. It’s huge compared to other dwarf planets.

There’s no reason that Pluto couldn’t have remained a proper planet. It’s big enough to be round and its largest moon is big enough to be round. Mars doesn’t have any round moons. Mars is still a planet.

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He didn't "do it", he was one voice among many astronomers who have been calling for a reclassification for years, the IAU voted and made the decision. It's a little silly calling him out for "doing it" for ego reasons when you are the one implicitly giving him credit for it... He didn't write the definition, he didn't chair the committee, he wasn't even on the committee. All he did was leave it off the list of planets at the Hayden Planetarium, where he was director.

> The definition for “planet” could change and Pluto could have been left alone, grandfathered in, in a way

This doesn't sound like a science way of doing things. The definition of planet would have to be literally changed to add "Or has to have been discovered before 19XX" in order to keep Pluto without becoming an unbounded set. If you're annoyed at all the pedants correcting kids or anyone else talking about the nine planets, I'd take it up with them for uselessly debating such a fine distinction, like a chemist arguing about the word "Sodium" on a Nutrition Facts label.

I would argue the colloquial definition has indeed been changed in the above way, in that most people would say that what Mars, Venus, and Pluto have in common is they're all planets, and only a few would remember the odd factoid that the dwarf planet designation was created.

It's okay for the colloquial definition to be different than the scientific one. There isn't any use case where that will harm anyone. It's not like we're chartering flights to "All Planets" where space tourists are going to be ripped off, limited to 8 planets by the technicality and missing out.

You’re probably right, but I still think there’s room for things like this.

What’s a “moon” versus a “planet”? Earth is a moon of Sol, is it not? Why is having a lot of planets a problem in the first place? Why do we have to restrict the definition at all? If 2-3 stars are at the center of a star system, are the planets in that star system planets, or something else? What if they’re small?

This whole scene is ripe for people who want to put their stamp of opinion on something to go nuts arbitrarily.

> What’s a “moon” versus a “planet”? Earth is a moon of Sol, is it not?

Planets orbit stars. Moons orbit planets. That's a clear and easy distinction. Planet vs dwarf planet isn't so clear to most.

Don't planets and moons both orbit their center of mass? The distinction only seems to make sense if the masses of the two bodies are far apart. If they have similar mass, which is the moon and which the planet?

Indeed, before the planet/dwarf planet debate, Pluto and Charon were sometimes called a “binary planet” because their center of rotation is outside the volume of either body.

What’s a moon that orbits a moon? Doesn’t that make the orbited moon a planet? Pluto has moons. But it’s not a planet? ???

If a super massive planet and two stars orbit each other in the center of a star system, all the planets that orbit those stars are moons then technically, right?

This is all super fuzzy and completely arbitrary. These concepts are constructs. Humans could make them better. Instead, everyone decided to make it all worse.

Don't forgot about moonlets!

No. A star is not a planet. The bodies orbiting the stars are planets, or dwarf planets, asteroids or comets. Bodies orbiting them are moons. Bodies orbiting the moons don't have a name.

> Bodies orbiting the moons don't have a name.

Satellites? Natural or manmade, small or big, doesn't matter.

A natural moon of a moon is called a subsatellite: https://en.m.wikipedia.org/wiki/Subsatellite

At present, purely theoretical: we don’t know of any. They are probably quite rare, but we don’t really know - maybe, in centuries to come, we’ll know of dozens of examples; maybe, there are none to find in this entire galaxy

What’s a “moon” versus a “planet”? Earth is a moon of Sol, is it not?

We already have the word "satellite" for "things that go around other things" right? I think "moon" is just "satellite of a planet" for convenience in discussing that subset.

> Why is having a lot of planets a problem in the first place?

I think keeping the number manageable is explicitly something we keep around to help kids grasp the main entities in the solar system. If we just said "there are 235 planets" it would be silly to try to teach them all, so we'd probably just settle for "The top 10 biggest planets" or something. Having a definition instead of a number to bound the set isn't much less arbitrary than teaching the "top 10," but since the long tail clearly starts after #8, "Top 8" would be the only guaranteed stable set to give special treatment to, which is what we've arrived at with the official definition.

> Earth is a moon of Sol, is it not?

No. The sun is a star, so it doesn't get to have moons. It has planets. If Jupiter started generating heat from nuclear fusion reactions, we'd call Io a planet right before we boiled to death, and with our dying breath we'd add "and also, it's no longer a moon".

Putting a leash on a cat doesn't make it a dog, and both of those creatures have four legs even if you call the tails of each a leg. A planet revolves around a star, a moon revolves around a planet (revolving around a star). There's further elements which make Ceres and Ed White's lost glove not a planet or a moon, respectively, but planets and moons are distinct and non-overlapping categories.

Why would we be boiling to death in this situation? Jupiter is much further from Earth than the sun is and Jupiter is also much smaller. Heat would increase, but probably not that much.

I would rather expect Earth to not have a stable orbit. Either ripped apart from fluctuating tidal forces, flung away or in one of suns (thus boiling would happen, briefly) or just generally a much more extreme place compared to now.

Eris is essentially the same size as Pluto and has a larger mass.

Then ADD Eros. Don’t remove Pluto.

Why?

> Why?

Why remove Pluto?

The definition of a Planet could be whatever we want. It could be "these named entities are planets, other things are not planets" if we wanted. That makes a hell of a lot more sense to me than anything else, because eventually we are going to find planets which really blur the boundaries we have currently. Until we observe the entire universe, any set of rules we come up with are going to appear to be wrong in some situations.

"It's not perfect so we should just do it arbitrarily instead" is a pretty silly scientific proposition.

Congrats, the solar system would then contain these planets:

Mercury, Venus, Earth, Mars, Ceres, Pallas, Juno, Vesta, Jupiter, Saturn, Uranus, Neptune, Pluto, Eris, Makemake

(Plus some more).

So what? Is 10 some mental limit of names for most population? If I can memorize 8 I can handle 12 or 15, or neither. Making up sub-categories is such a typical bureaucrat's approach to problems.

Why should giant planets be in same category as normal ones? Why mixing ringed with non-ringed? Why mixing moonless with moon-enabled? Gas/liquid ones and solids? I could go on for a long time.

> If I can memorize 8 I can handle 12 or 15, or neither.

Current estimates expect about 200 Pluto-sized objects in the Kuiper belt and about 10'000 in the surrounding region.

Compared with 4 rocky planets, 2 gas giants and 2 ice giants.

Isn't that kind of the issue though? Pluto's moon isn't just round it's about half the size of Pluto itself such that the Pluto-Charon system orbits around a point in space between the two bodies.

Jupiter and the sun orbit a barycenter, too. Jupiter is a planet.

And the sun is a star. The point is the category exists to be useful: if Pluto is a planet then a ton of other stuff is technically a planet.

and in cases where the star is binary with a huge rocky planet? what are the large satellites in that star system? are they planets of the star, or moons of the huge rocky planet?

Stop agonising over metadata. Pluto is still there and it’s not going anywhere.

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