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[–] 42 points 7 months ago* (last edited 7 months ago) (6 children)

People fighting for Pluto that it should be a planet instead of a dwarf planet

Ceres: 🥺

Context: Ceres is now considered a dwarf planet, and used to be considered just an asteroid, but when it was first discovered it was considered a planet. That was in 1801. There is no objective criteria for what a planet is and isn't. Like a lot of things in nature, things just exist, and as humans we categorize them. Ceres is round like a planet like Pluto. I'm not saying it should be considered a planet, I think dwarf planet fits them both nicely. As late as the 1950s Ceres was still sometimes considered a planet by some people.

I have a sort spot for it. I love it.


Edit: Because two people have misunderstood me now I'm going to say it more explicitly. I'm fully aware there is a scientific definition for dwarf planets. I'm not saying there isn't. I'm just saying compared to something else like prime numbers there isn't an obviously correct way to categorize them and the definition has changed over time. By stating the current definition of planets and/or dwarf planets you're missing my point. Those definitions change. See here for the history.

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  • [–] 20 points 7 months ago (2 children)

    I'm fighting for jupiter to also be a dwarf planet because it has not cleared its orbit of a few million asteroids.

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  • [–] 7 points 7 months ago (1 child)

    I found it interesting that Warframe, set in the Solar System (+ SciFi/Fantasy stuff) features not only the various planets (including Pluto), but also moons (Deimos, Phobos, Europa) and dwarf planets (Ceres, Eris, Sedna) and even an asteroid (though the original name isn't known, if it ever had one). Not relevant to the topic, just came to mind.

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  • [–] 5 points 7 months ago (2 children)

    it blew my tiny mind when i found out that there are multiple dwarf planets in long solar orbits in our system

    they might be small and enjoy solitude but why are we forgetting about them???

    and now apparently there's also a dwarf planet in the inner solar system that nobody talks about??? rude

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  • [–] 5 points 7 months ago

    there's also a dwarf planet in the inner solar system

    It's arguable about whether it's in the "Inner Solar System". Ceres is inside the asteroid belt, and the asteroid belt is the separator between the inner and outer system. It's like floating in the middle of The Rhine and debating whether you're in Germany or France

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  • [–] 4 points 7 months ago* (2 children)

    There is no objective criteria for what a planet is and isn’t

    There are - exactly three.

    1. is in orbit around a star,
    2. has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
    3. has "cleared the neighbourhood" around its orbit.

    The last one means that its gravitational pull has removed any smaller objects that might be in its orbit, either by kicking them out of it, or by catching them as moons.

    Pluto's orbit is full of debris.

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  • [–] 9 points 7 months ago (1 child)

    There is no objective criteria for what a planet is and isn't. Like a lot of things in nature, things just exist, and as humans we categorize them.

    You're the second person to ignore the sentence immediately following that.

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  • [–] -2 points 7 months ago (1 child)

    Because that sentence doesn't really make sense. "Criteria" is a human concept. Nature doesn't do "criteria", nor "objective" for that matter. So, yes, there's no "natural criteria" for when something is X or Y, we, humans, make those criteria. Doesn't matter if it's in relation to animals, plants, or planets.

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  • [–] 5 points 7 months ago (1 child)

    So you're saying things just exist, and as humans we categorize them? Because that's what I said.

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  • [–] -1 points 7 months ago (1 child)

    The idea of a "category" is inherently human. Just like "objective" and "criteria".

    Which means there is objective criteria for what is categorised as a planet - it's whatever we, humans, define them to be.

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  • [–] 6 points 7 months ago

    The scientific community was basically backed into a corner: either create a new category for Pluto and similar bodies, or we go from 9 planets to over 3,000 (iirc), lol.

    The only sensible choice was made, imo.

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  • [–] 2 points 7 months ago

    There is no objective criteria for what a planet is and isn’t.

    There is, though, or rather there should be another one.

    The official definition says

    But I also said,

    Like a lot of things in nature, things just exist, and as humans we categorize them.

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  • [–] -4 points 7 months ago* (last edited 7 months ago) (3 children)

    There is no objective criteria for what a planet is and isn’t.

    There is, though, or rather there should be another one.

    The official definition says it's a planet if it's big enough to be round, which IMHO is a bullshit definition because nobody cares whether your object's round, as in, for practical settlement purposes.

    What's important though is that it's large enough to hold an atmosphere (at least if it had one). That's only the case if the gravitational field is strong enough, which is the case roughly for objects of mass starting at around 10^23 kg. That definition fits surprisingly well the current actual classification of what is a planet and what isn't, though.

    Edit: I want to elaborate a bit more on this. Basically, if you consider a planet that has an atmosphere, like Earth, you see that the atmospheric density/pressure decreases exponentially with height. The concept of Scale Height discusses this: The atmosphere decreases exponentially, but if you take the total mass of the atmosphere and divide it by the density of the atmosphere at sea level, you get a height. That means, if the atmosphere had constant density up to that limit height and then cut off to zero, it would have the same mass as the actual atmosphere has. For Earth, that atmospheric scale height is about 8 km, about as high as the highest mountains on Earth btw.

    The same concept of a scale height also exists for the gravitational field. Planets have a gravitational potential, which is formally the integral of the gravitational acceleration from ground to infinitely far-away. But you can simplified imagine it as: If the gravitational field would be constant up to a limit height and then would cut off to zero, that's the scale height. For Earth, that gravitational scale height is about 8000 km, or about 1000x the atmospheric scale height.

    The consequence of that is that Earth can hold an atmosphere neatly. Because for every gas molecule in the atmosphere, it is affected by the field of gravity strongly enough to be certainly bound to Earth. We take that as a granted, but consider this:

    If the atmospheric scale height of another, fictional planet, was also 8 km but its gravitational scale height was only 4 km, then that would mean that a large part of the atmosphere would be exposed to being above-the-cutoff-height for gravity, so it would be effectively un-affected by gravity and would float away freely from the planet. This would actually not only imply that the planet would lose half of its atmosphere, but all of it. This is because, when the planet loses half its atmosphere, the atmospheric scale height actually doesn't decrease at all. This is because it's not like the atmosphere becomes less high, instead it just becomes half as thick everywhere. That also includes the ground level. So you have half the total mass of the atmosphere, but also half the thickness on ground level, so if you divide this, it's still the same atmospheric scale height (!). This would mean that again, half of it would be above the gravity field and would escape again, and this process would repeat indefinitely until the planet has lost practically all of its atmosphere. Thus the planet could not hold an atmosphere.

    That's why there's an important relationship between the gravitational potential of a planet and the fact whether the planet can hold an atmosphere at all. This isn't just about how big the atmosphere can be in total, but whether there's any atmosphere at all. Below a certain minimum planet mass, that's completely impossible. Above, it's possible.

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  • [–] 3 points 7 months ago

    The official definition says it’s a planet if it’s big enough to be round, which IMHO is a bullshit definition because nobody cares whether your object’s round, as in, for practical settlement purposes.

    That's the second out of the three points of the definition.

    As to why it's not bullshit - the roundness is a byproduct of the object achieving hydrostatic equilibrium (which is the actual criterion, not roundness).

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  • [–] 2 points 7 months ago (1 child)

    What's important though is that it's large enough to hold an atmosphere (at least if it had one).

    Define an atmosphere. Because there's multiple asteroids that technically have one, albeit extremely thin ones. And be careful about being too nitpicky, as Mercury's atmosphere is just it's rock being vaporized due to its proximity to the sun

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  • [–] 1 point 7 months ago (1 child)

    Because there’s multiple asteroids that technically have one, albeit extremely thin ones.

    can you please link an example?

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  • [–] 2 points 7 months ago (2 children)
  • [–] 1 point 7 months ago (1 child)

    And of course, scientists often just use the non-dimensional number characterizing this; gravitational scale height divided by the atmospheric scale height is the Gandolfi number (Gf). :-)

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  • [–] 1 point 7 months ago (1 child)

    Gandolfi number (Gf). :-)

    wait, fr?

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