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Pluto's Not Coming Back, But Astronomers Want To Redefine Planets Again (axios.com) 129

Posted by msmash from the shape-of-things-to-come dept.
A group of astronomers want to change the definition of a planet. Their new proposed definition wouldn't bring Pluto back into the planetary fold, but it could reclassify thousands of celestial bodies across the universe. From a report: The International Astronomical Union's (IAU) current definition of a planet, established in 2006, includes only celestial bodies that are nearly round, are gravitationally dominant and orbit our Sun. This Sun-centric definition excludes all of the bodies we've discovered outside our solar system, even if they may fit all other parameters. They are instead considered exoplanets. Those behind the new proposal critiqued the IAU's definition in an upcoming paper in the Planetary Science Journal, arguing it's vague, not quantitative and unnecessarily exclusionary.

Their new proposal would instead classify planets based on their mass, considering a planet to be any celestial body that:
1. orbits one or more stars, brown dwarfs or stellar remnants and,
is more massive than 10ÂÂ kilograms (kg) and,
is less massive than 13 Jupiter masses (2.5 X 10^28Âkg).
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Pluto's Not Coming Back, But Astronomers Want To Redefine Planets Again More

Pluto's Not Coming Back, But Astronomers Want To Redefine Planets Again

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  • Proofread (Score:5, Insightful)

    by MBGMorden ( 803437 ) on Friday July 12, 2024 @01:24PM (#64621473)

    10ÂÂ kilograms ? What the hell is 10ÂÂ ?

    Come on editors proofread this stuff before hitting submit.

    • Re: (Score:2)

      by msauve ( 701917 )
      The editors don't have time for that, they're late for their shift as a Walmart greeter.

    • 10ÂÂ kilograms ? What the hell is 10ÂÂ ?

      Come on editors proofread this stuff before hitting submit.

      It doesn't appear to be an iPhone-related mess either - I just checked on my iPhone, and it shows the same thing there as well.

      Looking at TFA, apparently someone copied-and-pasted some superscripts. To be fair, Slashdot probably can't handle "advanced" formatting like that.

    • Re: (Score:3)

      by suutar ( 1860506 )

      Apparently it should be 10^23

    • Re: (Score:2)

      by dbialac ( 320955 )
      Keep it simple: if it's round because of gravitational force and doesn't radiate light, it's a planet. At some point somewhere, we'll encounter two planets similar in size in orbit around one another. One won't be a moon of the other. That will kill our current definition of a planet and we'll have to redo it. Or just keep it simple. Pluto is a planet as are several other round bodies in the asteroid belt. Technically by our current model, Neptune isn't a planet because Pluto intersects its orbit so it hasn

      • Re: (Score:3)

        by Sique ( 173459 )

        Keep it simple: if it's round because of gravitational force and doesn't radiate light, it's a planet.

        So is a black hole. It's round (it is even perfectly round), and it does not radiate light.

        You have to be more careful with your definitions.

        • Re: (Score:2)

          by dbialac ( 320955 )
          It needs a little bit of tweaking, but the overall principle is sound.
          • By his definition, there could be thousands if not tens of thousands of "planets” in our solar system. Many objects in the Kuiper Belt cannot be confirmed to be round unless there is a probe sent out there to survey them.

            • Re: (Score:2)

              by dbialac ( 320955 )
              And?
              • And what do we call the 8 objects that are currently called "planets" to distinguish them from the thousands of others? Super planets? Special planets? Planet A+? This is your proposal to rename all those belt objects as planets.
                • We call them nothing special. Why should we ? They would be planets as well as the potential hundreds of other planets in the solar system. If you really want to call them something special just call them the 8 planets closest to Earth.

            • By his definition, there could be thousands if not tens of thousands of "planets” in our solar system.

              OK, so let's call objects that are round because of their gravitation, space-balls. We can subcategorize them into hot space-balls and dense space-balls. We can also add some arbitrary distinction to our 8 special space-balls, such as having a certain surface gravity, or escape velocity, or having a core (gravitationally round + layered = space onion).

          • Re: (Score:2)

            by Sique ( 173459 )
            If you are tweaking it until it works, it will be pretty close to the definition of the International Astronomical Union.

        • What is light?
          Radiation in the spectrum our (average human) limited biological light receptors can detect?

          No, the mass definition is better, where the cutoff for what's a failed star and a big planet needs to be sussed out though. I wish definition could be written with caveats that certain specified numbers should be changed when updated data supports such a change.

      • Re: (Score:2)

        by Opyros ( 1153335 )
        Wouldn't that make a lot of satellites into planets, though?

      • Re:Proofread (Score:4, Insightful)

        by MBGMorden ( 803437 ) on Friday July 12, 2024 @03:14PM (#64621847)

        Technically by our current model, Neptune isn't a planet because Pluto intersects its orbit so it hasn't cleared its orbit.

        Yeah that part of the current definition always struck me as odd. If Pluto isn't a planet because it hasn't cleared out Neptune, then the reverse also would apply.

        My personal definition would be:
        1) Has sufficient mass to pull itself into a roughly spherical shape
        2) Less than 13 Jupiter masses (that excludes everything from Brown Dwarfs on up)
        3) Its primary orbit is around a star, or pair of stars

        Basically, if its spherical and smaller than a brown dwarf, then its either a planet or a moon, with the differentiator being that a planet is orbiting a star and a moon is orbiting a planet.

        We could then have subcategories of icy, rocky, and gaseous planets. Yes this would bring back in Pluto (and Ceres, and many other Kupier belt objects), but realistically Earth has a heck of a lot more in common with Ceres than it does Jupiter.

        • Re: (Score:2)

          by dbialac ( 320955 )
          What about something that has escaped it's star and is aimlessly drifting around the universe?
        • https://www.astronomy.com/scie... [astronomy.com]

          Looking at a solar system diagram on paper, it appears the Pluto and Neptune orbits intersect. Of course, this is not the case. The orbits appear to intersect only because it’s a flat projection, and not the three-dimensional orbits. [...] The closest distance between the two orbits is 2.4 AU. If we could reach out magically and move Pluto and Neptune to any point in their orbits, the closest they could ever get is 2.4 AU. However, this minimum distance can never occur.

          By your argument the inner planets wouldn't be planets

      • The exact situation of Earth/Luna, cleverly eliminated by the lower limit on the weight (Luna is 7.3e22 Kg. Note that the Luna orbit is NEVER convex with respect to the sun. This whole redefinition is to clearly eliminate Luna as a planet, whereas in the previous definition, it was, although never admitted.

      • Your definition includes both large asteroids in the Kuiper belt and the asteroid belt between Mars and Jupiter, as well as black holes.

        This is why the one they're proposing has a mass-based facet to it. For example, Ceres is 9.3839×10^20 kg in mass, but still "round because of gravitational force and doesn't radiate light" as it has 0.27m/s of surface gravity.

        The best way to define a planet is going to be through a mass range, and that's what they're trying to do.

    • Wouldn't it be easier for Slashdot to finally support Unicode?

    • Avogadro's number?

  • How many kilograms in 10ÂÂ kilograms?

    • Why don't people use standard references and universally understood values - like Libraries of Congress or Football Fields?

      • Re: (Score:3)

        by g01d4 ( 888748 )

        Why don't people use standard references and universally understood values - like Libraries of Congress or Football Fields?

        You're not far off. Astronomers typically use units somewhat normalized to the scale they're dealing with. Planetary orbital distances are typically related in astronomical units. So it would make more sense if planetary sizes were defined similarly (e.g. Earth or even Neptune).

        • Astronomers do use Earth masses and solar masses. Both have their own symbol (U+1F728 or U+2641 for Earth, U+2609 for solar)
          the unit is M[symbol].

      • Why don't people use standard references and universally understood values - like Libraries of Congress or Football Fields?

        If we standardize on earth density and American Football, a 10^23 kg planet would have an area of 6.2e9 football fields.

        If we assume that a LoC is the commonly bandied quantity of 15TB, we can use the formula for a black hole's entropy WRT mass: 4 pi G M^2 / (hbar c ln(2)). For 10^23 kg, this gives 3.8e62 bits, or 3.2e48 Libraries of Congress.

        • Re: (Score:2)

          by hawk ( 1151 )

          >of 6.2e9 football fields.

          Unless Brady is playing, and then it's more like 5.5 . . .

      • Why don't people use standard references and universally understood values - like Libraries of Congress or Football Fields?

        At this point we might as well just go with the number of Americans. For example, at birth, an African elephant can weigh as much as the average American high schooler. When full grown, an African elephant can weigh as much as fifty American adults.
      • How much does a Library of Congress or a Football Field weigh?

    • Re: (Score:2)

      by ls671 ( 1122017 )

      How many kilograms in 10ÂÂ kilograms?

      10ÂÂ kilograms equals 120 ÉÊ kilograms

  • Oh hell... (Score:2, Troll)

    by cayenne8 ( 626475 )
    Why not just grandfather Pluto back in, eh?

    • Pluto certainly has mass greater than 10 kilograms!

    • ....beacuse then logically you would also have to include all the other dwarf planets like Eris, Ceres etc. The problem with science is that we don't know everything and sometimes, as we learn more, we find out that the way we initially classified things was wrong. For example, at one time distant galaxies were once through of as being just nebulae - the Andromeda Galalxy was once called the Andromeda Nebula - but clearly it would be silly to go back to calling all galaxies nebulae again because they now kn

      • Re:Things change as we Learn (Score:5, Funny)

        by dbialac ( 320955 ) on Friday July 12, 2024 @02:11PM (#64621647)
        Just call Eris and Ceres planets as well. Call anything gravetationally round a planet. Or create the MJ rule: if Michael Jordon lands on it and he can't jump off of it under his own power, it's a planet.

        • Or create the MJ rule: if Michael Jordan lands on it and he can't jump off of it under his own power, it's a planet.

          Well, problem is, that definition will change over time - eventually even tiny pebbles would qualify.

    • Because science is about being exact and doesn't give a shit about people's feelings. You don't have a definition of a planet being 3 specific characteristics + oh Pluto too because a bunch of people had a cry.

  • 10AA (Score:4, Informative)

    by sconeu ( 64226 ) on Friday July 12, 2024 @01:32PM (#64621511) Homepage Journal

    It's 10^23kg. I went to TFA to see.

    • The slashdot editors finally found how to trick the readers to at open TFA and even to read a sentence of it.

  • orbits one or more stars, brown dwarfs or stellar remnants and, is more massive than 10^23 kilograms (kg) and, is less massive than 13 Jupiter masses (2.5 X 10^28 kg).
  • it must be orbiting a star, must have enough mass and gravity to be/become rounded and capable of having a moon orbit it, anything smaller is just an asteroid or rock
  • Science is about numbers and relationships, not the name of a rose. But then again, it's important for education that common terms make a modest effort to be useful for average people.

    The closest thing to a purely scientific definition of "planet" would be something like "A stable system of gravitationally bound matter in hydrostatic equilibrium exclusively in solid, liquid, and gaseous phases." But that obviously is total gibberish to someone without a basic footing. It would also confuse people beca

    • Science is about numbers and relationships, not the name of a rose. But then again, it's important for education that common terms make a modest effort to be useful for average people.

      False. Definitions matter in science. You're thinking of math. The actual thing we call them are irrelevant, we could call them Blazfarts for all anyone cares, but the definition of how to combine and group common things is critical in science, especially since a lot of that numbers is about how certain groups of defined things are related to each other.

      • You're confusing the public presentation of science with the practice of it. The practice of science involves very long strings of meticulously-laid out logic and system modeling. The verbal definition of "planet" is public presentation. Scientists know what they mean when they say "planet," but the public gets confused when they hear that one planet is a blazing hot gas giant with iron rain, and another is a little ball of ice with no air.

        Phase diagrams are a little advanced for the casual observer,

  • It all depends on what you care about (Score:3)

    by Baron_Yam ( 643147 ) on Friday July 12, 2024 @02:05PM (#64621625)

    You classify things to make it easier to process information about large numbers of them.

    Mass is a good basic criteria for categorizing bodies in space - you can have dust, asteroids, rocks, gas balls, stars, and black holes using mass as your first classification method.

    After that, we tend to care about planets similar to ours and ones that aren't. Rocks and gas giants.

    Where I think the astronomers have it wrong is in how they've classified planets based on the details of their gravitational relationships. I think any non-fusing mass that has reached hydrostatic equilibrium should be a planet, and then further sub-categories should follow from that. Gas giants, large moons, rogue worlds, dwarf planets... They're all planets to me. Where it is and if and how it orbits something (alone or in a swarm of neighbours) is a circumstance, not an intrinsic quality of a body.

  • Scientists are more than welcome to bicker over the definitions of their own words. The word "planet" doesn't belong to the IAU or even to science, it belongs to everyone.

    Not only in my view does the IAU lack required legitimacy to unilaterally redefine words predating the existence of IAU their poor use of language isn't doing them any favors. A "flight computer" is a computer, a "rocket ship" is a ship, a "moon base" is a base, a "rocket engine" is an engine and a "space telescope" is a telescope yet a

  • Mickey Mouse is devastated.

  • Don't care what anyone else says. Popular culture says Pluto's a planet. End of story. It's not changing.

    • Popular culture can barely name Mars and probably Jupiter. And likely doesn't think of Earth as a planet because it's just that ever-present thing we stand on and don't think about in that context.

      Go ahead and ask a random person to name the known planets of the Solar system from the Sun outward, and unless you're hanging out in a crowd of astronomers it's going to take you a while before you find someone who knows to start with Mercury.

      In a generation, those who care will not be worried about classifying

  • I don't quite get why people are so hung up on the whole Pluto thing, since, with the current definition, Pluto *is* still a planet, after all Pluto didn't change! It's just been defined as a *small* (dwarf) planet by some propeller-heads, and sure, they can call it whatever they want, but I'm basically fine with that, since It's just sets and subsets: the whole set of planets in our solar system numbers at least 17. There's a subset of those called "dwarf planets" and another subset called "giant planets

    • with the current definition, Pluto *is* still a planet, after all Pluto didn't change! It's just been defined as a *small* (dwarf) planet by some propeller-heads

      Planet or dwarf planet. Make up your mind. It literally can't be both. In science definitions matter, and if you consider Pluto a planet then our solar system will have a hell of a lot more than 9 planets which was the whole reason this mess came up in the first place.

      • Yes, in science, definitions matter. And those scientists are already happy using their much MUCH more technical categorizations that the layperson doesn't need to know, and likely never thought about.

        The disconnect here is when laypersons hear that a group of people that nobody ever heard of decided to reclassify things because ${REASONS} and all of a sudden their 4th grade astronomy mnemonic has been invalidated, and they can't come to grips with it.

        Literally nobody would correct anyone in a reasonable l

    • It's what we usually refer to as ambiguation. And, because everyone on Slashdot loves a car analogy, here we go: You can disambiguate into full-size cars, mid-size cars, and compact cars. Even hatchbacks, coupes, 4-door, 2-door, crossover, wagons. But they're all cars.

      Mercury, Venus, Earth, Mars are mid-size planets.
      Jupiter, Saturn, Uranus, Neptune are full-size planets.
      Ceres, Pluto, Haumea, Makemake, and Eris are compact planets.

      But they're all planets. It's right there in the name.

      • Re: (Score:2)

        by eriks ( 31863 )

        Exactly. That's literally what I said. 17+ planets orbit the sun, some are big, some are medium and some are small. That's the 10000 foot categorization, if you're into that sort of thing. The reality though is complex and nuanced: "planetary-sized" moons that maybe *used* to be dwarf planets, crossing orbits, weird orbital resonances, and (I'm quite sure, but I'm not a planetary scientist) other things too, since most things seem to defy human-created definitions unless one is highly specific and litera

      • "compact" really? Did they cram as much into a smaller space?

  • I thought we got rid of that years ago.

  • In my heart, Pluto will always be a planet, no matter what the official astronomer bullies say. One of these days, Pluto is going to get tired of being called a dwarf, and go after his tormentors. And THEN we'll all be sorry.

  • IAU recategorizing deck chairs.

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