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Is Pluto a planet? (Pluto discussion thread, if there isn't already one)


Is Pluto a planet?  

49 members have voted

  1. 1. In your opinion, is Pluto a planet?

    • Yes.
    • No.
    • Well, the IAU (International Astronomical Union) declared that it's only a dwarf planet, so.....
    • I don't really know or care.

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Its not a planet. It only was deemed a planet because we didn't understand the context surrounding what Pluto was.

This follows along with how we used to think the Moon was a "planet", or how we used to think the planets were just "special stars" that moved, or how the Earth goes around the sun. 

As we learn more, our understanding of existing concepts may change. The change surrounding Pluto is just much more recent, so its still very fresh in our minds. 

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For 75% of my life Pluto was a planet. So I'm just going to give Pluto the benefit of the doubt for the remainder, and because we've come this far together already and it's like having a favourite old pair of comfortable shoes.

Science can continue on and do what it needs to do. 

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45 minutes ago, kerbiloid said:

Also still can't get, why not Hades.

Because Pluto (as in the roman god of the underworld) as opposed to Hades (the greek god of the underworld) fits  in with the existing theme of naming the planets (and big asteroids) after roman and not greek gods (they goofed up with Uranus though and used the greek name, that one should have been named Caelus to fit the theme).

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45 minutes ago, Minmus Taster said:

Ignore the science for once ya nerds!


45 minutes ago, Minmus Taster said:

Pluto at heart is still a planet, you can't just erase 70 or so years of something like that instantly anyways:valjoy:

Pluto's heart is cold nitrogen ice and does not care what you think.

Unless you would care to accept 13 planets?

Edited by cubinator
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There are some things that make me agree and disgree with the IAU about this. I getCceres, ya it shares an orbit with a gazailliion asteroids but, Plauto. It way out there and probbely the nereast atroid to it are really really far. So. The IAU's definition of a planet is a bit off. The third rules says this: it has to clear its orbit. But. How could any planet do that. Unless its Jupiter or Saturn. The space are so vast. How could any planet use gravity to attract or repel astroid that are really really raeely far away? It would take forever! So I dont agree or diagree, I just think the clarification on the rules should be a bit more clear. 

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6 hours ago, RKunze said:

Because Pluto (as in the roman god of the underworld) as opposed to Hades

Πλούτων is a Greek word. The Romans just adopted it for either one of their gods instead of his original Etruscan  name, or just adopted the god himself, like they did it later with Cybele and Isis (let alone the Blackstone, lol).

In the  Greek mythology, Pluto is a nickname of Hades, "Wealthy". I.e. Hades + Pluto = Unseen The Wealthy.

6 hours ago, Minmus Taster said:

Pluto at heart is still a planet



At Heart - it is.
In other places- it isn't.




Ignore the IAU! Dwarf planets are planets, too!

Stop discriminating the lesser worlds!
Treating them as something less significant is a racism of astronomical scale!

2 hours ago, Superfluous J said:

I'm more comfortable calling the "big 4" Jovian Moons and Titan planets, than I would be with Pluto.

You are just afraid of their boss.



While Pluto is lonely.




2 hours ago, SOXBLOX said:

and C) is roundish.

If it was a torus, it would stop being a planet.

Cuz it's not a healthy shape, but a who knows what.

1 hour ago, Dr. Kerbal said:

The third rules says this: it has to clear its orbit

Some call this rule an incitement to fratricide.

Why to be a planet means to be a bloody maniac?
Why several planets can't peacefully share same orbit.

There are Langrange points. They are at proper triangles.
So, a family of six planets could occupy same orbit and not just disturb each other, but vice versa, stabilize.

Looks like astronomers are even more cruel than chessplayers, who hide the passion for mass murder under mask of wooden toys, "taking" (or in colloquial Russian  - "eating") the another one's "pieces" (knights, bishops, even simple pawns), when there is twice as more room on the board than they need.

Edited by kerbiloid
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3 hours ago, SOXBLOX said:

Pluto is a planet because it is A) not a star, B) not a moon, @sevenperforce's comment notwithstanding, ^_^ and C) is roundish. This definition is no more arbitrary than the IAU's.

And yes, I'm happy to declare all the dwarf planets to be planets.

If you are averse to admitting Pluto's status as a moon of Neptune, let it at least accept the role of a planetary satellite of Neptune.

Y'all know my preferred definitions for natural objects in our solar system (or any solar system):

  • Star. Any object with sufficient mass to fuse hydrogen-1 in its core.
  • Stellar object. Any object which is or was once a star.
  • Stellar binary. Any pair of stellar objects which orbit a local common barycentre.
  • Planet. Any gravitationally-rounded object, other than a stellar object, which composes more than 50% of the mass of all objects crossing its orbit around a stellar object or stellar binary.
    • Giant planet. Any planet too large to have a defined surface.
    • Terrestrial planet. Any planet small enough to have a defined surface.
  • Satellite. Any object, other than a planet, which is in a permanent orbital resonance with a larger object around a stellar object or stellar binary.
    • Moon. Any satellite which remains permanently within the Hill Sphere of the larger object with which it is in orbital resonance.
    • Planetary satellite. Any gravitationally-rounded satellite.
      • Planetary moon. Any planetary satellite which is also a moon.
  • Comet. Any object in orbit around a stellar object or stellar binary, other than a satellite, which crosses the orbit of the planet nearest the stellar object or stellar binary.
  • Asteroid. Any object in orbit around a stellar object or stellar binary which is not a planet, satellite, or a comet.
    • Planetary asteroid. Any gravitationally-rounded asteroid.

These definitions are simple. Every object in our solar system (or, indeed, in any conceivable solar system) has a place, and nothing fits into multiple categories other than categories which are a subset of other categories. A fifth grader (or earlier) can readily explain all of these categories.

  • Ceres? It's a planetary asteroid.
  • Jupiter Trojans? Those are just satellites of Jupiter, along with all of Jupiter's satellites, including a number of moons, some of which are planetary moons.
  • Luna? It's a satellite, subset planetary satellite, and a moon, subset planetary moon.
  • Pluto? It's a planetary satellite of Neptune.
  • Charon? It's a planetary moon of Pluto and also a planetary satellite of Neptune.
  • Phobos? It's a satellite of Mars, subset moon of Mars.
  • Earth? It's a planet, subset terrestrial planet.
  • Uranus? It's a planet, subset giant planet.

Everything has a place. It's clear and impossible to confuse and matches the vernacular easily.

Edited by sevenperforce
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Phil Metzger makes many good arguments why Pluto should still be a planet, and I find them pretty convincing. That's ignoring the technicalities of the IAU decision, which seems to have violated their own rules (changing the specific definition they were voting on without having previously published it, and voting on the last day after a large number of attendees had left).


if a set of rules is to be designed, then they should produce zero broken results for any other solar system. If there is a possibility of an edge case that has to be characterized based on some subjective criteria because the rules don't work in that oddball case—the rules are bad in the first place, and Pluto could be called a planet in similar violation as well in the second place.

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In terms of size, Pluto is small in comparison to other planets in our solar system.

Why is Pluto so different from the other planets? - Quora

Callisto somewhat marks the boundary between "this is moon sized" and "this is planet sized". Pluto is on the far right of the main lineup and is smaller than Earth's Moon. Because of its size in comparison to other planets, it makes sense to label Pluto as a dwarf planet. There are also quite a few spherical, non-moon objects smaller than Pluto in orbit around our Sun. If we named all of those celestial bodies as "planets" and not "dwarf planets", the exact definition of a "planet" would become even more muddled, and classification would become confusing. Labeling an object as a "dwarf planet" doesn't exclude it from planet status; it specifies that the object is a really small planet.

Please note, this is my opinion.

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25 minutes ago, wpetula said:

Labeling an object as a "dwarf planet" doesn't exclude it from planet status; it specifies that the object is a really small planet.

That’s the dumb thing about the IAU definition: they say “planet” is ONLY for the big eight and “dwarf planet” is for anything smaller. But adding a diminutive modifier isn’t how the English language works. It would be like saying a “toy poodle” isn’t actually a type of poodle, or a “kiddie pool” isn’t actually a type of pool.

If the IAU had said “They’re all planets but some are minor planets and some are major planets” then that would have made much more sense, linguistically. 

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