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Tetrahedral Planets


Nazalassa
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5 hours ago, sevenperforce said:

Well, technically, this definition merely says "big enough" to have enough gravity to force it into a spherical shape. It doesn't have to be spherical.

If we ever encounter a planet in the shape of a Platonic solid I suspect it will quickly go the way of the dodo Pluto :D That quote is from a NASA site anyway, they tend to simplify stuff for the general public (makes sense of course, their target demographic is "The American tax payer" rather than "Professional Stargazers"). The literal quote from the IAU's resolution is thus:

(1) A planet1 is a celestial body that
(a) is in orbit around the Sun,
(b) has sufficient mass for its self-gravity to overcome rigid body forces
so that it assumes a hydrostatic equilibrium (nearly round) shape,
and

(c) has cleared the neighbourhood around its orbit.

Source: https://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf

So it looks like they accounted for SmartSes when it comes to (b) :P

The more interesting part is (a) I think. I guess an exoplanet is not considered a planet but is a class by itself, which seems a bit heliocentric and silly to me. By 2006 we had found a good handful of them so it's not like they didn't know exoplanets existed.

 

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5 hours ago, Nazalassa said:

Yes, but they see the world as flat, wherever they are -- they will see the world as flat if they live at -- or near -- the center of a face, and as flat, but sloppy, if they live further from the center of the face. It will always seem flat. If people are travelling at random on a face, they will experience different gravity orientation, even if they see the world as flat, and I guess they won't really understand what's happening.

If they grew up there I would assume they would have evolved in a way to deal with it, and perhaps even take advantage of it. It would provide some interesting options for navigation. Imagine living on a world where you could determine your altitude simply by measuring the angle of the ground with the gravity-normal. Global navigation on Earth required the development of extremely accurate clocks as well as many observations of stars. On a Platonic solid you'd have a much easier job as there is the extra information of the angle between the horizon and the direction of gravity. On a sphere that angle is always 90 degrees, so it doesn't give you any information (other than that you are, in fact, on a sphere, dear flat-earthers :D), but on a Tetrahedron it would tell you how far away you are from the center of one of the polygons.

 

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3 hours ago, Beamer said:

The more interesting part is (a) I think. I guess an exoplanet is not considered a planet but is a class by itself, which seems a bit heliocentric and silly to me. By 2006 we had found a good handful of them so it's not like they didn't know exoplanets existed.

Well so actually that’s one of the reasons the definition is what it is. We wanted to be able to decide whether exoplanets should be characterized as planets or not, and so the concept of “big enough to be round” and “has the right location and size to clear its orbit” were chosen because those things could be measured from far away.

Planetary astronomers are divided to some degree between the study of body composition and the study of orbital mechanics and system evolution. The current definition largely reflects consideration of the latter before the former. 

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4 minutes ago, sevenperforce said:

Planetary astronomers are divided to some degree between the study of body composition and the study of orbital mechanics and system evolution. The current definition largely reflects consideration of the latter before the former. 

Well I guess I am a bigger proponent of rigorous definitions than most astronomers. I'm a database specialist by trade, a lot of my work revolves around trying to bring structure into badly indexed data. And that, my astronomer friends, is a BAD index :D But I suppose they don't worry about the fact that they have to revise the definition every X years or so, that's for people like me to fix afterwards.

There's a clear natural index which flows from the orbital mechanics. Planets orbit stars, moons orbit planets, moon-moons orbit moons. If you want to add a mass limit on top of that, sure no problem, over X mass follow the above, under X mass call it a dwarf-something or make up names like 'comet' or 'asteroid' or 'ring system', those are just attributes of each specific body. But the (c) rule always seemed pointless and useless to me. It's a moment in time, maybe it hasn't had the time yet because it migrated orbits or is still young. And of course there's the Uranus/Pluto problem. What about Jupiter, aren't the trojan asteroids considered 'in the neighborhood'? Is that a rigorously defined volume in astronomy anyway? Worst of all it doesn't say anything about the body, it tells you something about its neighborhood, it's in the wrong table! :P And there are almost more exceptions to it than confirmations of it. No idea why that's there.

As for (a), I'm not an expert in the lingo, perhaps they mean 'Sun' as in "central star of a solar system". Do astronomers refer to the Sun as 'Sol'? The fact that it's spelled with a capital makes me think they mean "our star" which is just a revision waiting to happen.

 

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4 minutes ago, Beamer said:

But the (c) rule always seemed pointless and useless to me. It's a moment in time, maybe it hasn't had the time yet because it migrated orbits or is still young. And of course there's the Uranus/Pluto problem. What about Jupiter, aren't the trojan asteroids considered 'in the neighborhood'?

Oh, it’s terribly hackneyed.

FWIW, it’s strictly the Pluto/Neptune problem, not the Pluto/Uranus problem. Virtually all of the Plutoids and other bodies in the Kuiper belt are in resonance with Neptune. By all rights, Pluto should be considered a satellite of Neptune, just like the trojans of Jupiter should be considered satellites of Jupiter.

It feels very icky to think that Pluto would be a planet if it was close to the orbit of Mercury, and Earth would be a dwarf planet if it was out there near the orbit of Pluto.

I’m all for categorizing things based on the role they play in stellar system evolution, but you’ve gotta have a limit somewhere. 

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12 hours ago, sevenperforce said:

FWIW, it’s strictly the Pluto/Neptune problem, not the Pluto/Uranus problem.

Yep, that's what I get when posting at 4 AM :)

10 hours ago, kerbiloid said:

We have a hexagonal planet, why not cubic or tetrahedric one.

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1490

 

Awesome image, don't think I had seen the colorized version before. I must admit I was slightly disappointed when it turned out not to be aliens... again :D

 

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4 hours ago, Beamer said:
17 hours ago, sevenperforce said:

FWIW, it’s strictly the Pluto/Neptune problem, not the Pluto/Uranus problem.

Yep, that's what I get when posting at 4 AM :)

We've tossed this about before, but if I had my druthers, I'd use something like the following nomenclature:

  • Star. Any body which currently or in the past sustained nuclear fusion in its core.
    • Main sequence star. Any star in hydrostatic equilibrium which is currently fusing hydrogen in its core.
    • Giant star. Any star in hydrostatic equilibrium which was previously a main sequence star but has exhausted the hydrogen in its core.
      • Red giant. Any giant star with an inert core which is fusing hydrogen or helium in shells.
      • Supergiant. Any giant star large enough to fuse helium into heavier elements via the alpha process.
    • Failed star. Any star which was unable to sustain sufficient nuclear fusion to reach hydrostatic equilibrium.
    • Stellar remnant. Any star which formerly reached hydrostatic equilibrium but no longer sustains nuclear fusion.
      • Degenerate star. Any stellar remnant which is supported by quantum degeneracy pressure.
        • White dwarf. A degenerate star supported by electron degeneracy pressure.
        • Neutron star. A degenerate star supported by neutron degeneracy pressure.
      • Black hole. Any stellar remnant which has collapsed to be smaller than its Schwarzschild radius.
      • Stellar nebula. Any stellar remnant or portion of a stellar remnant which has been ejected into a diffuse cloud.
  • World. Any gravitationally-rounded body which is not a star.
    • Rogue world. Any world which is not in orbit around a star.
    • Giant world. Any world too large to have a solid surface or crust, in which the transition between gas and liquid occurs above the critical point.
      • Gas giant. giant world comprising primarily hydrogen and helium.
      • Ice giant. giant world comprising primarily elements heavier than helium.
    • Terrestrial world. Any world with a solid surface.
      • Ethereal world. A terrestrial world with a persistent troposphere.
      • Ice worldA terrestrial world with a surface composing of icy volatiles and no persistent troposphere.
      • Rocky world. A terrestrial world which is neither an ethereal world nor an ice world.
  • Planet. Any world in orbit around a star which is not a natural satellite.
    • Major planet. Any planet which, together with its natural satellites, makes up the vast majority of the mass in its orbital neighborhood.
    • Planet binary. Any pair of planets in orbital resonance, neither of which are a major planet, but which together with their natural satellites make up the vast majority of the mass in their orbital neighborhood.
    • Minor planet. Any planet which is neither a major planet nor a member of a planet binary.
  • Natural satellite. Any body in orbit around a star which is in orbital resonance with a larger body, other than a planet binary.
    • Moon. Any natural satellite which stays within the Hill Sphere of a larger body.
      • Planetary moon. Any moon which is also a world.
      • Dwarf moon. Any moon which is not a world.
    • Trojan satellite. Any natural satellite, other than a moon, which is in a 1:1 resonance with a larger body.
      • Leading trojan. A trojan satellite at the L4 point of a larger body.
      • Trailing trojan. trojan satellite at the L5 point of a larger body.
    • Resonant satellite. Any natural satellite with a resonance other than 1:1 with a larger body.
  • Orbital neighborhood. The orbital neighborhood of a body is the set of objects orbiting the same star as that body which crosses the orbit of the body or of one of its natural satellites.
  • Comet. A body with a sufficiently eccentric orbit that it experiences cycles of visible mass loss near periastron.
  • Asteroid. Any body in orbit around a star other than starscometsplanets, and the natural satellites of planets.
    • Belt asteroid. Any asteroid which orbits entirely between the orbits of adjacent major planets
    • Centaur. Any asteroid which crosses the orbit of a major planet.
    • Distant asteroid. Any asteroid which orbits a star at a greater average distance than any major planet.
Edited by sevenperforce
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1 hour ago, sevenperforce said:

We've tossed this about before, but if I had my druthers, I'd use something like the following nomenclature:

  • Star. Any body which currently or in the past sustained nuclear fusion in its core.
    • Main sequence star. Any star in hydrostatic equilibrium which is currently fusing hydrogen in its core.
    • Giant star. Any star in hydrostatic equilibrium which was previously a main sequence star but has exhausted the hydrogen in its core.
      • Red giant. Any giant star with an inert core which is fusing hydrogen or helium in shells.
      • Supergiant. Any giant star large enough to fuse helium into heavier elements via the alpha process.
    • Failed star. Any star which was unable to sustain sufficient nuclear fusion to reach hydrostatic equilibrium.
    • Stellar remnant. Any star which formerly reached hydrostatic equilibrium but no longer sustains nuclear fusion.
      • Degenerate star. Any stellar remnant which is supported by quantum degeneracy pressure.
        • White dwarf. A degenerate star supported by electron degeneracy pressure.
        • Neutron star. A degenerate star supported by neutron degeneracy pressure.
      • Black hole. Any stellar remnant which has collapsed to be smaller than its Schwarzschild radius.
      • Stellar nebula. Any stellar remnant or portion of a stellar remnant which has been ejected into a diffuse cloud.
  • World. Any gravitationally-rounded body which is not a star.
    • Rogue world. Any world which is not in orbit around a star.
    • Giant world. Any world too large to have a solid surface or crust, in which the transition between gas and liquid occurs above the critical point.
      • Gas giant. giant world comprising primarily hydrogen and helium.
      • Ice giant. giant world comprising primarily elements heavier than helium.
    • Terrestrial world. Any world with a solid surface.
      • Ethereal world. A terrestrial world with a persistent troposphere.
      • Ice worldA terrestrial world with a surface composing of icy volatiles and no persistent troposphere.
      • Rocky world. A terrestrial world which is neither an ethereal world nor an ice world.
  • Planet. Any world in orbit around a star which is not a natural satellite.
    • Major planet. Any planet which, together with its natural satellites, makes up the vast majority of the mass in its orbital neighborhood.
    • Planet binary. Any pair of planets in orbital resonance, neither of which are a major planet, but which together with their natural satellites make up the vast majority of the mass in their orbital neighborhood.
    • Minor planet. Any planet which is neither a major planet nor a member of a planet binary.
  • Natural satellite. Any body in orbit around a star which is in orbital resonance with a larger body, other than a planet binary.
    • Moon. Any natural satellite which stays within the Hill Sphere of a larger body.
      • Planetary moon. Any moon which is also a world.
      • Dwarf moon. Any moon which is not a world.
    • Trojan satellite. Any natural satellite, other than a moon, which is in a 1:1 resonance with a larger body.
      • Leading trojan. A trojan satellite at the L4 point of a larger body.
      • Trailing trojan. trojan satellite at the L5 point of a larger body.
    • Resonant satellite. Any natural satellite with a resonance other than 1:1 with a larger body.
  • Orbital neighborhood. The orbital neighborhood of a body is the set of objects orbiting the same star as that body which crosses the orbit of the body or of one of its natural satellites.
  • Comet. A body with a sufficiently eccentric orbit that it experiences cycles of visible mass loss near periastron.
  • Asteroid. Any body in orbit around a star other than starscometsplanets, and the natural satellites of planets.
    • Belt asteroid. Any asteroid which orbits entirely between the orbits of adjacent major planets
    • Centaur. Any asteroid which crosses the orbit of a major planet.
    • Distant asteroid. Any asteroid which orbits a star at a greater average distance than any major planet.

Seems pretty comprehensive, although I'm missing fall-backs for binary star systems :) Anywhere you say "orbits a star" I would go for "... or the barycenter of multiple stars" or some such.

 

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14 minutes ago, sevenperforce said:

How about "in orbit around one or more stars"?

Sounds good to me, it should get us by until we find a Tetrahedral world that shouldn't exist. Not bad for a day's work. Of course the hard and time-consuming part is getting it past an international committee, which is probably why we have a definition that seemed barely sufficient half a century ago :D

 

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