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The International Astronomical Union defined a planet [0] as an object that: (a) orbits the sun, (b) has sufficient mass to be round, or nearly round, (c) is n

The International Astronomical Union defined a planet [0] as an object that: (a) orbits the sun, (b) has sufficient mass to be round, or nearly round, (c) is not a satellite (moon) of another object, and (d) has removed debris and small objects from the area around its orbit The dwarf "planets" often suffer from not being round (enough), and even more from not clearing out their debris in their orbit. [0]: https://www.space.com/25986-planet-definition.html https://www.space.com/25986-planet-definition.html

yitchelle 8y ago
Is a plant being round a subjective thing? Just trying to understand how roundness can be measured.
- simonh 8y ago
  Yes, but if we want to distinguish between say asteroids and planets, or orbital pebbles and planets, we need criteria with which to do so and those are going to be at least somewhat arbitrary. The 'cleared it's orbit' thing is also arbitrary. Pluto is sometimes closer to the Sun than Neptune for example but does that mean they share an orbit? It's mainly to exclude Ceres, which is pretty close to spherical. The problem is any rules that exclude Ceres and also exclude the other trans-Plutonian planetoids also end up excluding Pluto. If Pluto is a planet and this new thing, why not Eris, Haumea, Sedna, orcus, etc, etc, etc.There are many dozens of them. We do have applicable terms - planetoid, dwarf planet, so why not use them?
  - lmm 8y ago
    Why does one want to exclude Ceres? What I don't like about the "cleared its orbit" criterion is that it's something you can't tell from looking at the object itself. If a planet is ejected from its orbit, it becomes a ... well, we don't even have a name for what it would be. (This problem exists for "moon" to a certain extent, but a moon that's thrown out of planetary orbit would become a planet if it was big enough and an asteroid if it wasn't). It's also hard to apply: a lot of extrasolar planets we're discovering will have to be reclassified once we get better at finding smaller bodies, just like in this solar system.
    - KnightOfWords 8y ago
      > What I don't like about the "cleared its orbit" criterion is that it's something you can't tell from looking at the object itself. Dwarf planet is a helpful term in some contexts but not others. The solar system is a dynamic system, when talking about the whole thing or its evolution it's useful to distinguish between planets and dwarfs.
  - KnightOfWords 8y ago
    > The 'cleared it's orbit' thing is also arbitrary. Not really, the degree to which a body has cleared its orbit can be calculated: https://en.wikipedia.org/wiki/Clearing_the_neighbourhood https://en.wikipedia.org/wiki/Clearing_the_neighbourhood > Pluto is sometimes closer to the Sun than Neptune for example but does that mean they share an orbit? Pluto is locked in a resonant orbit with Neptune, a position it reached due to the influence of Neptune's gravity: https://en.wikipedia.org/wiki/Pluto#Relationship_with_Neptune https://en.wikipedia.org/wiki/Pluto#Relationship_with_Neptun...
- ProblemFactory 8y ago
  In more detail it's not about the shape, but mass. Planets are massive enough for gravity to overcome the internal bonds in rocks and metals, making even these materials flow into a round shape. An asteroid could be perfectly round, but its shape is based on material strength and not just gravity.
hibbelig 8y ago
Well, Farout orbits the sun, is round, is not a moon. The article is silent on item d. I don't know what to do with the information that its diameter is 500km. I guess that makes it 1/5 of Pluto's diameter, and given that Pluto is called a dwarf planet...
- walrus01 8y ago
  500 km is about the size of one of the bigger asteroid belt objects: https://en.wikipedia.org/wiki/4_Vesta https://en.wikipedia.org/wiki/4_Vesta Notably, Ceres is about 950km and much more spherical.
- zamadatix 8y ago
  "round" is a bit oversimplified in the above the actual criteria is: "(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, " If Farout is highly icy 500km could be enough to qualify it. Regardless it isn't nearly massive enough to clear it's orbit.
- jessriedel 8y ago
  > The article is silent on item d. I believe the mass and orbital parameters necessary to clear an orbit of debris are known pretty well known theoretically, and farout won't satisfy it. This is why astronomers can "be looking for Planet 9/X/10" without searching for debris everywhere. (Likewise, knowing the rough mass is enough to know whether a body has a hydrostatic shape; you don't have to actually resolve it's shape in your telescope.) Edit: yep: https://en.wikipedia.org/wiki/Clearing_the_neighbourhood#Criteria https://en.wikipedia.org/wiki/Clearing_the_neighbourhood#Cri...
LanceH 8y ago
The flip side is the IAU came up with a definition that only allows 8 planets in the universe and ceased to be relevant for a common sense definition of planet which existed well before they did.
- zipwitch 8y ago
  There are a lot of things out there that orbit stars, and it increasingly looks like just one, or even a handful, of formal definitions are going to be inadequate, even for our own solar system. We need a detailed and extensive classification system.... https://en.m.wikipedia.org/wiki/Star_Trek_planet_classification https://en.m.wikipedia.org/wiki/Star_Trek_planet_classificat...

yitchelle 8y ago

Is a plant being round a subjective thing? Just trying to understand how roundness can be measured.

simonh 8y ago
Yes, but if we want to distinguish between say asteroids and planets, or orbital pebbles and planets, we need criteria with which to do so and those are going to be at least somewhat arbitrary. The 'cleared it's orbit' thing is also arbitrary. Pluto is sometimes closer to the Sun than Neptune for example but does that mean they share an orbit? It's mainly to exclude Ceres, which is pretty close to spherical. The problem is any rules that exclude Ceres and also exclude the other trans-Plutonian planetoids also end up excluding Pluto. If Pluto is a planet and this new thing, why not Eris, Haumea, Sedna, orcus, etc, etc, etc.There are many dozens of them. We do have applicable terms - planetoid, dwarf planet, so why not use them?
- lmm 8y ago
  Why does one want to exclude Ceres? What I don't like about the "cleared its orbit" criterion is that it's something you can't tell from looking at the object itself. If a planet is ejected from its orbit, it becomes a ... well, we don't even have a name for what it would be. (This problem exists for "moon" to a certain extent, but a moon that's thrown out of planetary orbit would become a planet if it was big enough and an asteroid if it wasn't). It's also hard to apply: a lot of extrasolar planets we're discovering will have to be reclassified once we get better at finding smaller bodies, just like in this solar system.
  - KnightOfWords 8y ago
    > What I don't like about the "cleared its orbit" criterion is that it's something you can't tell from looking at the object itself. Dwarf planet is a helpful term in some contexts but not others. The solar system is a dynamic system, when talking about the whole thing or its evolution it's useful to distinguish between planets and dwarfs.
- KnightOfWords 8y ago
  > The 'cleared it's orbit' thing is also arbitrary. Not really, the degree to which a body has cleared its orbit can be calculated: https://en.wikipedia.org/wiki/Clearing_the_neighbourhood https://en.wikipedia.org/wiki/Clearing_the_neighbourhood > Pluto is sometimes closer to the Sun than Neptune for example but does that mean they share an orbit? Pluto is locked in a resonant orbit with Neptune, a position it reached due to the influence of Neptune's gravity: https://en.wikipedia.org/wiki/Pluto#Relationship_with_Neptune https://en.wikipedia.org/wiki/Pluto#Relationship_with_Neptun...
ProblemFactory 8y ago
In more detail it's not about the shape, but mass. Planets are massive enough for gravity to overcome the internal bonds in rocks and metals, making even these materials flow into a round shape. An asteroid could be perfectly round, but its shape is based on material strength and not just gravity.

hibbelig 8y ago

Well, Farout orbits the sun, is round, is not a moon. The article is silent on item d. I don't know what to do with the information that its diameter is 500km. I guess that makes it 1/5 of Pluto's diameter, and given that Pluto is called a dwarf planet...

walrus01 8y ago
500 km is about the size of one of the bigger asteroid belt objects: https://en.wikipedia.org/wiki/4_Vesta https://en.wikipedia.org/wiki/4_Vesta Notably, Ceres is about 950km and much more spherical.
zamadatix 8y ago
"round" is a bit oversimplified in the above the actual criteria is: "(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, " If Farout is highly icy 500km could be enough to qualify it. Regardless it isn't nearly massive enough to clear it's orbit.
jessriedel 8y ago
> The article is silent on item d. I believe the mass and orbital parameters necessary to clear an orbit of debris are known pretty well known theoretically, and farout won't satisfy it. This is why astronomers can "be looking for Planet 9/X/10" without searching for debris everywhere. (Likewise, knowing the rough mass is enough to know whether a body has a hydrostatic shape; you don't have to actually resolve it's shape in your telescope.) Edit: yep: https://en.wikipedia.org/wiki/Clearing_the_neighbourhood#Criteria https://en.wikipedia.org/wiki/Clearing_the_neighbourhood#Cri...

LanceH 8y ago

The flip side is the IAU came up with a definition that only allows 8 planets in the universe and ceased to be relevant for a common sense definition of planet which existed well before they did.

zipwitch 8y ago
There are a lot of things out there that orbit stars, and it increasingly looks like just one, or even a handful, of formal definitions are going to be inadequate, even for our own solar system. We need a detailed and extensive classification system.... https://en.m.wikipedia.org/wiki/Star_Trek_planet_classification https://en.m.wikipedia.org/wiki/Star_Trek_planet_classificat...