Re: "Pluto Now Called a Plutoid"

In message <PYKdnToYLvdOlenVnZ2dnUVZ_rbinZ2d@xxxxxxxxxxx>, K_h <KHolmes@xxxxxxxxx> writes
The IAU's definition of a planet is extremely flawed for a number of
reasons. For some examples, the IAU definition:



* defines a planet as "nearly round" suggesting that only nearly spherical
bodies qualify as planets. But Saturn is over 12% oblate and so it is not
clear if it qualifies as a planet.

It does, the nearly round is hydrostatic equilibrium so it is round modulo any distortions caused by spin.


* defines a planet as having "cleared the neighborhood around its orbit"
which would exclude Jupiter type planets in other solar systems. For
example, consider a solar system with a 6 solar-mass star orbited by a 25
Jupiter mass brown dwarf (at 20AU from the star) and one Jupiter mass planet
at each LaGrange point L4 and L5. Neither Jupiter mass planet would have
"cleared the neighborhood around its orbit" and so they would not qualify as
planets.

That is an actual potential ambiguity, at the moment we don't know of any such bodies if we find one then we will have to decide. This is true of most possible definitions. How you might classify a giant Trojan can await actually discovering one.



* requires that a Kuiper belt world with the same size and mass as the Earth
is not a planet. A world with 70 percent the mass of the Earth may very
well reside within the Kuiper belt.

The minimum size for orbit clearing depends on distance from the primary; and this is a problem why?



* defines a dwarf planet as not being a planet and this contradicts English
usage. In English, an adjective like "dwarf" modifies a noun like "planet".
What the IAU has done is like defining a woman to be a young female and then
claiming that there are old women.

In English the noun with the modifier often denotes a subset, it can also denote a disjoint set. A few examples: In Alpine Skiing the Slalom Giant Slalom and Supergiant Slalom are three distinct events and there is no overlap between the types of courses used. Moles, Golden Moles and Marsupial Moles are three quite distinct, if superficially similar, groups of burrowing mammals again they are entirely disjoint sets and are not closely related.


* defines a planet in such a way that it only applies to Earth's solar
system. This is bad; a definition should be applicable throughout nature.
Claiming one definition for Earth's solar system, and another definition for
the rest of the universe, implies there is something special about Earth's
solar system and that is a violation of the Copernican principle.

We have a lot more information about the solar system than about any other system, we can actually resolve individual objects that are too small to be planets in the solar system, with one exception this is not true of any exoplanetary system, so for the moment we don't need to worry about the lower limit of planetary size outside the solar system.



* has intrinsic ambiguity in its phrase "cleared the neighborhood around its
orbit". There is no way to know how much clearing is necessary.

* fails to specify what kind of orbit a planet must take. It is unclear if
a world, with the same size and mass as the Earth, traveling on a hyperbolic
orbit around the sun qualifies as a planet.

A hyperbola isn't actually an orbit, as it isn't closed so no that would not have cleared its orbit. Orbits are either elliptical or circular and repeat. Parabolas and hyperbolas do not repeat.




The IAU's definition of a planet is not based on good science and is not
very useful. The best way to define a planet is to define it in context
with other bodies in the universe. Below is an example of one possible way
of doing this.

The IAUs definition does this, a planet has achieved a particular status in terms of its orbital dynamics, it has cleared a region of space of pretty much all other bodies.

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A star is a celestial body that sustains, has sustained, or is capable of
sustaining nuclear fusion.



A brown dwarf is a star that is only capable of fusing deuterium and/or
lithium even if neither is present.



A celestial body that has its barycenter continuously inside the body of
another celestial body is a satellite of that celestial body.



A planetary body is a celestial body that has sufficient mass for its self
gravity to overcome rigid body forces so that it assumes a hydrostatic
equilibrium, is nearly spherical or nearly spheroidal in shape, and is not a
star.



A planet is a planetary body that is not the satellite of another planetary
body.

That would include a bunch of small spherical bodies which are too small, such as Ceres. Planet are large and should dominate their region of space.




A moon is a satellite of a planet.

Wrong, a fair number of non-planets have moons for example Dactyl is a moon of the Asteroid Ida. Not acceptable for that reason




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With the above definitions, there is the option of having additional
definitions that divide planets and moons into further categories:



Giant planet: Diameter >= 30,000 Km.
Midsize planet: 30,000 Km > Diameter >= 3,000 Km.
Dwarf planet: 3,000 Km > Diameter

Giant moon: Maximum width >= 3,000 Km.
Midsize moon: 3,000 Km > Maximum width >= 300 Km.
Dwarf moon: 300 Km > Maximum width



Primary planet: Giant or midsize planet
Secondary planet: dwarf planet

Primary moon: Giant or midsize moon
Secondary moon: dwarf moon



A classification scheme like this should satisfy the anti-Pluto people since
only 8 planets in the solar system qualify as primary planets. It also
prevents the number of primary planets from growing too large; which seemed
to be a big concern for the IAU.


It doesn't use any fundamental characteristic to define a Planet, diameter is an arbitrary number while orbit clearing is a dynamic process which produces a clear natural gap. Diameter depends a lot on composition, Mercury has a smaller diameter than Ganymede but is almost twice the mass. Icy bodies also achieve hydrostatic equilibrium at a lower mass than rocky bodies.




There is my 2 cents on the planet definition problem and one possible way to
fix it.


There are fewer problems with the IAU definition than with yours. Orbit clearing is a straightforward and significant orbital dynamic process.
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