Re: "Pluto Now Called a Plutoid"
- From: "K_h" <KHolmes@xxxxxxxxx>
- Date: Tue, 8 Jul 2008 17:39:53 -0700
"Jens Egon Nyborg" <jens@xxxxxxxxxxxxxxx> wrote in message
news:g4bdcl$kho$1@xxxxxxxxxxxxxxxxxxxxxxxxxxxx
Brett Paul Dunbar skrev:
(2) A "dwarf planet" 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[2], (c) has not cleared the neighbourhood around
its orbit, and (d) is not a satellite.
If I may, (c) is were /I/ think the definition becomes downright silly.
Extend this definition to other systems (we will have to, someday) by
substituting "exoplanet" for planet and "the local star" for "the Sun".
Now we can ask "How many 'dwarf exoplanets' with a mass greater than
Jupiter are there in the Milkyway?"
We may or may not be able to give a sensible answer, but I see it as a
problem that the question actually makes sense.
Consider rogue planets, planets in independent orbit around the galaxy
(if they exit), are they "Dwarfs"? If not then Ceres would become a
"normal-sized" rogue planet if expelled from the solar system, but if
they are dwarfs, then surely there are dwarfs out there that are just
shy of being stars themselves.
In short, taking a word that denotes 'smallness' and using it to define
something often associated with smallness invites problem of
communication further down the line
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.
* 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.
* 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.
* 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.
* 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.
* 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.
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.
-----------------------------------------------------------------------------
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.
A moon is a satellite of a planet.
-----------------------------------------------------------------------------
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.
There is my 2 cents on the planet definition problem and one possible way to
fix it.
K
.
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