Re: "Pluto Now Called a Plutoid"

In message <obidne-_XekouB3VnZ2dnUVZ_uqdnZ2d@xxxxxxxxxxx>, K_h <KHolmes@xxxxxxxxx> writes

"Brett Paul Dunbar" <brett@xxxxxxxxxxxxxxxxxx> wrote in message
news:M0nc6VDlrzfIFwxd@xxxxxxxxxxxxxxxxxxxxxxxxx
In message <pcOdna4DibNJH-PVnZ2dnUVZ_hudnZ2d@xxxxxxxxxxx>, K_h
<KHolmes@xxxxxxxxx> writes

"Brett Paul Dunbar" <brett@xxxxxxxxxxxxxxxxxx> wrote in message
news:4htBfzPOWVfIFw2Z@xxxxxxxxxxxxxxxxxxxxxxxxx
In message <jridnfxvzcbapuDVnZ2dnUVZ_uqdnZ2d@xxxxxxxxxxx>, K_h
<KHolmes@xxxxxxxxx> writes

"Brett Paul Dunbar" <brett@xxxxxxxxxxxxxxxxxx> wrote in message
news:2OPy0vGFPIfIFwET@xxxxxxxxxxxxxxxxxxxxxxxxx
In message <ALOdnR7-EYk6dObVnZ2dnUVZ_r_inZ2d@xxxxxxxxxxx>, K_h
<KHolmes@xxxxxxxxx> writes

It should be called a planet. Pluto, for example, has polar caps, an atmosphere, seasons, three moons, and interesting geological activity at its surface.

Being interesting does not make it a planet. Comets also have
atmospheres sometimes very big atmospheres. Pluto looks to be a
gargantuan comet.

Again, the nomenclature should cover everything that is bound to be out
there. Pluto looks nothing like a "gargantuan comet".


Actually it does, it appears to have been formed from the same bit the sun's circumstellar disk as the short period comets and seems to be made of the same stuff. Which, on compositional grounds, make it a giant comet. If it were on perturbed onto an orbit which took it into the inner solar system it would look like a comet forming a coma and tail as its ices vaporised. Several other outer solar system bodies have previously displayed comet like behaviour, Chiron has displayed a coma and is one of a number of bodies designated as both a comet and an asteroid.

Not at all. Suppose there is an Earth sized world, made out of largely ice and a little rock, that has "clear out its orbit" out in the Kuiper belt. It too would form a tail, if it got close to the sun, but it would qualify as a planet under the IAU's definition. So your compositional argument makes no sense.

It does make sense. If you define a comet by what it is made of then
Pluto is a comet. This is based purely on the intrinsic qualities of
Pluto. Pluto is apparently a product of the accretion of a very large
quantity of cometary material, hence it is a gargantuan comet. What
doesn't make sense is to claim that Pluto looks nothing like a comet.

The Kuiper Wall planet would fit in both the class planet and the class
comet.

But there is no simple way to define what a comet is made out of. For
example there are a continuum of possible ratios for things like ices,
rocks, metals, etc in what we could define as a "comet". How much iron must
be in a comet before it loses its cometary status? Ratio ranges for all the
materials must be given in any definition of a comet and such definitions
quickly become unwieldly.

So far it hasn't posed much of a problem, primarily icy bodies from the outer solar system are comets while rocky or metallic inner system bodies are asteroids. A few are officially classified as both, Chiron for example.

The class of comet was originally defined based on their behaviour in the inner solar system, forming comas and tails &c. without knowledge of structure, origin or composition. The behaviour happens to derive from the composition so the composition is a key part of the definition.

Asteroids are basically debris which doesn't show cometary behaviour. Some asteroids are dead comets as they have lost enough ice that they have a thick non-volatile crust and no longer erupt under heating. This doesn't happen at any specific ratio, for example a comet might have a very thick impermeable crust over most of its surface with a single active spot and be a comet while a different body might have a much thinner impermeable crust with no active spots and be a cometary remnant type asteroid.

Pluto was, we think, formed from the same stuff that the short period comets were, which means that its composition falls into the cometary range.


I note that you chose only to comment on one case where the terminology has been changed to reflect a better understanding of the facts. In many other cases, Moles, Golden Moles and Marsupial Moles or Crabs, Hermit Crabs and Porcelain Crabs or Chimps and Pygmy Chimps or Hippopotamuses and Pygmy Hippopotamuses, the noun with the modifier is not a subset of the noun without the modifier. It is perfectly normal English.

Those are still exceptions to the rule and is not the best of English.

There are so many exceptions that it isn't really possible to say that
there is a rule. Both the inclusive and the exclusive cases are
perfectly normal English.

No excuse for not fixing the IAU definition since most of the time
adjectives modify nouns.

There isn't any great prevalence of one or the other, not that it matters materially what label you apply. Planet as currently defined is a clear natural category. Labelling is a separate matter. Dwarf Planets could be renamed, You could call them planetoids for example, or medium solar system bodies, or asteroids.

Unclear what your point is. Generic definitions of planets are possible that will not change with new discoveries. I believe Mark Sykes has suggested defining a planet as an object in hydrostatic equilibrium that is not a star. Simple definitions like these are broad enough that they will cover everything that is out there.


Bad definition. It includes a large number of bodies that nobody ever
calls Planets, like Ganymede, it thus fails to follow established usage.

I did not fully characterize Sykes definition, obviously it would need to
distinguish between moons and planets. If I recall, he defined a planet as
a celestial body, in hydrostatic equilibrium, that is in orbit around a star
and that would exclude Ganymede (which orbits Jupiter).


All he is doing is making Dwarf Planet a sub class of Planet, this puts an excessive number of objects in the class of planet and replaces an unambiguous lower limit with a much harder to detect criterion.

Ceres is not a planet, this was settled 150 years ago. It was intended that the term "Minor Planet" be a subset of planet along with "Major Planet". Usage quickly developed so that "Major Planet" became simply Planet and therefore planet and minor planet were mutually exclusive categories.

There might be a reason for defining a class of sub-stellar objects in
hydrostatic equilibrium. It should not be labelled planet.

The IAU definition both defines a natural class, objects that have
cleared their orbits, and applies a label whose traditional usage
matches well to that class.

No. I listed some of the most glaring problems of the IAU's definition in
my original post.

Your objections were either ridiculous grammatical quibbles, or just plain wrong. Orbit clearing is a dynamic process that produces a clear gap, it is a natural category whatever you choose to call the members of the category. It happens that the normal usage of the term Planet matches pretty well.


The term star is used to include White Dwarfs and Neutron Stars, both
discrete stellar remnants, Black Holes, even when they are stellar
remnants, are not counted as stars.

Yet another problem with the IAU's definition, since it requires a planet to be something that is orbiting a star. The example definitions I provided do not require that a planet orbit a star. Suppose there is a Jupiter like world orbiting a black hole and further suppose it has even "cleared out its neighborhood". Under the IAU's definition it is not a planet. According to the IAU, then, what the heck is it?

It is orbiting a stellar remnant, therefore it would be a planet. The
IAU working definition explicitly states this.

<http://www.dtm.ciw.edu/boss/definition.html>

1 Objects with true masses below the limiting mass for
thermonuclear fusion of deuterium (currently calculated to be 13
Jupiter masses for objects of solar metallicity) that orbit
stars or stellar remnants are "planets" (no matter how they
formed). The minimum mass/size required for an extrasolar object
to be considered a planet should be the same as that used in our
Solar System.

2 Substellar objects with true masses above the limiting mass for
thermonuclear fusion of deuterium are "brown dwarfs", no matter
how they formed nor where they are located.

3 Free-floating objects in young star clusters with masses below
the limiting mass for thermonuclear fusion of deuterium are not
"planets", but are "sub-brown dwarfs" (or whatever name is most
appropriate).

But the problem is with the two definitions. One definition for the solar
system, and one for everywhere else, makes no sense. We don't have one
definition of a tree, for trees in America, and another definition for a
tree everywhere else. So the IAU's definition of a planet (for the solar
system) does have the problem I mentioned when we try to combine it with the
definition of a planet (everywhere else). That is the point, those
definitions are inconsistent with each other.


Paragraph 1 indicates that the solar system definition should usually be used, with appropriate modifications.

Paragraph 2 imposes an upper limit on planet size, which isn't needed in the solar system as Jupiter is about on thirteenth the mass needed to fuse Deuterium. Incidentally there is another potential lower limit for Brown Dwarfs, a body becomes fully convecting at about 7 Jupiter masses and there is some support for making that the lower limit of Brown Dwarf status.

Paragraph 3 deals with a class of objects we haven't seen yet but do expect to detect in the not too distant future. They are theorised to form in a rather different manner to solar system planets and have currently been excluded from the class of planet.

The working definition is the solar system definition; with a couple of extra specifications to deal with types of bodies absent from the solar system.
--
Great Internet Mersenne Prime Search http://www.mersenne.org/prime.htm
Livejournal http://brett-dunbar.livejournal.com/
Brett Paul Dunbar
To email me, use reply-to address
.

Relevant Pages

  • THE COMET KINGS (Edmond Hamilton)
    ... COMET KINGS by Edmond Hamilton. ... Every planet was inhabited by variations ... Captain Future has booby-trapped the Moon so no one else can land there. ... Cap gathers his team of hulking metal robot, ...
    (rec.arts.sf.written)
  • Re: THE COMET KINGS (Edmond Hamilton)
    ... COMET KINGS by Edmond Hamilton. ... Every planet was inhabited by variations ... Captain Future has booby-trapped the Moon so no one else can land there. ... Cap gathers his team of hulking metal robot, ...
    (rec.arts.sf.written)
  • Re: Discounting Pluto?
    ... > The whole Pluto thing is phenomenally stupid. ... > demotion from planet status and completely oblivious to and unaffected ... > beings on Pluto who are miffed by the so-called demotion. ... motions with the other planets to determine solar system structure and ...
    (sci.astro.amateur)
  • Re: "Pluto Now Called a Plutoid"
    ... decided that an object too small to fuse Deuterium formed in a star like ... manner is a Sup-Brown Dwarf not a planet, ... Pluto looks nothing like a "gargantuan comet". ... If it were on perturbed onto an orbit which took it into the inner solar ...
    (rec.arts.sf.science)
  • Re: Are we ruining the solar system?
    ... >> It struck me the other day that we might be ruining the solar system. ... >> This, of course, brings the planet closer to the sun. ... Maybe we should have a moratorium on using gravity ... > manouvers around Pluto with it a million times it would ...
    (sci.space.policy)
Loading