Of Stars, Pluto and Planetary Classification

The "Planet" Problem:

The IAU has come up with a planetary classification scheme that is
*not* based solely upon the physical nature of the objects being
classified. The orbital details of an otherwise qualified body can
exclude it from the "planet" classification.

This means a body can be planet if located in one orbit (and
gravitationally bound to one star) while an identical body would fail
to be classified as a planet if located in a different orbit (or
gravitationally bound to a different star).

Furthermore, the IAU classification scheme cannot be applied to bodies
orbiting stars other than the Sun. This ignores the vast majority of
potential planetary bodies.

About Stars:

Stars remain stars regardless of where they are. A star in orbit
about another star is still a star. This is the case regardless of
anything occupying a nearby orbit. This is the case even if other
stars occupy the same orbit. Even a star found between two widely
separated galaxies would still be a star.

Concerning Pluto:

Current events have placed Pluto on center stage. Either Pluto is a
planet or Pluto is not a planet. Some favor one outcome. Some favor
the other outcome. A planetary classification scheme should *not* be
doctored to specifically include nor to specifically exclude Pluto as
a planet. The classification scheme needs to be more objective.

Concerning the Number of Planets:

A planetary definition should not be judged on how many or how few
planets our solar system contains. Four planets or four hundred
planets -- it makes no difference so long as all qualified bodies are
included and all unqualified bodies are excluded.

Properties of a Good Planetary Definition:

1) It should be based solely upon one or more (preferably easily
determined) physical property of the candidate body.

2) It should permit all qualifying bodies to remain planets
regardless of where they are and regardless of where nature or future
technology might move them (with due allowances for boiling off mass,
etc.)

3) The cut-off between planet and non-planet should not be arbitrary.
It should be based upon a real, naturally occurring, physical
transition.

4) It should be applicable to our own solar system as well as to
extra-solar planetary systems.

Further Thoughts on a Planetary Definition or Planetary Classification
Scheme:

In light of the above points, the upper cut-off point could be related
to thermonuclear fusion. We wouldn't want to include stars as
planets! Optionally, (and recommended) a transition classification
could also be included.

The lower cut-off point could be related to the mass associated with
the transition between reasonably spherical and reasonably
non-spherical objects. Once agreed upon, the cut-off *mass* (not the
degree of sphericalness) will be the deciding factor. Optionally,
(and recommended) a transition classification (specifying a range of
mass) could also be included.

If so desired, the classification "Planet" could be subdivided into
Classes, with each class having upper and lower mass limits.

Pay no special attention to Pluto! Let it fall wherever in (or
outside) the definition or classification scheme. Pay no special
attention to orbits! (See "About Stars" above.). It's the physical
body we're interested in, regardless of where that body might be
found. If this means that some planetary satellites are also Planets,
then so be it!

It's not the job of science to keep the number of planets in our solar
system small for easy memorization by school children. It's more
important to have definitions and classification schemes that science
and astronomy can be proud of.

Willie R. Meghar

In article ,
Willie R. Meghar wrote:

The IAU has come up with a planetary classification scheme that is
*not* based solely upon the physical nature of the objects being
classified. The orbital details of an otherwise qualified body can
exclude it from the "planet" classification.

That's nothing new! Consider Titan, which is bigger and more massive
than Mercury and has an atmosphere as dense as the Earth's atmosphere.
Yet, it's not a planet, because of its orbital characteristics: it
orbits Saturn, not the Sun.

Based solely of the physical nature of the object itself, Titan would
be an obvious planet. Right?

--
----------------------------------------------------------------
Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stockholm dot bostream dot se
WWW: http://stjarnhimlen.se/

(Paul Schlyter) wrote:

That's nothing new! Consider Titan, which is bigger and more massive
than Mercury and has an atmosphere as dense as the Earth's atmosphere.
Yet, it's not a planet, because of its orbital characteristics: it
orbits Saturn, not the Sun.

Based solely of the physical nature of the object itself, Titan would
be an obvious planet. Right?

Right, unless the limiting mass for a planet is reduced to a level
that would exclude Titan, in which case Mercury would not be a planet
either.

Willie R. Meghar

In article ,
Willie R. Meghar wrote:

(Paul Schlyter) wrote:

That's nothing new! Consider Titan, which is bigger and more massive
than Mercury and has an atmosphere as dense as the Earth's atmosphere.
Yet, it's not a planet, because of its orbital characteristics: it
orbits Saturn, not the Sun.

Based solely of the physical nature of the object itself, Titan would
be an obvious planet. Right?

Right, unless the limiting mass for a planet is reduced to a level
that would exclude Titan, in which case Mercury would not be a planet
either.

Willie R. Meghar

I think you mean "increased" rather than "reduced"..... :-)

Anyway, no-one has seriously suggested THAT - right?

--
----------------------------------------------------------------
Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stockholm dot bostream dot se
WWW: http://stjarnhimlen.se/

(Paul Schlyter) wrote:

In article ,
Willie R. Meghar wrote:

(Paul Schlyter) wrote:

Based solely of the physical nature of the object itself, Titan would
be an obvious planet. Right?

Right, unless the limiting mass for a planet is reduced to a level
that would exclude Titan, in which case Mercury would not be a planet
either.

Willie R. Meghar

I think you mean "increased" rather than "reduced"..... :-)

Some suppositions were required in order to answer your initial
question. I used "reduced" based on the suppositions that mass was
used as "the" physical nature criteria, and there existed some minimum
mass less than or equal to the mass of Mercury that a body must have
in order to be called a planet. Without these suppositions (that I
thought were self-evident) it would not have been possible to answer
your question.

In this context, I believe "reduced" was the correct word to use.

Anyway, no-one has seriously suggested THAT - right?

If by "THAT" you mean the classification of solar system bodies based
on mass without regard to orbital details, then I reply that my
suggestion that this be done is a serious suggestion; but if by
"serious" you're asking if the IAU has considered it -- I don't know.

Willie R. Meghar

In article ,
Willie R. Meghar wrote:
(Paul Schlyter) wrote:

In article ,
Willie R. Meghar wrote:

(Paul Schlyter) wrote:

Based solely of the physical nature of the object itself, Titan would
be an obvious planet. Right?

Right, unless the limiting mass for a planet is reduced to a level
that would exclude Titan, in which case Mercury would not be a planet
either.

Willie R. Meghar

I think you mean "increased" rather than "reduced"..... :-)

Some suppositions were required in order to answer your initial
question. I used "reduced" based on the suppositions that mass was
used as "the" physical nature criteria, and there existed some minimum
mass less than or equal to the mass of Mercury that a body must have
in order to be called a planet. Without these suppositions (that I
thought were self-evident) it would not have been possible to answer
your question.

In this context, I believe "reduced" was the correct word to use.

You wrote "limiting mass for a planet is reduced", not "physical nature
criteria for a planet is reduced". Reducing a mass means making it smaller,
not larger. Right?

Anyway, no-one has seriously suggested THAT - right?

If by "THAT" you mean the classification of solar system bodies based
on mass without regard to orbital details,

No ... instead I meant creating a classification such that Mercury too
would have become a non-planet. Increasing the limiting mass sufficiently
could be a way to accomplish that.

then I reply that my
suggestion that this be done is a serious suggestion; but if by
"serious" you're asking if the IAU has considered it -- I don't know.

Willie R. Meghar


--
----------------------------------------------------------------
Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stockholm dot bostream dot se
WWW: http://stjarnhimlen.se/

Willie R. Meghar wrote:
snip

About Stars:

Stars remain stars regardless of where they are. A star in orbit
about another star is still a star. This is the case regardless of
anything occupying a nearby orbit. This is the case even if other
stars occupy the same orbit. Even a star found between two widely
separated galaxies would still be a star.

This applies elsewhere. Things are what they are, and only rarely do
they change what they are because of their location. A watch is a
watch whether it's on the retailer's shelf, strapped to my wrist, or in
a thief's pocket. It's a watch wheter or not it has a jeweled
movement, digital movement (yes, there have been such things), a
quartz oscillator, or some other mechanism for keeping time. It takes
very special circumstances for it's location to have an impact. Under
the wheels of a bus, it stops being a watch and becomes a piece of
junk, not because of it's location, but because the bus did something
dramatic to it, changing the nature of what it was.

snip

Pay no special attention to Pluto! Let it fall wherever in (or
outside) the definition or classification scheme. Pay no special
attention to orbits! (See "About Stars" above.). It's the physical
body we're interested in, regardless of where that body might be
found. If this means that some planetary satellites are also Planets,
then so be it!

It's not the job of science to keep the number of planets in our solar
system small for easy memorization by school children. It's more
important to have definitions and classification schemes that science
and astronomy can be proud of.

I think the real problem is trying to put a scientific definition to a
non-scientific term. Astronomers typically don't use the term
"shooting star" because it is a non-scientific term. Instead, new
terms were invented and defined. It makes no sense to try to define
"shooting star" scientifically because it is such a misleading term.

What's really needed is a new term (or, more likely, a set of new
terms) to define non-star bodies in space. I think astronomers need to
stop using the term "planet" exept when dealing with the public. We
figured out that calling man-made sattelites "moons" was not
appropriate, perhaps even before there were any.

Maybe it's time to throw out the term "planet" and replace it with a
new set of terms rather than try to force a scientific definition on a
term that has long outlived it's scientific usefulness.

Austin

"AustinMN" wrote:

I think the real problem is trying to put a scientific definition to a
non-scientific term. Astronomers typically don't use the term
"shooting star" because it is a non-scientific term. Instead, new
terms were invented and defined. It makes no sense to try to define
"shooting star" scientifically because it is such a misleading term.

What's really needed is a new term (or, more likely, a set of new
terms) to define non-star bodies in space. I think astronomers need to
stop using the term "planet" exept when dealing with the public. We
figured out that calling man-made sattelites "moons" was not
appropriate, perhaps even before there were any.

Maybe it's time to throw out the term "planet" and replace it with a
new set of terms rather than try to force a scientific definition on a
term that has long outlived it's scientific usefulness.

Yes, this makes a great deal of sense!

Willie R. Meghar

Willie R. Meghar wrote:
"AustinMN" wrote:

I think the real problem is trying to put a scientific definition to
a non-scientific term. Astronomers typically don't use the term
"shooting star" because it is a non-scientific term. Instead, new
terms were invented and defined. It makes no sense to try to define
"shooting star" scientifically because it is such a misleading term.

What's really needed is a new term (or, more likely, a set of new
terms) to define non-star bodies in space. I think astronomers need
to stop using the term "planet" exept when dealing with the public.
We figured out that calling man-made sattelites "moons" was not
appropriate, perhaps even before there were any.

Maybe it's time to throw out the term "planet" and replace it with a
new set of terms rather than try to force a scientific definition on
a term that has long outlived it's scientific usefulness.

Yes, this makes a great deal of sense!

Willie R. Meghar

Let us test this. I propose "cabody"

Let the discussion begin: "what is a cabody?"

Do you think it is easier to agree what a "cabody" is than to define what a
"planet" is? I think I know the answer.....

Clear skies
Carsten A. Arnholm
http://arnholm.org/
N59.776 E10.457

Carsten A. Arnholm wrote:
Willie R. Meghar wrote:
"AustinMN" wrote:

I think the real problem is trying to put a scientific definition to
a non-scientific term. Astronomers typically don't use the term
"shooting star" because it is a non-scientific term. Instead, new
terms were invented and defined. It makes no sense to try to define
"shooting star" scientifically because it is such a misleading term.

What's really needed is a new term (or, more likely, a set of new
terms) to define non-star bodies in space. I think astronomers need
to stop using the term "planet" exept when dealing with the public.
We figured out that calling man-made sattelites "moons" was not
appropriate, perhaps even before there were any.

Maybe it's time to throw out the term "planet" and replace it with a
new set of terms rather than try to force a scientific definition on
a term that has long outlived it's scientific usefulness.

Yes, this makes a great deal of sense!

Willie R. Meghar

Let us test this. I propose "cabody"

Let the discussion begin: "what is a cabody?"

Do you think it is easier to agree what a "cabody" is than to define what a
"planet" is? I think I know the answer.....

This is a wrong approach. First you come up with a definition, then
apply a term. That does not mean you can't have a term in mind, but
the truth is, this whole discusion has proven that the term "planet" is
useless.

On the other hand, if you start with "A cabody is any object that has
sufficient mass for its self-gravity to overcome rigid body forces so
that it assumes a hydrostatic equilibrium (nearly round) shape and
would not otherwise be 1) classified as a star or 2) be primarily
composed of a form of matter not naturally occuring on earth" then
everyone knows what that means (assuming we don't need to define
"nearly round," "star," or "earth").

If the definition works, you can then use any appropriate term. Call
it a lubite or glyph or a Taumaugh, as long as we can agree on the
definition, (and is not already in use or would be misleading) it
works. Using a term like "space rock" or "pond water" would not work
because they would, by their nature, be misleading. We will not likely
ever agree on the definition of a "wandering star" or "planet" and the
term will always (OK, perhaps only for a very long time) be interpreted
by the public differently from any scientific definition.

It could then be qualified with other terms - solar cabody, extra-solar
cabody, jovian cabody, kaiper belt cabody, short-period cabody, even
sub-cabody - and everyone in the astro sciences community still knows
what you mean. The definition is based on what the object is, not
where it is (but can be qualified that way) and can be applied
immediately.

The appropriate response to the question "Is it a planet?" is
relatively simple.

"Astronomers can't agree on exactly what a planet is so we just don't
use the term any more."

People will get used to it. There are other commonly used terms that
are no longer used, even by the public, because they had no useful
scientific meaning.

It just doesn't make sense to call them "wandering stars" any more.
Planets are for poets.

Austin