Pluto is out from planet dictionary

"Paul Schlyter" wrote in message
...
In article . com,
Jeff Root wrote:

http://www.bautforum.com/showthread.php?t=34542

The questions I asked:

Were the Sun and Moon commonly referred to as "planets" in
ancient times?

When was Earth first called a "planet", or described as a body
comparable to a planet, even if it wasn't thought to wander?

When were the stars first recognized as being like the Sun?

Probably around Copernicus' time. One prevailing argument against
a heliocentric solar system was that the Earth's orbital motion
around the Sun ought to cause a quite visible yearly parallax
among the stars --- unless of course the stars were extremely
distant and not just a little farther away than Saturn, as was
commonly believed in that time.

When the stars did turn out to be that vastly distant, they
must also be very bright --- like the Sun.

Herschel was one of the first trying to find the actual distance
to some stars, and he thought Sirius was some 3 light years away.


Christiaan Huygens did this experimentally about a century before Herschel,
by comparing Sirius's brightness with the Sun (an ingenious experiment,
BTW). The main reason he got a distance that was too small, was his
assumption that Sirius and the Sun had the same absolute brightness.

I can't recall Herschel's method but it may have been similar.


The answers I got were basically just:

The Sun and Moon were considered planets by early Greek
astronomers and are still considered planets in astrology.

Giordano Bruno suggested or asserted that that other stars
were suns with planets, like our own Sun.

I pointed out:

Lucian, a Syrian writing in Greek, in Athens, in the
second century AD, wrote a satire titled 'True History'
which described inhabitants of the Moon and the Sun.
That would seem to imply that he thought of them as
places something like the Earth.

And of course, the fact that the ancients knew that Earth
must be a sphere because of the shape of its shadow on the
Moon in lunar eclipses. But no answer to the question.

Erathosthenes (from ancient Greece) was the first to try
to measure the circumference of the Earth. His walue was
correct to within one percent or so -- much due to luck,
since he used a (by today's standards) very crude method.

Margo Schulter wrote:

On the "is not a satellite," apart from the Pluto-Charon
question, one possible reason for not including it in
the definition of "planet" (i.e. my "major planet") is
that it might be redundant: a dynamically dominant body
which "has cleared the neighborhood around its orbit"
can hardly be a "satellite" of another body sharing that
neighborhood.

Obviously. No reason to mention an impossibility.

George Dishman replied to Margo Schulter:

My point on that is that (d) is redundant because an
object cannot be a satellite and also meet criterion
(a) that it be in orbit around the Sun.

All known satellites are in orbit around the Sun. There
is nothing redundant about (d).

-- Jeff, in Minneapolis



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e-mail: pausch at stockholm dot bostream dot se
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Mike Dworetsky

(Remove "pants" spamblock to send e-mail)

"Jeff Root" wrote in message
ups.com...
....
George Dishman replied to Margo Schulter:

My point on that is that (d) is redundant because an
object cannot be a satellite and also meet criterion
(a) that it be in orbit around the Sun.

All known satellites are in orbit around the Sun. There
is nothing redundant about (d).

You see no difference in the gravitational binding
of Ceres and Moon, both are orbiting the Sun? The
sentence "The Moon orbits the Earth and the Earth-
Moon system orbits the Sun while Ceres orbits the
Sun." appears to recognise a significant difference
regarding the hierarchy of gravitational binding IMO.

George

In sci.astro George Dishman wrote:

Margo Schulter wrote:

Margo, I'm going to trim severly and 'cherry pick'
your comments. Primarily this is because I have
given some thought to your alternative suggestions
and have gelled my own views a bit. The post was
getting rather long and I have no real disagreement
with the remainder of your comments.


Hi, George, and that's fine; selective quoting can make things
more readable and save bandwidth, especially when there's mostly
agreement.

On the "is not a satellite," apart from the Pluto-Charon
question, one possible reason for not including it in
the definition of "planet" (i.e. my "major planet") is
that it might be redundant: a dynamically dominant body
which "has cleared the neighborhood around its orbit"
can hardly be a "satellite" of another body sharing that
neighborhood.

My point on that is that (d) is redundant because an
object cannot be a satellite and also meet criterion
(a) that it be in orbit around the Sun.

Yes, "directly in orbit around the primary, or 'fusor,'
or whatever we choose to call it."

OK, here's my alternative. Consider first Ceres, Pallas
and the other largest main belt obects. If say the top
tem had merged and were collecting the rubble then
they would approach being classed as a planet albeit
of very low mass. We already have a name for objects
which subsequently merge to form planets, that being
"planetesimal". As a result of our discussion, I would
suggest that Ceres etc. should be classed as remnant
planetesimals.

That's certainly one usage of "planetesimal" with
precedent. Personally the term tends to suggest for me
more specifically something maybe around 1-10 km that's
assumed to be one of the bodies serving by accretion to
form some kind of larger planet -- in my terms, it could
be a major planet (the "Big 8" in our Solar System); or
a minor planet (dwarf or smaller). However, I certainly
agree that there's a usage where any kind of minor planet
(notably an asteroid) can be called a planetesimal.

Here we get to the question of set or subset definitions
versus the names to give. To me, "planetesimal" could be
both evocative and "cosmogonically correct" for lots of
the smaller minor planets -- "here we're seeing a living
fossil, as it were, of those 1-10 km planetesimals that
were the elementary building blocks of the larger planets,
major or dwarf, etc." It suggests to me something smaller
than a "gravito-spheroidal" planet, or "spheroidal" for
short, which has likely accreted from lots of planetesimals
in the narrower sense -- whether a dwarf planet, a major
planet, or even a larger Small Solar System Body (SSSB)
or "microplanet" as I call it, say Vesta, which isn't
quite massive enough to be (gravito-)spheroidal.

However, your usage seems to have lots of company, and I
realize that from a certain dynamical view, anything other
than a major planet (IAU "planet") could be viewed as
"uncleared rubble."

If I wanted to propose some term other than "dwarf planet"
or "mesoplanet" for the spheroidal minor planets, maybe
it would be "planetoid" (carrying some science fiction
associations, as has been pointed out) or possibly
"planetino." Then people who wanted could view this
term as referring to a type of planet, and others could
argue the analogy that a neutrino is certainly not a type
of neutron.

Again, it's a matter of taste -- and it seems that
"planetesimal," like "planet," can evoke lots of distinct
semantic preferences.

If Ceres and Pallas at some time came close and
became a binary, that doesn't change their individual
nature so I would further suggest they should then be
classed as a binary planetesimal system.

An interesting question, indeed! If the barycenter (my
provincial spelling, just to let everyone know I'm aware)
is outside the radius of either body, then a binary
planetesimal system -- or in my lingo "binary dwarf planet
or mesoplanetary system" -- would indeed seem correct.

If the barycenter is within the radius of one of the bodies,
then we have the "not a satellite" question -- does this
apply to satellites of minor as well as major planets?

If I were to propose a distinction, maybe based in part on
precedent (catalogued minor planets, it seems to me, should
stay minor planets, although they might also be satellites of
another minor planet), I might argue that maybe for one
minor planet to be a satellite of another in a belt environment
is a bit more subtle of a relationship than the contrast
between major planet and satellite. However, there's a problem
there, too: with minor planet (and more specifically mesoplanet
or dwarf planet) Pluto, Nixie, for example, is in a role much
like that of a satellite of a major planet.

Do we maybe use a mass ratio test, with Ceres-Pallas a "binary"
but Pluto-Nixie a planet plus satellite?

If the barycenter is between the two bodies, of course, then I'd
say "binary" is the right answer. This is a subtle line of
questions.


Looking then at Charon, it appears that if it were not
associated with Pluto, it would also warrant being
classed as a planetesimal hence the Pluto/Charon
system should be considered a binary planetesimal
(with two moons).

Yes, and here I'm applying the barycenter test, which
suffices to yield this result.

The boundary between binary and object/satellite is
a bit more difficult but could perhaps be resolved by
the mass ratio or location of the barycentre.

Yes, or some combination -- minor planet systems (other
than the well-noted Pluto-Charon situation) are one area
maybe illustrating that taxonomic schemes, including either
yours or mine, can have certain "knotty problems."

Similarly the distinction between a satellite and a
moon is unclear but let me suggest as a minimum
that a planetesimal in orbit around a planet should be
called a moon. The terms satellite and moon relate
to orbital configuration rather than mass and shape
so it would be both a moon and a planetesimal.

Yes, a "moon" would then be what I recall that Stern/Levison
(2002) call a "planetary-mass satellite" or the like, or what
I might call a "spheroidal satellite."

By the way, I'd guess that the major/minor distinction might
not apply for a satellite, since it's "circumstantial" as
Basri would put it -- unless someone wants to estimate
whether the satellite, if a planet in a comparable orbit,
_would_ have sufficient mass to "clear its neighborhood"
(if the major planet it is orbiting weren't there!). That
would get into "what-if" cosmogonic scenarios, I guess.

I'm developing a new typology which does address extrasolar
objects, as do your new ones, so maybe I'll have more coherent
views to present soon -- _relatively_ more coherent, anyway
grin.

Objects not meeting the 'nearly round' criterion would
still be "Small Bodies" and further sub-divided into
the usual myriad classes (TNO, KBO etc.).

Sure, those "myriad classes" are still very useful!

The criteria used by the IAU would still be applicable,
hence there would be eight planets in the solar system.
The definition could be easily rationalised to allow for
extra-solar planets by replacing "the Sun" by "a star",
even with the proviso that the current definition is
limited to the solar system since AFAIK there is only
one star in it ;-)

True, unless we want to get into the "Nemesis" hypothesis of
the 1980's (as I recall) when the whole mass extinction
connection with asteroid or comet impacts (especially
Cretaceous-Tertiary at around 65 Ma or "Mega-anni ago,"
to use a geological style) led to the hypothesis of an
"invisible companion" to the Sun which every 26 My ("million
years" as duration or interval rather than distance from
present) or so was diverting Oort Cloud objects or the like
toward the inner Solar System, including Earth.

That theory wasn't found persuasive, I guess -- the "Nemesis"
part, as opposed to the impact theory of the Cretaceous-Tertiary
or "K-T" mass extinction, which now seems generally accepted as
at least one main cause of the extinction (with the discovery of
an impact crater that fits the geological timing and the scale
of the hypothesized event).

I will try to find time to draw this up as a flowchart, but
I have very limited opportunity over the next week, and
perhaps also add a test for fusion to identify stars
(including brown dwarf stars), free-floating 'planemos'
and binary planemo systems.

Those are neat charts!

I would appreciate your views on the this proposal, in
particular the criteria for distinguishing binary from
object/satellite and what should qualify a satellite to
be raised to the status of a moon.

For a major planet and a satellite, or for two minor planets
where the barycenter is between the two bodies, we might both
be comfortable with the usual tests, although tending toward
different names for some of the categories. With two minor
planets like Pluto-Nixie, object/satellite seems fine. The
harder situation might be when we have two minor planets
not too different in size with the orbit of one within the
radius of the other. Maybe we say, "Once a minor planet,
always a minor planet," and put a code like the letter "S"
for "satellite" after its minor planet number.

Thus it seems that we both find mass ratio relevant if the
barycenter test doesn't indicate a binary system.

Note that this would mean that many of the moons of
the planets would also be classified as planetesimals
so the precedence of being a satellite and other
definitions might be contentious.

How about planetisimo-satellite or the like? I would say
"spheroid satellite," or "gravitospheroid satellite" if
we want to make it explicit that the "near-roundness"
must result from self-gravity approximatinng hydrostatic
equilibrium.

The more I consider this, the more I lean toward a convention
where minor planets systems with the barycenter within the radius
of one of the bodies get classified as "planet/satellite," but
the satellite still keeps or gets its minor planet number, with
a code like "S," as I described above.

George


Most appreciatively,

Margo

In sci.astro Jeff Root wrote:
http://www.bautforum.com/showthread.php?t=34542

The questions I asked:

Were the Sun and Moon commonly referred to as "planets" in
ancient times?

When was Earth first called a "planet", or described as a body
comparable to a planet, even if it wasn't thought to wander?

When were the stars first recognized as being like the Sun?

The answers I got were basically just:

The Sun and Moon were considered planets by early Greek
astronomers and are still considered planets in astrology.

True, they are "wanderers," the literal meaning of the term,
in contrast to the "fixed stars."


Giordano Bruno suggested or asserted that that other stars
were suns with planets, like our own Sun.

I pointed out:

Lucian, a Syrian writing in Greek, in Athens, in the
second century AD, wrote a satire titled 'True History'
which described inhabitants of the Moon and the Sun.
That would seem to imply that he thought of them as
places something like the Earth.

This gets into a number of interpretations of "the plurality
of worlds." I'm going to look over again some writings of
Nicolas of Cusa (or Cusanus) from the mid-15th century,
which get into things like what we call the Cosmological
Principle that basic physical laws are the same in different
parts of the physical universe, and the idea of a universe
with "its center everywhere and its circumference nowhere."

There's also a book called _De ludo globi_ or, freely translated,
_The Game of Spheres_, which takes a heliocentric view (after
the teachings of the Pythagoreans, for example).

And of course, the fact that the ancients knew that Earth
must be a sphere because of the shape of its shadow on the
Moon in lunar eclipses. But no answer to the question.

There's a book on theories of _The Plurality of Worlds_ which
I looked at around 1986 or 1987, and this might be one good
place to start.

[further points trimmed out, but discussed in following posts]

Most appreciatively,

Margo Schulter

In sci.astro Paul Schlyter wrote:
In article . com,
Jeff Root wrote:

http://www.bautforum.com/showthread.php?t=34542

The questions I asked:

Were the Sun and Moon commonly referred to as "planets" in
ancient times?

When was Earth first called a "planet", or described as a body
comparable to a planet, even if it wasn't thought to wander?

When were the stars first recognized as being like the Sun?

Probably around Copernicus' time. One prevailing argument against
a heliocentric solar system was that the Earth's orbital motion
around the Sun ought to cause a quite visible yearly parallax
among the stars --- unless of course the stars were extremely
distant and not just a little farther away than Saturn, as was
commonly believed in that time.

"Show us the stellar parallax" was also one of the lines of
rebuttal to Galileo -- since if one followed the view expressed
by St. Roberto Bellarmino (1615) that the traditional geocentric
interpretation of certain Bible passages should be altered only
on the basis of indubitable proof, the "missing" parallax could
still be a reason for doubt. As I recall, the Church gradually
grew more and more reconciled to Galileo's perspective as the
18th century progressed, but didn't make it fully "official"
until the early 19th century, when parallax was confirmed for
one or more of the nearest stars to our Sun.

When the stars did turn out to be that vastly distant, they
must also be very bright --- like the Sun.

Herschel was one of the first trying to find the actual distance
to some stars, and he thought Sirius was some 3 light years away.

Interesting! Maybe that's about the same degree of accuracy as
Roemer's estimate of the speed of light (1676) using the moons of
Jupiter. It was an awesome leap -- I'm trying to find a worthy
adjective -- from the state of things earlier in the century,
when Galileo had tried the experiment of having two people a few
miles away show lamps to each other and try to estimate any delay
in seeing them at that distance.

[...]

Erathosthenes (from ancient Greece) was the first to try
to measure the circumference of the Earth. His walue was
correct to within one percent or so -- much due to luck,
since he used a (by today's standards) very crude method.

Yes; some of the concepts, if not the quantative methods,
may have influenced medieval thought, or at least been
paralleled in it. Thus Dante spoke of "the round threshing
floor" of Earth in the _Divine Comedy_; and later in the
14th century, Nicolas Oresme spoke of the Earth as shaped
like an "apple." Once I misquoted this as "pear," and got
someone excited about an anticipation of the concept of
oblation -- but that was my error, as I confirmed when I
checked. Oresme also suggested that it would be less
complicated to have the Earth rotating diurnally than to
have the whole stellar universe doing so.

Most appreciatively,

Margo Schulter

In sci.astro Mike Dworetsky wrote:

Herschel was one of the first trying to find the actual distance
to some stars, and he thought Sirius was some 3 light years away.


Christiaan Huygens did this experimentally about a century before Herschel,
by comparing Sirius's brightness with the Sun (an ingenious experiment,
BTW). The main reason he got a distance that was too small, was his
assumption that Sirius and the Sun had the same absolute brightness.

Ah, so this was in the same era as the Olof Roemer experiment to
approximate the speed of light.

BTW, Huygens is also known for his theory in the area of music,
where he advocated the consonant nature of integer ratios using
the prime 7 -- which reminds me of the 7:2 orbital resonances
now being described between certain bodies in our Solar System.

I can't recall Herschel's method but it may have been similar.

Thanks for a fascinating item to add to my picture of the later
17th century -- it might interesting to compare Huygens and
Herschel in more detail as to their techniques.

Most appreciatively,

Margo Schulter

In sci.astro George Dishman wrote:

[On newer version of IAU resolutions from 24 August 2006]

http://www.iau.org/fileadmin/content...n_GA26-5-6.pdf

There are two changes I have noticed. In the introduction
the wording ".. other bodies in our Solar System, except
satellites, be defined .." has been reordered as ".. other
bodies, except satellites, in our Solar System be defined
.." which is better grammatically but makes no difference.

This is a fine nuance, but maybe the second is more felicitous
because "except satellites" is in closer apposition to "other
bodies."

Commas have been added in definition (3) changing
".. objects[3] except satellites orbiting the Sun shall .."
into "objects[3], except satellites, orbiting the Sun shall .."
which now means all satelites are excluded rather than
just those orbiting the Sun which makes a lot more sense.

Yes, agreed.

Other minor points:

The quotes have been removed from "planets" in footnote 1.

Footnote 2 is still wrong, "either .. and" should of course be
"either .. or".

Well, I know how I can miss things in proofreading (maybe especially
my own writings) grin.

"Trans-Neptunian" has been capitalised in Resolution 6A.

I'd tend to favor that style, maybe to make the acronym more obvious:
Trans-Neptunian Object (TNO) -- although I'd need to check to see whether
"Object" is also capitalized.

George


Best,

Margo

In sci.astro Cardman wrote:
On 05 Sep 2006 21:22:22 GMT, Margo Schulter
wrote:

In sci.astro George Dishman wrote:

--------------------------------
Possible compromise proposal
for future consideration
--------------------------------

------------------- Solar system bodies ----------------
| | |
Hydrostatic equilibrium, Not enough mass for Satellite
self-gravity produces hydrostatic equilibrium; of another
"nearly round" shape; and not a satellite of another body [other
not a satellite of another body [other than Sun] than Sun]
body [other than Sun] | |
| | |
Planet in broad sense | |
| | |
/ \ | |
/ \ | |
/ \ | |
/ \ | |
/ \ | |
Major Dwarf Small Solar System Body Satellite
Planet Planet (e.g. most Solar System (e.g. Moon,
or (doesn't asteroids, TNOs, comets) Titan)
Planet clear
in strict orbit)
sense
(clears orbit)

I agree that this looks better. Using Major Planet instead of
Classical Planet overcomes at least two known problems. One being that
these astronomers have forgot their astronomical history.

I should point out though that the IAU have already rejected Dwarf
Planets being classed within a collective planet group, which means
that this idea you will find the hardest to gain general support for.

Hi, and I'd tend to suspect also that the issue was likely not just the
term "Classical" but the "collective planet group" concept. Thus, indeed,
people who supported 5A (and rejected 5B) might not find "major" vs.
"classical" as a distinction making a difference in their votes.

More and more, I'd favor "planet" as an even wider umbrella covering
everything customarily called a "major planet" or "minor planet," and
subdivide from there, with the IAU concepts readily applied. I'm
developing some formal definitions and more informal presentations
on how such a scheme might be applied.

Curiously, while "planet" for everything larger than a meteroid and
smaller than a brown dwarf might be a bit radical, it could also
in effect "neutralize" or at least play down the differences between
people who have proposed usages where "planet" is defined so as to
imply that either "orbit-clearing" or "hydrostatic equilibrium" is
_the_ proper criterion for "planethood."

If everything traditionally called a "major planet" or "minor planet"
qualifies for the generic sense of planet, then the arguments about
what _kind_ of planet Ceres or Pluto or 2003 UB313 is might become a bit
calmer and less charged.

Thus "major planet" would be equivalent to "planet" in 5A, and "minor
planet" to what it means now, an object in a belt population ranging
from a 100m asteroid to a spheroid minor planet like Ceres or Pluto,
etc. If a minor planet is spheroid (by self-gravitational forces), it
would also be a "dwarf planet" -- in effect, a definition identical
to 5A, but under the umbrella of "planet" along with smaller minor
planets and the major ones too.

The idea of "planet = major planet or minor planet" occurred to me in
the course of a discussion with a planetary scientist who brought this
generic definition to my attention -- easy to document in the OED and
some astronomical reference books also.


Also while I am about it I consider all three diagrams flawed when
there is no size split in the satellites section, even though the
satellite grouping is fine. What I mean is that I always hate it when
some astronomer goes that they found another moon around a gas giant
and it turns out to be a oddly shaped pebble.

Maybe this usage is so widespread that it would be hard to change,
especially if we take "moon" as a more informal term -- but there have
been proposals to say things like "a planetary-scale satellite" to indicate
that it approximates a spherical shape brought about by hydrostatic
equilibrium.

Or, as I'd say, a "spheroid satellite" or "gravitospheroid satellite" if we
want to make it explicit that the near-roundness is gravitationally
constrained (I'm not sure what the probability is of finding a
"near-round" satellite of insufficient mass for self-gravity to be the
constraining factor).

So I have always believed that there should be a split based on
hydrostatic equilibrium here as well. So that only your nearly round
objects are allowed to be called moons, where everything more minor
gets called as an orbiting asteroid instead.

Or maybe:

macrosatellite = moon (hydrostatic equilibrium)
microsatellite = insufficient mass for hydrostatic equilibrium

So maybe that is just my personal gripe. Still, once you have finished
playing "What is a planet?" then you can start on "What is a moon?".
As if they don't stop their current method then soon enough your next
moon of Saturn will be the size of a football.

Yes, this satellite question is mentioned in some of the recent literature
on defining a planet, but could well deserve a focus in its own right,
both on its own merits and for a creative change of pace.

Most appreciatively,

Margo Schulter

In sci.astro Anthony Buckland wrote:

"Margo Schulter" wrote in message
...
... A curious question: Why did the supporters of 5B choose
"classical planet" rather than "major planet"? The latter
term seems to me much more familiar, comprehensible, and
pertinent to the issue of dynamical dominance.


Possibly because some would regard only Jupiter, Saturn, Uranus and Neptune
as
"major". The inner planets might as well be grouped with Ganymede and Titan
as "minor" bodies, along with Pluto, Xena, etc.


Hello, and I agree that that is a valid and ingenious point, although I tend
to accept the traditional major/minor planet usage, and agree that dynamical
dominance seems a valid criterion to make the distinction.

Anyway, comparing the masses of Earth and Jupiter or Saturn _does_ make your
point rather nicely!

Most appreciatively,

Margo

George Dishman replied to Jeff Root

George Dishman replied to Margo Schulter:

My point on that is that (d) is redundant because an
object cannot be a satellite and also meet criterion
(a) that it be in orbit around the Sun.

All known satellites are in orbit around the Sun.
There is nothing redundant about (d).

You see no difference in the gravitational binding
of Ceres and Moon, both are orbiting the Sun? The
sentence "The Moon orbits the Earth and the Earth-
Moon system orbits the Sun while Ceres orbits the
Sun." appears to recognise a significant difference
regarding the hierarchy of gravitational binding IMO.

Yes, there is a significant difference, but Ceres, the
Earth, the Moon, and the Earth-Moon system all orbit the
Sun. Saying that a body is in orbit around the Sun does
not determine whether it is a satellite or not. On the
other hand, saying that a body orbits another body which
in turn orbits the Sun means that the first body orbits
the Sun.

-- Jeff, in Minneapolis