Margo Schulter wrote:
In sci.astro George Dishman wrote:

Margo Schulter wrote:

more trimmed

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.

Having looked into it a little more, I think I agree with
your criticism. From Wiki, the term "protoplanet"
would be more fitting, though the "proto" prefix is
perhaps not optimum.

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."

"Planetesimal" has the advantage of being clearly a
single word. "Protoplanet" could be written "proto-planet"
implying a subtype the definition of planet again leaving
the ambiguity unresolved. That is unfortunate as the term
is more accurate.

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.

On that basis, they could also argue that an asteroid is
a type of star ;-)

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

I doubt there can be any existing term that doesn't
and really this whole exercise is 'damage limitation'.

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.

My concerns with the barycentre argument are first
that it sets a size limit that depends on the ratio of
separation to diameter. A simple mass ratio limit,
while arbitrary, could make the distinction more
consistent. Secondly, it requires some knowledge
of the density (to convert mass to radius) which
will be a problem for extra-solar objects where size
will not be directly measurable other than under
fortuitous circumstances (e.g. transit detections).

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?

Ah, well spotted, I had missed that. It's going to
mess up my flowcharts :-( Yes, the test needs to
apply at all levels so you can get a binary asteroid
just as we already have a few asteroids with satellites.
I probably need to include "not a member of a binary"
as well as "not a satellite".

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?

For the reasons given above, I prefer the mass ratio
test but whichever we use I have no doubt that Nix
and Hydra should be seen as satellites (or moons?).

The question then is whether there are two satellites
of the Pluto/Charon binary system or are there three
moons of Pluto?

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.

I think mass ratio versus barycentre is one question
and moon versus satellite is another.

snip - agreed

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."

That is consistent since we now have a definition
of "planetary mass" that is based on "nearly round".
It also mirrors my own view that a "moon" should be
a substantially larger object than the generic term
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 think the question is "not applicable", an object too
small to achieve a round shape is almost certainly
incapable of clearing its orbit.

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.

Excellent, I look forward to seing it.

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 believe there was another paper in the last year or so,
it hasn't entirely gone away yet.

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 am preparing a page of notes to explain the
flowchart but it's not in a state where I can upload
yet. For reference for anyone following previous
posts, my flowchart is here (but needs some work
to incorporate the changes discussed above):

http://www.georgedishman.f2s.com/astronomy/GAD.png

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.

I think we need to keep the names simple and understandable
for the wider public appreciation while the criterion should be
explicit, unabiguous, scientific and measurable. A press
release saying "New gravitospheroidal satellites discovered!"
just doesn't have the impact of "New moon discovered!"

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.

I would say "protoplanet/moon" if both are "nearly round"
or "protoplanet/satellite" if the smaller is a rocky peanut
or "asteroid/satellite" if both are irregular.

Existing small body numbering should be undisturbed by
any of this, though perhaps a few new numbers could be
allocated for consistency.

George