CCNet SPECIAL: PLUTO - PLANETESIMAL, PLANET, OR
SIMPLY A DOG?
(1) PLUTO - A PLANETESIMAL
Jacqueline Mitton <firstname.lastname@example.org>
(2) AS FAR AS I'M CONCERNED, PLUTO IS A PLANET
Dave Tholen <tholen@IfA.Hawaii.Edu>
(3) PLUTO'S A DOG
Bob Kobres <email@example.com>
(1) PLUTO - A PLANETESIMAL
From Jacqueline Mitton <firstname.lastname@example.org>
David Hughes says he is inclined to 'put his money' on Pluto
being an escaped satellite. However, it is now generally regarded as
virtually impossible, on dynamical grounds, that Pluto was ever a
satellite of Neptune. There is no evidence that any other large planet
ever existed in this part of the solar system. Far more plausible on the
basis of current knowledge is the idea that Triton is a Pluto-like object
large icy planetesimal) that was captured by Neptune when much larger
of such objects were present in the trans-Neptunian region, and that
Pluto has survived where it is because of its dynamical 'niche' in a 2:3
resonance with Neptune. It is an astonishing fact that Pluto can
approach nearer to Uranus than it ever gets to Neptune. Thus it is hard
to see how they could ever have belonged to the same system.
May I refer interested readers to 'Pluto and Charon: Ice worlds on the
ragged edge of the solar system' by Alan Stern and Jacqueline Mitton
(Wiley 1998), particularly chapters 5 and 6, which elaborate on this
point in a non-technical way.
Best wishes - Jacqueline Mitton
8A Canterbury Close
Cambridge CB4 3QQ, UK
Phone +44 (0)1223 564914 Fax +44 (0)1223 572892
(2) AS FAR AS I'M CONCERNED, PLUTO IS A PLANET
From Dave Tholen <tholen@IfA.Hawaii.Edu>
I want to comment on two aspects of David Hughes' item regarding Pluto's
> (b) a planet is more massive than a minor planet.
> Here again one tends to be somewhat arbitrary but I think a dividing
> line at about 1/25 the mass of Earth is about right.
I agree that the choice is somewhat arbitrary. However, I tend to look
at the distribution of masses and draw dividing lines based on gaps in
that distibution, rather than using a fixed number. The situation is
quite similar to the assigning of grades. If I have students with total
point scores of 977, 903, 899, 825, 807, 791, and so on, I'm not going
to draw the dividing line between letter grades at 900! Would you like to
be the student with the 899 and the grade lower than the student with
903? The problem with Pluto is that it falls roughly in the middle of
a rather large gap in the mass distribution. I've always had the most
difficulty assigning a grade to a student whose point total falls in
the middle of a large gap.
> We all know that Earth, Jupiter, Saturn etc are planets, because that
> is the term we have always applied to them.
So in the case of Pluto, this is exactly what I've used to decide a
difficult issue. Pluto is a planet because that is the term we have
always applied to it. I'm letting history decide the matter, much as
I might decide the isolated student's grade by comparing the point
total to those of students in past terms.
> As far as I am concerned Pluto is not a planet, simply because it is
> not similar enough to the objects like Earth, Jupiter, Saturn
And as far as I am concerned, Pluto is a planet, simply because it is
not similar enough to the objects like Ceres and 1992 QB1. Does any
asteroid have an atmosphere? Maybe Chiron, but it is really a comet and
would not be in the asteroid catalog if it had been discovered in 1988.
Does any asteroid have a satellite? Ida does, and there is
circumstantial evidence for a few others, but Dactyl comes nowhere close to
the size of its primary (indeed, the Earth's Moon comes closest to
Pluto-Charon in that regard). And when it comes to mass and diameter,
we simply don't know enough about the other trans-Neptunian objects. Yes,
some have size estimates of more than 800 km, but their albedos were
assumed to be low, thus they have a distinctly different surface from
Pluto's high albedo one. And if we assume a comparable albedo, then the
trans-Neptunians shrink to the point that Pluto stands out by virtue of
(3) PLUTO'S A DOG
From Bob Kobres <email@example.com>
If the generic 'Trans Neptunian Objects' (TNOs) will not do to describe
this type of solar-system-citizen...Why not ROVERS or maybe SEADOGS?
;^) On the other hand we might want to revive some older terminology so
as to make it all a bit more mysterious to an already often confused
Personally I think that, excluding the eight true BIBBUs (see below),
components of the solar system need more descriptive names like 'Neptune
Orbit Crossing Object' (NOCO) or 'Just Outside Earth Orbit Object'
(JOEOO) etc. The Greco-Roman-Egyptian god naming scheme is not directly
informative, nor is it expandable. Besides, how often are people going
to pronounce Aten correctly if they have only read about this type
object in a paper or book? Then there is Chiron and Charon--oh well.
Although it does have an interesting history, figurative sky terminology
can get pretty twisted: Like when I noticed a Comet from Mercury
passing in front of the Subaru, which I had been following that night.
Anyway--a reminder of our ancestors' view, which may have been a bit
different than ours today:
A number of cultures retained stories of impact induced winter. Most
telling of such lore this author has read are these amazingly
informative tales of the Yakuts: [note that [CH] is actually 'c' with
the diacritic 'v' bobk]
[CH]OLBON . . . is said to be "the daughter of the Devil and to have had
a tail in the early days". If it approaches the earth, it means
destruction, storm and frost, even in the summer; . . .
[CH]OLBON, the daughter of the Devil is a beautiful girl ... she is the
bride and the sweetheart of Satan's son URGEL (Pleiades). When these two
stars come close to one another, it is a bad omen; their eager
quivering, their discontinuous panting cause great disasters: storms,
blizzards, gales. When they unite, fathom deep snow will fall even in
the summer, and all living beings, men, animals and trees will perish .
Both folk memories were recorded by ethnographer V.L. Serosevsky, the
first in 1877, the next in 1885. The Yakuts identified Venus as colbon;
however, as a later student of this culture, G.V. Ksenofontov, observed:
The Yakuts have two words for the "star": SULUS and [CH]OLBON. The first
means simply "star", the second refers to stars that change their place
in the sky, sometimes appearing and disappearing. Nowadays, however, it
no longer--or very seldom--refers to other planets than Venus and has
almost become its name. Yet, as we have seen, in legends also other
[CH]OLBONS (i.e. planets) are mentioned.
What is remarkable about these particular tales is the conjunction of
several pieces of information. From these lines we gather that a comet
([CH]OLBON with a tail) came close enough to influence weather on
Earth--i.e. deadly storms, frost and deep snow in summer. Also, we are
told that this is most likely to occur if the comet appears close to the
Pleiades. In short, these legends accurately describe what can now be
inferred from astronomical data on comet Encke and the ring of debris
its progenitor strew about the Sun.
As the above example suggests, contemporary researchers need to be wary
of assuming our predecessors' folk memories of astral events relate to
bodies familiar to our time. There is considerable reason to suspect
that the majority of the planets namesakes were comets--probably of the
A conventional view comes from W.M. O'Neil's TIME AND THE CALENDARS
The word planet comes from the Greek PLANETES, the wanderers; these
seven celestial bodies moved among the fixed stars. The Babylonians had
a more picturesque name, BIBBU, the wild sheep, as these bodies broke
through the fixed formation in which the tame sheep crossed the sky.
To call into question Greek continuity of planet identity I refer to
Leonardo Taran's work on the "Pseudo-Platonic" Epinomis (1975) where in
commentary on lines 986 A 8-987 D 2 Taran states:
Having previously proved to his own satisfaction that all the heavenly
bodies are the greatest divine living beings and having pointed out that
they are not yet honored as gods, the author explains who these visible
gods are and why they are not honored in Greece. They are the eight
interrelated sidereal revolutions and the heavenly bodies which travel
on them, for they are all gods of the same kind. And the contemplation
of this divine cosmic order is what will make a man happy both in this
life and in the next. But the lack of this wisdom in Greece is due to
ignorance of the true paths of the planets, a knowledge which comes from
the Orient and which must be incorporated into our laws. That the
knowledge of the planets comes from the Orient is to be seen in the very
fact that the planets lack proper names and are called after the
(traditional) gods, for this kind of appellation is due to the
barbarians who first discovered the planets.
The Epinomis, which dates from around the 4th century BCE, is the
earliest extant record of Greek planet names; each is given as "the star
of": Cronos, Zeus, Aphrodite, etc. Clearly the planets did not inspire
the earlier stories which championed these gods. The mythology
associated with these names certainly better describes the break-up of a
comet with an orbit that crossed Earth's path than the monotonous
behavior of planets.
As for the BIBBUS, as well as the Oriental influence alluded to above I
call attention to J.K. Bjorkman's article in METEORITICS (1973) which
deals with much earlier texts:
We move now to a discussion of a word which probably refers to comets,
BIBBU. . . BIBBU has a variety of astromantic and non-astromantic
meanings. There is a lengthy omen text, the 56th tablet of Enuma Anu
Enlil, which deals with various features of the BIBBU, and some of these
seem to describe comets. For example:
If a BIBBU continues one day, two days in the sky and does not
If three or four BIBBUS rise one after the other at sunrise
The latter text might refer to a comet which has broken up into three or
four comets . . . .
There are many more references to BIBBU, but in them the translations
"unspecified planet" or "meteor" could be proposed.
Confusion of planet terminology is also evident further to the east as
can be demonstrated by James Legge's translation of a passage concerning
the emperor Kwei in the ANNALS OF THE BAMBOO BOOKS:
In his 10th year, the five planets went out of their courses. In the
night, stars fell like rain. The earth shook. The E and Loh became dry.
Still hoping we learn to avoid future bad-heir-days before one leads to
And being a bit silly.
University of Georgia
Athens, GA 30602
CCCMENU CCC for 1999