Space Forum / Astronomy / August 2005

Toward a rational Definition of what is a Planet
                     or
How many KBOs have diameters greater than 390 km?

Two thesis to start with:

1-The ordering propensity of the hunter/gatherer mind produced by human
brains makes the ordering of our perceptions into categories not only
legitimate but necessary. Furthermore, for convenience=B4s sake our
minds tend to order concepts into Super- and Subconcepts.

2-Definitions of scientific categories must be based exclusively on
rational criteria. Furthermore, it is not permissible if concepts which
have originated outside of the scientific realm are utilized within
scientific thinking.

This said:

In the wake of recent Trans-Neptunian Object (TNO) discoveries, there
makes itself felt an amazing resiliency not only of astrologers but
even of people within the science scene to admitting that that today we
simply don=B4t know any more how many planets there are in our solar
system.

There have been two alternative proposals up to this date which try to
save the day for those unwilling to change their mind on this issue,
one which I believe to not be justified and one that is outright
amusing. The amusing one says that planets are those which we now
consider as such and that we will never ever be willing to to change
our mind because we don=B4t want to. Full stop. I will not name anyone,
but it is a fact that even in the scientific community there exist
people who maintain such a position. The consequence of this would be
that Pluto is a planet and all TNOs further out are not planets but
simply Kuiper Belt Objects (KBOs). And an even more absurd consequence
of this would be that Pluto, the prototype of the Plutinos, could by
itself not be considered to be a Plutino -which, of course, it is.
What should be clear to everyone reading this article is that as far as
we know, Pluto is indistinguishable from other typical KBOs as far as
size, composition and orbit are concerned.

The other alternative proposal is in fact worth debating, even though I
believe that in the end it will turn out to be unjustified: In order to
not end up with an indefinite number of planets, some people argue that
there are only eight planets, and that all TNOs should be considered as
KBOs instead of as planets. This would of course imply that we would
have to deny the status of a planet to Pluto. The three arguments which
I have seen so far against considering a KBO as a planet are the
following:

One objection is based on the the composition of KBOs being very
different from that of the inner eight planets, mostly due to their
high water content. But this only seems to be so at first glance: The
composition of terrestrial planets is so radically different from that
of the Gas Giants, that there can be no reason to not accept more
bodies with a similarly radical difference in composition to both into
the family of planets.  In fact, when formerly we have been talking
about four Gas Giants in our solar system, today we differenciate
between two Gas Giants (Jupiter and Saturn) and two Ice Giants (Uranus
and Neptune). And  Ice Giants are only superficially similar to Gas
Giants, i.e. mainly composed of Hydrogen and Helium. When we look
inside of an Ice Giant, we see that its mass is predominantly composed
of Water, Amonia and Methane, and not of Hydrogen and Helium as in the
Gas Giants. Therefore, the composition of Ice Giants is actually more
similar to that of the KBOs than to that of the Gas Giants. So there
exists a composition gradient in the outer solar system which cannot
justify a strict separation between giant planets and KBOs (including
Pluto) based on their composition.

The second objection is based on the orbital characteristics of KBOs.
KBOs and especially the Scattered Kuiper Belt Objects (SKBOs) tend to
have more eliptical orbits than the established planets (exept Pluto),
and the planes of their orbits are often highly inclined with respect
to the plane of the Ecliptic. But the established planets also have
eliptical orbits, enough to almost drive Kepler into desperation before
he finally grasped it. And just like with the established planets, the
orbital motion of KBOs has the Sun at one of its foci.

As far as I can see, the inclination of the orbital plane of many KBOs
with respect to the plane of the Ecliptic is the only criterion which
clearly distinguishes the KBOs from the established planets (exept
Pluto). Nevertheless, this high inclination of the orbital plane does
not apply to all KBOs. Even though these orbital planes are not bound
to the plane of the Ecliptic, they are in fact strongly concentrated
towards it. Or in other words, they are simply less strongly bound to
the plane of the Ecliptic than are the orbital planes of the eight
inner planets. So what in effect we are dealing with here is more of a
quantitative than a truly qualitative difference between KBOs and the
eight inner planets.

The third objection is based on the size of KBOs, which is far below
that of the inner eight planets. But this also only seems to be so at
first glance: Note that the diameter of Mercury is only 3.4% that of
Jupiter, while the diameter of 2003 UB313 is about 53% that of Mercury
(Pluto has 47%). So the range in size of the inner eight planets is so
enormous, that it would not be significantly widened by the inclusion
of certain bigger KBOs. The size argument therefore does not hold.

A fourth objection which I could think of is the interesting aspect
that KBOs can collide among themselves, while the eight inner planets
cannot - or at least not without a catastrophic influence from
without the Solar System. So Pluto could one day collide with another
KBO of considerable relative size. But this argument by itself in my
opinion cannot justify the denial of planetary status to certain larger
KBOs. Rather, it just goes to show that the number of planets in our
Solar System is variable.

In summary, there can only exist two rational answers to the question
of how many planets there are in our Solar System: Either there are
eight or we don=B4t know. And even though there exist certain general
differences between KBOs and the eight inner planets, these differences
are of a more quantitative than of a true qualitative nature and thus
IMHO cannot justify the denial of planetary status to certain larger
KBOs. In my opinion, it would make much more sense in this context to
differenciate between Cis-Neptunian and Trans-Neptinian Planets (CNPs
and TNPs, respectively).

But the above says nothing about the problem of how big a KBO should
be, in order to to be included into the family of planets. Somehow the
size criterion has to play a role in the definition of a planet,
because otherwise we might end up having to call every zodiacal dust
particle a planet. But "big" and "small" tend to be hard to
define in a scientific, i.e. rational manner. Arbitrary border values
like "everything bigger than 1000 kilometers" - for continental
Europeans or "everything bigger than 1000 miles" - for the
Anglosaxon world should be rejected for what they are (see second
thesis above).

As of right now, I can see only one rational criterion for the lower
size limit of a planet, and this criterion is not an explicit but an
implicit one: Its shape. Spherical shape is one of the attributes we
associate with planets, and in order to aquire a spherical shape a
natural body needs to have a certain minimum mass and thus also a
corresponding minimum diameter. So the definition of a planet which I
would like to propose here is:

"Any spherical object of natural origin which is not a star, but
which has a star or the gravitational center of a multiple star system
at one of the foci of its orbital motion."
This definition of a planet is not too different from those definitions
already in existence, it is just more precise than they are since the
existing definitions have turned out to be insufficient in the light of
recent discoveries.

So what is the minimum diameter which a KBO/TNO needs in order to
become spherical? Unfortunately, I don=B4t know.

The smallest natural and spherical solar system body I know is Mimas,
the sixth moon of Saturn, with a diameter of 8.0% that of Mercury (392
km). So the diameter of Mimas compares 2.4 times more favourable to
that of Mercury, than the diameter of Mercury compares to that of
Jupiter. If we would define a lower size limit of 390 km for TNOs in
order to be called planets, then we would probably soon end up with
hundreds of planets in our Solar System. But since as it is we already
do not know how many planets there are, this would also not make a
significant difference. The above arguments show that the dam assuring
a limited number of planets has already broken. So it does not hurt to
extend their permissible size range a bit more, as long as this helps
us to end up with a rational definition for what is a planet.

Furthermore, whether TNOs with a diameter around the lower size limit
of a (spherical) planet as defined above would be round or not could
ultimately only be determined by measurement. But the direct
measurement of the shape of a TNO is exeedingly difficult. As far as I
can see, this could only be done by timing star occultations by TNOs.
And timing star occultations requires very precise knowledge of the
orbital path of the occulting object. So here we have another
difficulty in determining what is a planet and what not. But as I have
already implied above: One difficulty more or less doesn=B4t really
matter when it comes to the determination of the number of planets in
our Solar System.

So what I really would like to know at this point is the (approximate)
lower size limit for a spherical object with the composition of a KBO.
Only when I have this information will I be capable of estimating (*)
how large is the approximate number of sperical bodies in our Solar
System which have the Sun at one of the foci of their orbital motion.

Peter Holm

* by estimating the diameter from the assumed albedo and the absolute
brightness.