Daggaz wrote:
How about we forget about trying to keep Pluto as a planet. Pluto has
significantly lower mass than at least 6 of the solar systems moons and is
part of a large population of bodies (the Kuiper Belt) that includes objects
of similar size to Pluto.


I must preface this post by saying that I have no more than a lay
interest in, or understanding of, planetary astronomy (although it has
been a longstanding lay interest). So please feel free to take my
comments with a grain of salt.

From my position of relative ignorance (or perhaps because of it), I'm
having difficulty getting my head around the reasons for the strong
debate and lack of consensus on a planetary definition.

Based on my limited understanding, there would seem to be little issue
with the upper boundary for planetary size, with this being covered by
the IAU statement of 2001, as amended in 2003:

http://www.dtm.ciw.edu/boss/IAU/div3...efinition.html

Similarly, the distinctions within the planetary realm, between gas
giant and ice giant, and ice giant and the largest rocky/terrestial
bodies (based on composition) don't seem to be contested. Without
touching on the subject of "planet" it would seem appropriate to me if
this categorisation by composition was extended to include "ice dwarfs"
to differentiate between smaller bodies that are primarily made of ices
rather than rock.

I don't understand the reasons for considering trans-Neptunian as a
descriptor for Kuiper Belt or Oort Cloud bodies, or cis-Jovian for
terrestial planets, as some are advocating, as such definitions
obviously can't be applied to bodies outside our solar system.

In comparing those known bodies orbiting the Sun that are smaller than
Neptune to arrive at a lower boundary for a planetary definition, there
seems (from my lay position) to be only one scientifically solid point
of distinction, and that is at the point where bodies have sufficient
mass to have a differentiated interior and to tend to towards a sphere
(or whatever other oblated or extruded variation its rate of spin
dictates). While it may be difficult to set a prescriptive lower limit,
for mass or size for this point of sphericity, I expect a nominal value
could be derived (as I assume was the case for the 13 Jupiter masses
upper boundary).

Obviously, that would leave us with rather a lot of "planets", and with
a growing number of bodies that, due to our limited knowledge of them,
would be "potentially" planets. Given that we're dealing with continuum
in terms of sizes/masses, I guess we would also have transitional
bodies that are somewhat planet-like but of sub-planet mass. I don't
see the problem with this. When dealing with a continuum, why would
anyone want to pretend that they were not?

In addition to using say mass to determine planetary status, and
applying adjectives based on composition (ice giant, terrestial, ice
dwarf, etc.), if there was a need to further distinguish between bodies
that have ejected all other bodies from their orbit, or forced them
into Lagrange points, and those that haven't, the latter could I
suppose be referred to as "Asteroidal". So Pluto could be an Asteroidal
Ice Dwarf planet, and Ceres an Asteroidal Terrestial planet, or
whatever.

This makes far more sense to me than relying on an historical approach
that will someday seem merely silly or naive, or on dynamical
properties that have no context outside our solar system.

Or am I just missing the whole point?

Cheers,

Steve