On 11 Sep 2006 00:47:18 GMT, Margo Schulter
wrote:
In sci.astro Cardman wrote:
On 05 Sep 2006 21:22:22 GMT, Margo Schulter
wrote:
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.
The use of the word "Classical" was clearly unhelpful in 5B, but if
the removal of this word would have changed support for 5B remains to
be seen. I would have assumed that these IAU members would have well
considered the implications for rejecting 5B with or without this
word.
My honest view would be that this was a meeting to kick out Pluto and
leave everything else exactly the same.
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.
I see the major fault in the IAU definition of "planet" is in their
concept that a Dwarf Planet is not a planet. This is not just due to
the contradiction in terms, but that I am sure that the vast majority
of people would have been happy with Pluto being a Dwarf Planet had it
still been a planet.
The mistake that a lot of astronomers are making is in saying that
Pluto should not be a planet when this 2306km diameter object cannot
compare to the likes of Mars or Earth. The mistake in this logic is
that Mars and Earth hardly compare to Jupiter and Saturn either. So if
you are running a singular planet definition then only Jupiter,
Saturn, Uranus and Neptune would be planets, where all the smaller
objects would be given a different suitable name.
And so once you entertain the concept that the "planet" word is
currently defined to include two very different classes of objects can
you see how this could be extended to three classes. And there should
be no comparison between members of these three sub-groups when they
are after all vastly different.
The second excuse I hear is that they would be upset if there was like
47 planets in our solar system. This I find odd when I was under the
assumption that astronomers were scientists. So the number will be the
number there are and not the number that you want there to be.
This usually trails on into what you would teach school children. That
should be obvious enough, when you teach them about the 4 inner
planets, the 4 gas giants, and then the teacher points out how many
more dwarf planets there are with picking out a few choice examples.
My Very Educated Mother Just Served Us Nachos, and a whole load more!
It is now a sad day to see that our solar system, in the eyes of
school children, has not got 1/9th smaller. I am sure that our solar
system must go nearly half way to the next star and it seems a
worthwhile classroom exorcise for the teacher to point out how BIG
that actually is followed by "and this little section is what we have
explored to date".
The next problem I see with this IAU planet definition is that it is
very warm planet biased, meaning that it only covers the area of our
solar system that was subject to a collapse in the gas cloud. The
evidence for that is obviously in the nice circular motions of the
inner 8 planets. So I am in fact doubtful that Earth has cleared
anything much when this "clearing" seems more a product of our local
star and our closest gas giant neighbour. The trophy in how badly our
planet clears local objects can be clearly seen in that planet-sized
sphere above our heads.
Also I suspect that it is true to say that had even Earth been located
out at about 150 AU, or better yet 120 to 180 AU, then even Earth
would have been unlikely to have cleared the region. The reason for
this is that you no longer have your easy record shaped disk to vacuum
up, when now you work in the 3D. So has anyone actually calculated the
odds of two objects going head to head out in this vastness? And so
this act of clearing could simply be an on-going event due to simply
there not being enough time since our solar system began.
So I am quite looking forwards to them finding something nice and big
out there and seeing them try to label this a "dwarf".
I would in fact say that the whole clearing concept is flawed and
should be removed as such. Or at minimum this is clearly an attribute
of the gas giants, and maybe smaller planets, and this attribute
should be contained to the definition of these two groups.
I can see why they want to strip Pluto of planethood in the belief
that these objects are the left-overs from the creation of the solar
system. I even read a lot of astronomy books saying exactly that, but
time did not stop and there are planets formed out there as well.
So one side of these astronomers needs to accept that the likes of
Pluto and "Xena" are indeed planets, then the other half need to
accept that these are dwarf planets are not as big/important as the
larger planet classes.
I think that is where you will find your agreement.
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."
Astronomers are mostly split into two groups. Both groups seem rather
biased and hot-headed in defending their position. You can add a third
group into this containing the majority who wisely avoid the entire
debate.
The 300 plus astronomers who next year will hold their own meeting to
redefine "planet", in the way that they are happy with, is a clear
indication that both side desire to remain in complete opposition.
This seems fair proof that the third group was right after all.
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.
True. Neither side will easily give ground though.
Also having real planets out there waiting to be found will create
increased interest in the exploration of this region. The IAU already
know that they have the public interest, so why not milk it for all
its worth?
That public can help get them increased budget to explore this region
after all. And we could do with a few more fly-by probes, or better
yet ones that can survive a high speed impact.
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.
The IAU desires anything 800km or above to be ranked for
dwarf-planethood. Anything less is simply a solar system object.
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 "minor planets" term has been killed off for being obsolete and
has now been replaced with "solar system objects".
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.
That sounds exactly like the old system, which explains why the minor
planet center keeps track of all these lesser objects. They obviously
have had to do some recent renaming as well.
You are overlooking that being round has importance over non-round
objects in the sense that the two should be independent groups.
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.
Yes, I prefer a three level system, but if the IAU wants to go as low
as 800km, and possibly lower, then I won't argue with them.
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).
You should check out 2003 EL61. This is a dwarf planet sized object
that got deformed by its rapid rotation.
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.
I can only hope.
Cardman
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