Planetary classification

Some time ago I came across this page
(http://arcbuilder.home.bresnan.net/PCLMaster.html) (I don't remember
how) and was quite interested. Though unfortunately we have only our
own solar system to study right now, contemplating the possibilities
is certainly worthwhile.

I had once imagined making such a list myself but was never confident
of my knowledge.

As I was reading it, I found many apparent inaccuracies - some I'm
sure
of, and others I'm not quite so sure of. I have written up my
criticisms and comments in a rather long list below. I hope you might
read it - I have done this kind of thing before, and you shouldn't
take
offence - after all I wouldn't even have bothered if I considered it
totally worthless.

You might want to take a look at my recent post 'Liquid ammonia
in space'
(http://groups.google.com/group/sci.a...thread/thread/
27a0eb982fef9030/76a1f9a71e472f5a#76a1f9a71e472f5a)
as some of my comments below pertain to it.

Well, here's the list. To read it you need a copy of the PCL open,
as the list is indexed by order in that.

If any of my points are clearly wrong, please tell me, and provide
literature references if
possible - I'd like to know more about it, in any case.

This post is adapted from an e-mail I sent to the author of the PCL,
John Dollan, two weeks ago, to which he never replied. The following
list is what I showed him.

--------

Small Body Group: Is 0.0001 Earth masses correct? An icy body can
sustain hydrostatic equilibrium considerably smaller than that.

Asteroidal Class: The 50% thresholds are arbitrary. Is there any
justification for them?

Gelidacous Type: These need to be defined as closer to the sun than
Cometary Class objects.

Aggregate Type: Probably volatile-rich bodies should be excluded here.

Cometary Class: What does 'can be' mean?

Passive Type: These need to be defined as farther from the sun than
Asteroidal Class objects.

ActiveBrevis Subtype: Two words. Same for next two.

Dwarf Terrestrial Group: Again the same comment about the 0.0001 Earth
threshold. The masses for this class are allowed to overlap the
Terrestrial class, so maybe there should be overlap between this and
the Small Body class. Also, all (not some) of these sustain
hydrostatic
equilibrium, and geological activity need not be due to 'tidal
forces',
e.g. Mars.

Protothermic Class: 'In the process of forming' is vague. Also, the
smaller worlds in this class will never have a H2/He atmosphere, or be
molten in the silicate phase.

ProtoFerrinian Type: Actually, high-metallicity stars will not give
worlds with a higher proportion of iron, since O and Si are increased
along with Fe and Ni; I believe the Fe/Si ratio is nearly the solar
value (0.8 by number) everywhere. However, worlds richer in metal are
obtained by condensation at high temperature and pressure e.g.
Mercury;
or perhaps by large impacts dispersing the silicates.

ProtoCarbonic Type: 'High-mass', not 'high-massed'.

Selenian Subtype: There's no reason to assume no permanent atmosphere
here, especially for those of larger mass or cooler than Earth's moon.

Cerean Subtype: These must necessarily be fairly cold; Ceres itself is
about the warmest possible to have accreted a high proportion of
volatiles.

Carbonian Subtype: The name is inconsistent with 'ProtoCarbonic'
above.
One of them should change. 'Hydrocarbon compounds' should be deleted
as
they are neither a major constituent, nor restricted to carbon worlds.

Phaethonic Type: Only a small mass of crust could melt during one
stellar passage. To have such heating cause vulcanism in the normal
sense would require very special conditions.

Sethian Type: Hydrocarbons will be decomposed by vulcanism into
graphite + methane or hydrogen. How are they renewed?

Erisian Type: The name is fine, but Eris (2003 UB313) is not such a
body any more than Pluto is.

GeoTidal Class: These will almost certainly be moons of giant planets.

Hephaestian Type: There is no way to get a molten surface from tidal
heating.

EoPromethean Subtype: 'between 800 million years and 3 billion years'
etc. - There is no reason to assume that all planets will develope on
the same time scale as Earth's. And even if the existence of life is
inevitable (doubtful, I say) it won't develope on the same time scale
either. These figures should be omitted, and the same throughout your
classification.

ThioPromethean Subtype: 'Thio' refers to sulphur, I believe, so is
inaccurate here. Also methane does not mix with water! This mistake is
repeated several times. In fact, the ammonia/water eutectic is lower
than any other - -100C/-148F - though the highest concentration to be
expected on an oxidised planet is ~15% (and you can't enrich that much
by freezing without icing over the world), which freezes at -23C/-6F.

Lokian Type: Molten _what_? The surface of a carbon planet will be
graphite!

Idunnian Type: Unfortunately, ammonia has severe stability problems in
contact with graphite or carbides - if it is ever subducted, it will
be
irreversibly decomposed. This applies to all the ammonia examples
here.

Burian Type: 'Liquid water is not possible' - since ammonia/water
mixtures freeze at lower temperatures than pure ammonia, this doesn't
make sense. Any liquid ammonia will dissolve all water ice that it's
in
contact with.

Atlan Type: Again, water and methane don't mix. Remove the statement
about water mixed with methane.

Enceladusian Subtype: 'Enceladian' would be better.

GeoCyclic Class: I believe the cyclic nature of such worlds is
unproven. Also, if it is possible, I don't see why it would be
restricted to low-mass worlds only.

Arean Type: What does 'be maintained for billions of years' mean?
Probably you mean the cycle can be maintained, but the words could
also
be read to mean that one cycle might occupy billions of years. Also
note that the cycle requires solar input within a fairly narrow range
-
significantly colder than Mars there could never be liquid water, much
warmer and there would be continuously (until it's all lost to
photolysis).

MesoArean Subtype: The 'rise to' and 'fall from' the height must be
different, or there wouldn't be a cycle.

Utgardian Type: Again, ammonia is no more stable than water on carbon
planets.

EuUtgardian Subtype: Methane should be lost to photolysis fairly
quickly on small planets that are too warm for liquid methane, as it
has no significant surface or subsurface reservoirs.

Titanian Type: Low solar input does not directly reduce the greenhouse
effect. The reason Titan has no significant greenhouse effect is that
it is compensated for by the sunlight-blocking hydrocarbon haze; in
fact, Titan has a net negative greenhouse effect. That situation
should
be essentially the same on any liquid-methane world. (The early Earth
was not cooled by having methane because it also had water vapor, a
more effective greenhouse gas, which also destroyed most of the haze.)

Terrestrial Group: The 'and/or' should probably be 'and', unless you
want Mars to be in this group. Geological activity is not always
necessary for an atmosphere; it is possible to imagine a stable
nitrogen or oxygen atmosphere on a dry, tectonically dead world.

Vesperian Type: 'K-type' should be capitalised like 'M-type'.

BathyVesperian Type: Why can't there be a global ocean without such
extreme temperatures? Also, it's unlikely that any world that did have
one would have very large temperature differences, so the '250
degrees'
comment should be removed - it's true that there could be a 'dead
zone', however. (And of course the entire night side will be pretty
much dead!)

ChloriVesperian Subtype: There does NOT have to be free HCl (which is
incompatible with silicates due to its acidity). Chlorine-releasing
photosynthesis is perfectly possible in oceans like the Earth's. The
reason it never evolved is likely because it would be very difficult
for an organism to produce chlorine without killing itself, as
chlorine
is highly destructive to all organic matter.

Telluric Class: It should be stated is the major atmospheric gas is
almost certainly CO2.

Cytherean Type: Venus's conditions have more to do with its high solar
input than any unusual geologic conditions; I don't know if you meant
to say that.

Asimovian Type: Again, water and methane don't mix! They can't exist
at
the same temperature, either. Also, liquid methane is not restricted
to
'dimmer M-type dwarf stars'; remove that comment.

Tectonic Class: Are you sure that all such planets will have oceanic
crust?

Gaian Type: There doesn't appear to be any reason stars brighter than
F8 couldn't have such planets, though they don't last as long. Also,
given that the event that created Earth's moon was extremely unlikely,
most Gaian worlds won't have any large moons.

EoGaian Subtype: You're correct that the presence for both CO2 and CH4
in large amounts probably requires life. However, the photochemical
haze won't be that thick if H2O is present, as it generates hydroxyl
radicals that react with hydrocarbons.

MesoGaian Subtype: I reiterate that you can't blindly use the time
scale of Earth's evolution for all similar worlds.

EuGaian Subtype: 'Byproduct', not 'bi-product'.

GaianXeric Subdivision: A high greenhouse effect makes the planet
warmer _at a given solar input_. That's no reason to think that these
worlds would be warmer _on average_ than other Gaian subtypes, since
they must have mean temperatures within a certain range to have stable
liquid water, anyway.

BathyGaian Subtype: This class of planet is impossible. It would
likely
evolve into a runaway greenhouse with surface temperatures above the
critical point. If solar input wasn't sufficient for that, then CO2 in
the atmosphere would be rapidly depleted by reaction of dissolved CO2
with silicates.

ChloriticGaian Subtype: Should probably be 'chloridic', not
'chloritic'. Presumably these have free chlorine in their atmosphere,
like the ChloriVesperian above. Why would these preferably be found
around warmer stars?

GaianGelidian Subtype: Actually, if the continents are high enough,
they will accumulate ice despite no liquid water source.

PostGelidian Subtype: I'm not sure what you intended here. Stars
become
brighter as they evolve, which warms planets around them. Therefore,
the later stages of this would only be found on rather water-poor
worlds. Those with as much water as Earth would instead experience a
runaway greenhouse. It is also possible for a planet to lose its water
before the star has evolved much. In this case it would skip the
'humid
greenhouse' phase and just become desert-like. In either case, the
desert-like phase might actually be stable for a long time as further
water loss could be balanced by water supplied through vulcanism.

Amunian Type: Again, carbon and carbides are not compatible with
ammonia, and there would be no solid water on an ammonia world.

BathyAmunian Subtype: Carbon monoxide?? Where would that come from?
Also, no 'sulfuric gases' are likely on an ammonia world; H2S
dissolves, S and SO2 react with ammonia.

Pelagic Type: These worlds most likely formed in the outer regions of
the system (which is why they got so much water) and then migrated
inward. Indeed, if the inward migration went too far, the water would
vaporise and one would have a gas giant-type world with no apparent
surface.

EuPelagic Subtype: What are the 'several ocean-related factors'?
Oxygen
content of the atmosphere should be expressed as partial pressure, not
as %.

PelagicGelidian Subtype: NO STARS dim over time! Also, oxygen and
nitrogen would be expected to be produced only if life still survived.

Helian Group: The question is not whether the planets can hold onto
helium, but whether they would have gotten it in the first place.
Radioactive decay can only produce a few tens of millibars at most, so
any helium-dominated atmosphere would have to be primordial, and in
fact, any planet massive enough to accrete helium would also get
larger
amounts of hydrogen, becoming a gas giant. This class should not
exist.

Jovian Group: The core can actually be heavier than 'several' Earth
masses. There is one extrasolar planet estimated to have a core of 70
Earth masses.

Sokarian Type: To have _no_ upper cloud layers, silicates and metals
must be completely vaporised, which requires a minimum temperature
about 2,750 K (4,500 F). This seems quite unlikely, though perhaps
just
possible around hot enough stars.

MesoJovian Class, SuperJovian Class: You didn't put any class for
intermediate temperatures here. Did you forget?

Chthonian Class: It's questionable that these should be classified as
Jovian. I think they would be practically identical to hot terrestrial
planets.

--------

Andrew Usher

On Mar 20, 8:31 am, Andrew Usher wrote:
Some time ago I came across this page
(http://arcbuilder.home.bresnan.net/PCLMaster.html) (I don't remember
how) and was quite interested. Though unfortunately we have only our
own solar system to study right now, contemplating the possibilities
is certainly worthwhile.

I had once imagined making such a list myself but was never confident
of my knowledge.

As I was reading it, I found many apparent inaccuracies - some I'm
sure
of, and others I'm not quite so sure of. I have written up my
criticisms and comments in a rather long list below. I hope you might
read it - I have done this kind of thing before, and you shouldn't
take
offence - after all I wouldn't even have bothered if I considered it
totally worthless.

You might want to take a look at my recent post 'Liquid ammonia
in space'
(http://groups.google.com/group/sci.a...thread/thread/
27a0eb982fef9030/76a1f9a71e472f5a#76a1f9a71e472f5a)
as some of my comments below pertain to it.

Well, here's the list. To read it you need a copy of the PCL open,
as the list is indexed by order in that.

If any of my points are clearly wrong, please tell me, and provide
literature references if
possible - I'd like to know more about it, in any case.

This post is adapted from an e-mail I sent to the author of the PCL,
John Dollan, two weeks ago, to which he never replied. The following
list is what I showed him.

The referenced scheme (and your objections) appear to concentrate on
many variables but leave others out; frinst a body's distance from its
primary, how long it's been there, and the characteristics of the
primary. I mean, take Venus and snuggle it up close to Sol and you'll
get something more Mercury-like, but toss it out near Neptune and its
atmosphere will become just another layer of weird ices. Then there're
comets (those that do close approaches to their primaries)- they're
(broadly speaking) ice/rock aggregates until they get warm, then they
grow atmospheres and maybe even (small) bodies of liquid volatiles.
Well, are they fundamentally different from comets that stay way out
there? How about comets that come near and get trapped, then "evolve"
into "asteroid swarms" as they lose their volatiles? Do they deserve a
different classification for each stage in their evolution? Should
classifications be limited to stable states of planets etc.? What does
"stable" mean in this context considering that the system primary can
change, thus altering the characteristics of orbiting bodies?


Mark L. Fergerson