Classification of Worlds System In the Interplanetary
Calendar System (ICS) there is a Classification of Worlds System
(CWS). This is described below. Students should realize this is a
proposal for a system that has not yet been adopted by the scientific
community.
Calendar System (ICS) there is a Classification of Worlds System
(CWS). This is described below. Students should realize this is a
proposal for a system that has not yet been adopted by the scientific
community.
It is the year 9 SD (Sol Date) in the
Interplanetary Calendar System. New
Year's Day for year 10 SD is December
22, 2009 CE.
Interplanetary Calendar System. New
Year's Day for year 10 SD is December
22, 2009 CE.
A Proposal for a System of Classification of Worlds
Will Napoli
In my work developing the interplanetary calendar system I found it necessary to have more than just a definition of a planet, but of
course, getting one from the IAU in ’06 helps. Still, there are many worlds and world types and conditions I needed to consider in creating
a system to encompass the known worlds and accommodate potential new world types in our expanding discovery of the universe.
So it is that I set out earlier this year to once again attempt to codify the calendar system for all possibilities and equipped with the new
IAU definition and recent discoveries beyond Neptune and other data I was finally able to create a Classification of Worlds System to
enable the task. I had been trying to do so for 10 years, knowing I couldn’t for another 12 before that. But finally, there was enough
information and I was familiar enough with it (after 6 years of using StarLab, a portable planetarium, as a NASA educator to students of all
ages all over town) that it could be done. Following is a presentation and explanation of that system.
WHAT WORLDS THERE ARE!
Indeed, what worlds are there? We have a working definition of a planet (and herein a similar variant). But, although it may be possible
in the future to visit the cloud layers of gas giants it’s far more likely that humans will seek residence on the giant moons of gas giants
over the oversized planets themselves. So then, moons are worlds to consider in creating calendars (for more than just a native month).
But it’s even conceivable that comets and their oversized kin in the Kuiper Belt, Scattered Disc, and Oort Cloud could one day be
inhabited, although true comets would be hazardous in their perihelial periods, some are so long-term it might not be a deterrent to long-
term temporary research, although, the physical integrity of such low density objects could be an issue for any physical exploration or the
establishment of surface bases.
Further, it is proposed here that large trans-Neptunian objects (TNOs) are similar enough to comets, though giant-sized in comparison,
that they ought to be called “super cometoids” and that all comet-like objects (icy rocks or even just ice and only traces of anything else)
are “cometoids” unless they have a cometary orbit, which results in cometary tails. If the tails are gone, but the eccentric orbit remains
then the object has probably lost its ice and is only a rock, thus a planetoid with a cometary orbit.
I prefer to do away with the earthbound misnomer “asteroid” and instead use the alternate “planetoid”. Thus, there are planets,
planetoids, comets, cometoids, moons, and co-orbitals: the worlds of the universe. Moons are a special case of co-orbital, which orbit
their primaries and thus anything their primaries orbit as well. Other co-orbitals include like-sized objects that revolve around a center
point that is between them and not inside either object, objects that have similar orbits that on occasion swap these orbits, some small
objects that appear to be touching but may not, as well as Trojans. Here I should introduce my variant definition of “planet” for
clarification. But first, the IAU’s definitions:
1. A planet[1] is a celestial body that:
a. is in orbit around the Sun,
b. has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round)
shape, &
c. has cleared the neighborhood around its orbit.
2. A “dwarf planet” is a celestial body that:
a. is in orbit around the Sun,
b. has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round)
shape[2],
c. has not cleared the neighborhood around its orbit, &
d. is not a satellite.
3. All other objects[3], except satellites, orbiting the Sun shall be referred to collectively as “small solar system bodies”.
[1] The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
[2] An IAU process will be established to assign borderline objects into either dwarf planet or other categories.
[3] These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small
bodies.
Unfortunately, the IAU has yet to attempt to include exoplanets, planets outside the Solar System because having “cleared the
neighborhood” would be difficult to establish across interstellar distances. Furthermore “clearing the neighborhood” has to be defined
as a process during the early life of the Solar System so that the many thousands of Trojan planetoids don’t count against Jupiter’s claim
to planethood, or the Martian Trojans against Mars, or the Neptunian Trojans, Pluto, and other plutinos against Neptune. Also, the use of
the term “satellite” is limited, it would seem, to satellites of non-stellar bodies, i.e., moons, not satellites of the Sun.
Here are presented the initial definitions used in the Interplanetary Calendar Project’s Classification of Worlds System.
1. A planet is a celestial body that:
a. is composed of any combination of metal, rock, ice, liquid, or gas, and may contain organic compounds on or in it, AND
b. has sufficient mass for its own gravity to overcome rigidity so that it necessarily assumes a near-spherical shape, AND
c. orbits a star but is neither a planetoid, comet, or a moon, AND
d. is the primary or co-primary of its orbit, AND
e. lies in and originated in the planetary disc of a star.
2. A planetoid is a celestial body that:
a. is composed of rock and/or metal, and may contain organic compounds on or in it, but does not contain significant amounts of ice,
AND
b. does not have sufficient mass for its own gravity to overcome rigidity so that it is not necessarily near-spherical in shape, AND
i. orbits a star but does not orbit a planet, planetoid, comet, or cometoid, AND
1. is alone in its orbit, OR
2. orbits with one or more co-orbitals and is thus a “planetoid co-orbital”, OR
ii. orbits a planet, planetoid, comet, or cometoid, AND
1. is alone in its orbit and is thus a “planetoid moon”, OR
2. orbits with one or more co-orbital moons and is thus a “planetoid co-orbital moon”, OR
iii. is an interstellar or intergalactic traveler without a star, thus a “rogue planetoid”, OR
c. has sufficient mass for its own gravity to overcome rigidity so that it is necessarily near-spherical in shape, AND
i. orbits a planet or cometoid, AND
1. is alone in its orbit and is thus a “super planetoid moon”, OR
2. orbits with one or more co-orbital moons and is thus a “super planetoid co-orbital moon”, OR
ii. is an interstellar or intergalactic traveler without a star, thus a “rogue super planetoid”.
3. A comet is a celestial body that:
a. is composed of rock, dust, ice, and may have organic compounds on or in it, which is called the nucleus, AND
b. orbits a star in a highly elliptical orbit with an approach of the star sufficiently close to create:
i. a coma, or temporary atmosphere,
ii. a dust tail that points away from the star but also shows curvature according to the orbital path, AND
iii. a gaseous ion tail that points directly away from the star being more fully influenced by the stellar wind, AND
c. originated beyond the planetary disc or in its outermost region, AND
d. travels between regions of the solar system, AND
i. is alone in its orbit, OR
ii. orbits within a group of bodies as a cluster and is thus a “co-orbital comet”, OR
iii. orbits a larger member of a cometary cluster and is thus a “moon comet”.
4. A cometoid is a celestial body that:
a. is composed of rock and/or ice and may have organic compounds on or in it, but does not contain significant amounts of metal,
AND
b. does not have sufficient mass for its own gravity to overcome rigidity so that it is not necessarily near-spherical in shape, AND
i. orbits a star but does not orbit a planet, planetoid, comet, or cometoid, AND
1. is alone in its orbit, OR
2. orbits with one or more co-orbitals and is thus a “cometoid co-orbital”, OR
ii. orbits a planet, planetoid, comet, or cometoid, AND
1. is alone in its orbit and is thus a “cometoid moon”, OR
2. orbits with one or more co-orbital moons and is thus a “cometoid co-orbital moon”, OR
iii. is an interstellar or intergalactic traveler without a star, thus a “rogue cometoid”, OR
c. has sufficient mass for its own gravity to overcome rigidity so that it is necessarily near-spherical in shape and either:
i. orbits a planet or super cometoid, AND
1. is alone in its orbit and is thus a “super cometoid moon”, OR
2. orbits with one or more co-orbital moons and is thus a “super cometoid co-orbital moon”, OR
ii. is an interstellar or intergalactic traveler without a star, thus a “rogue super cometoid”.
5. A moon is a celestial body that:
a. is either a planetoid, comet, or cometoid, AND
b. is a special case of co-orbital, AND
c. orbits a planet, planetoid, comet, cometoid, another moon, or co-orbital, which primary has sufficient mass that the center of the
orbiting moon’s revolution lies within the mass of the primary.
6. A co-orbital is a celestial body that:
a. is either a planet, planetoid, comet, cometoid, or moon, AND
b. shares its orbit with another co-orbital of the same or similar size such that the two either:
i. alternate between nearly identical orbits, OR
ii. share a center of mutual revolution outside of either co-orbital’s mass, OR
iii. are clustered together with one or more co-orbitals as a group, OR
c. shares its orbit with a larger primary body and possibly other co-orbitals closer to its own size where it is not the moon of another
co-orbital or the primary, AND
i. lies alone in or near a liberation point of the primary, OR
ii. lies in or near a liberation point of the primary in a group of similarly sized co-orbitals.
7. A world is a celestial body that:
a. is either a planet, planetoid, cometoid, moon, or co-orbital, AND
b. has a diameter of at least 1 kilometer, AND
c. has an orbit within a single region of a star system.
In examining the size ranges and clusters of examples of various types of celestial bodies, it becomes apparent that diameter can be
used as a primary sorting criterion. This also gives us a chance to create simple universal definitions of “subworld”, “minor world”,
“middle world”, “major world”, and “ultra world”. Subworlds are too small to gravitationally attract one another. Though, they are not
entirely negligible.
Although this system classifies planets and planetoids based on diameter, there are additional factors in designating planethood that if
not present could make a large otherwise planetary object a planetoid, for instance, if it were a moon or a co-orbital. Also, if such an
object were no longer in the planetary disc of its (or any) star it would be relegated to being a planetoid or a rogue planetoid.
It should also be noted that once a body of the cometoid class attains major world status (and this includes ultra worlds, which are just
the largest of the major worlds) it is eligible for planethood. It need only have originated in the planetary disc and remain there without
being the moon of another large body and without crossing stellar regions to be considered a planet despite having the composition of
the comet and cometoid class.
There have already been names given to types of planets based on finer categories of composition and their states. Those designations
are continued here but also expanded. The term “gas dwarves” is coined to account for planets like Venus that have atmospheres so
significant that they create a different type of environment than we see on planets with lesser atmospheres. What are currently called “hot
Jupiters”, gas giants larger than Jupiter but very near their stars, must be even less dense than Saturn to exist as observed. These are
little understood at this time, so naming their type is difficult. However, to make at least a preliminary distinction between “hot Jupiters”
and Jupiter types the term “gas monsters” is used in this system. They mark the known limit of the size of a planet with Jupiter near the
small end of the class being at the previously thought maximum size.
Diameters (d) are in kilometers.
Note that Uranus and Neptune do not appear in the examples of the planetoid and planet class. This is because they have no metal
content and are ultra worlds of the cometoid class, but they are still considered planets here because of the previously stated caveat that
major world cometoids, including ultra world cometoids, are eligible for planethood in this system.
Again, the only differences between cometoids and comets are the orbital characteristics and the presence of tails. Thus, super comets
and even ultra comets are conceivably possible and may one day be observed, but their longevity as such may be of a different order than
the comets we observe in the Solar System.
The transition to ice giants seems likelier from “megaprotocometoids” than from terrestrial protoplanets. The Jovian system gives us
indication that even for jovian worlds when there are planetary materials in a protoplanetary system vast enough to form a gas giant the
denser material is relegated to making planetoid moons rather than a terrestrial planetary core of the gas giant. Beyond the inner
planetary region of the planetary disc of stars like the Sun lies a very interesting transitional zone that crosses regions. [For more on the
regions of the Solar System see Overt Science #2.]
The heavy planetary material that might have been a denser core to gas giants than we see in the Solar System instead was divided
amongst three of the four Galilean moons in the Jovian system. Thus, along with the Moon of Earth, Jupiter’s Io, Europa, and Ganymede
are super planetoid moons. Of them, only Ganymede is large enough to qualify as a planet if it were not already a moon. By size then,
Callisto and Titan join Ganymede as being large enough to be planets if they were not moons, but Callisto and Titan would be the
smallest of the super cometoid planets at the other extreme of Uranus and Neptune in that class.
The recent example of the periodic comet Holmes, which in late October had its coma increase to become larger than the planet Jupiter
shows that there is the ability for the smallest worlds in the Solar System to become, even if only briefly, the largest. During such events a
comet is reclassified here as an “ice ghost” with “super comas”. Perhaps in a star system in a different stage of its life ice ghosts might
be more common. What might it seem like to aliens on a distant world with comets flaring up to become short-lived “stars” again and
again all over the sky?
In the table of examples of moons "many, many" is used to indicate both the planetoid variety and that of the cometoid. Terrestrial planet-
sized moons have already been mentioned, but it is conceivable that other star systems might have even larger moons. And, although it
seems unlikely, it might be that there are found one day even gas giant or ice giant moons to even larger gas giants or gas monsters.
A number of new names for types within classes of celestial bodies can be found in these tables of worlds. They are helpful to
identification of these types and proposed here as well as the overall system, but I have let their definitions be deduced from their
locations in the tables and in respect to the previous definitions of classes due to space restrictions here.
While the demarcation of subworlds allows us to limit the number of moons and planetoids we need attempt to remember or name in
any family or system, the reasons for developing this system are not geocentric, but rather in anticipation of human interplanetary travel.
So, instead of ignoring objects barely viewable from Earth entirely, if at all, we can consider the implications of having them in the path of
future peopled exploration. Therefore “millimetric” refers to objects about a thousandth the size of the smallest major bodies, but still
worth noting as they could be problematic to travel as well as being valuable for research into the evolution of the solar system and as
resources and even small ferries for future explorers.
In creating the dividing lines between classes of moons and other celestial bodies by diameter, groupings of naturally occurring sets
were observed and used to guide the choices for these dividing lines. Further developments in the realms of celestial mechanics,
computer modeling, and telescopic observation, which are outside the realm of the work thus far undertaken here, will help to adjust
these numbers as needed or to establish them as sufficient and useful: good guesses.
Again, this work is not undertaken strictly as scientific speculation for the sake of advancing theory, but initially was begun to create a well-
developed backdrop of speculative science for use in science fiction. Then it was soon realized that this work could be helpful in
introductory astronomy education, then later as herald of a “new astronomy”.
In classifying types of moons as either planetoid or cometoid their composition is examined. Possessing a metallic core qualifies a body
as a planetoid in this system as it is a requirement of the class. The same is not true of the presence of a significant, lasting global
atmosphere, which does not include the comas of comets because they do not conform to the shape of the nucleus, except when ice
ghosts, and are not always present. Thus Titan is off the list of planetoid moons due to its lack of a metallic core, lesser density, and
significant quantities of ice. Entirely silicate, rocky worlds without a layer of ice more than a surface dusting qualify as planetoids. The
same is true of smaller bodies that are largely metallic in content but are not large enough for differentiation of components into specific
layers, thus Himalia is classified here as a planetoid while the other large minor moons listed all have significant ice content, making
them cometoids.
Density is an indicator, in a graduated sense, of the classes of non-stellar celestial bodies, but not an absolute factor. That is, for minor
worlds a density greater than about 2.5 suggests a planetoid, while about 2.5 or less suggests a cometoid. However, small major
worlds can have a density even less than 2.0 but qualify as a planetoid because of the presence of a metallic core, Ganymede being the
example here.
Very large planets, the gas giants, all have densities less than 2.0 (Saturn’s is even significantly less than 1.0) and they have non-metallic
cores, but Saturn and Jupiter are believed to have metallic hydrogen mantles where hydrogen has taken the lattice form of metal under
extreme pressure. The gas giants also do not have solid surfaces. Instead there is a graduation from dense gases to liquids which is a
subtle transition.
The ice giants, Uranus and Neptune, are not believed to have metallic hydrogen, but they do have hot ice, water under great pressure
turned solid despite the high temperature. We find that “gas monsters” (hot Jupiters) must be the least dense of all planets and their
composition is not yet understood. Perhaps they are plasma spheres, a kind of fractal offspring of the stars they are so very near. If so,
they would mark a new transition between planet and star and the debate between the classification of both would renew.
The exact dimensions of the TNOs are not yet known and Eris appears to be on the cusp of being a giant super cometoid. It will be
interesting to see if there are any notable differences between Eris and Pluto or any of the other simply large super cometoids. If so, it
would make an interesting point in favor of the division line where it is, but it is not absolutely necessary for significant differences to exist
between neighboring categories. Diametric differences are a sufficient starting point at least.
Now having classified the six major non-stellar celestial bodies that typically orbit a star or stars, we are better able to examine the
systems of other stars that are starting to emerge within our expanding galactic view. [For more speculation on co-orbitals see Overt
Science #2.]
This Classification of Worlds System, if adopted, would allow educators to make this emerging information better understandable in early
science and introductory astronomy classes. It also gives the space industry a metric system for its purposes as well as the scientific
community for its continuing research. It is hoped that all sectors will respond to the proposal of adopting this system with comments
and data that could help to improve this first model.
Also, there are some interesting theories on the very nature of the universe that if accepted would change our models of comets and
planetary formation. So it is possible that there will be a major breakthrough that will significantly alter the details of this classification
system and the definitions that it is based on. However, use of diameters as the primary criterion for differentiation of classes allows that
the system will maintain usefulness to a notable degree even if there were to come a major shake-up in our thinking about the
composition and history of celestial bodies.
(c) 2007 - Will Napoli, Cleveland, Ohio. First appearance in Overt Science #1, November 2007.
Back to ICs
Off to C/Overt Science
Will Napoli
In my work developing the interplanetary calendar system I found it necessary to have more than just a definition of a planet, but of
course, getting one from the IAU in ’06 helps. Still, there are many worlds and world types and conditions I needed to consider in creating
a system to encompass the known worlds and accommodate potential new world types in our expanding discovery of the universe.
So it is that I set out earlier this year to once again attempt to codify the calendar system for all possibilities and equipped with the new
IAU definition and recent discoveries beyond Neptune and other data I was finally able to create a Classification of Worlds System to
enable the task. I had been trying to do so for 10 years, knowing I couldn’t for another 12 before that. But finally, there was enough
information and I was familiar enough with it (after 6 years of using StarLab, a portable planetarium, as a NASA educator to students of all
ages all over town) that it could be done. Following is a presentation and explanation of that system.
WHAT WORLDS THERE ARE!
Indeed, what worlds are there? We have a working definition of a planet (and herein a similar variant). But, although it may be possible
in the future to visit the cloud layers of gas giants it’s far more likely that humans will seek residence on the giant moons of gas giants
over the oversized planets themselves. So then, moons are worlds to consider in creating calendars (for more than just a native month).
But it’s even conceivable that comets and their oversized kin in the Kuiper Belt, Scattered Disc, and Oort Cloud could one day be
inhabited, although true comets would be hazardous in their perihelial periods, some are so long-term it might not be a deterrent to long-
term temporary research, although, the physical integrity of such low density objects could be an issue for any physical exploration or the
establishment of surface bases.
Further, it is proposed here that large trans-Neptunian objects (TNOs) are similar enough to comets, though giant-sized in comparison,
that they ought to be called “super cometoids” and that all comet-like objects (icy rocks or even just ice and only traces of anything else)
are “cometoids” unless they have a cometary orbit, which results in cometary tails. If the tails are gone, but the eccentric orbit remains
then the object has probably lost its ice and is only a rock, thus a planetoid with a cometary orbit.
I prefer to do away with the earthbound misnomer “asteroid” and instead use the alternate “planetoid”. Thus, there are planets,
planetoids, comets, cometoids, moons, and co-orbitals: the worlds of the universe. Moons are a special case of co-orbital, which orbit
their primaries and thus anything their primaries orbit as well. Other co-orbitals include like-sized objects that revolve around a center
point that is between them and not inside either object, objects that have similar orbits that on occasion swap these orbits, some small
objects that appear to be touching but may not, as well as Trojans. Here I should introduce my variant definition of “planet” for
clarification. But first, the IAU’s definitions:
1. A planet[1] is a celestial body that:
a. is in orbit around the Sun,
b. has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round)
shape, &
c. has cleared the neighborhood around its orbit.
2. A “dwarf planet” is a celestial body that:
a. is in orbit around the Sun,
b. has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round)
shape[2],
c. has not cleared the neighborhood around its orbit, &
d. is not a satellite.
3. All other objects[3], except satellites, orbiting the Sun shall be referred to collectively as “small solar system bodies”.
[1] The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
[2] An IAU process will be established to assign borderline objects into either dwarf planet or other categories.
[3] These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small
bodies.
Unfortunately, the IAU has yet to attempt to include exoplanets, planets outside the Solar System because having “cleared the
neighborhood” would be difficult to establish across interstellar distances. Furthermore “clearing the neighborhood” has to be defined
as a process during the early life of the Solar System so that the many thousands of Trojan planetoids don’t count against Jupiter’s claim
to planethood, or the Martian Trojans against Mars, or the Neptunian Trojans, Pluto, and other plutinos against Neptune. Also, the use of
the term “satellite” is limited, it would seem, to satellites of non-stellar bodies, i.e., moons, not satellites of the Sun.
Here are presented the initial definitions used in the Interplanetary Calendar Project’s Classification of Worlds System.
1. A planet is a celestial body that:
a. is composed of any combination of metal, rock, ice, liquid, or gas, and may contain organic compounds on or in it, AND
b. has sufficient mass for its own gravity to overcome rigidity so that it necessarily assumes a near-spherical shape, AND
c. orbits a star but is neither a planetoid, comet, or a moon, AND
d. is the primary or co-primary of its orbit, AND
e. lies in and originated in the planetary disc of a star.
2. A planetoid is a celestial body that:
a. is composed of rock and/or metal, and may contain organic compounds on or in it, but does not contain significant amounts of ice,
AND
b. does not have sufficient mass for its own gravity to overcome rigidity so that it is not necessarily near-spherical in shape, AND
i. orbits a star but does not orbit a planet, planetoid, comet, or cometoid, AND
1. is alone in its orbit, OR
2. orbits with one or more co-orbitals and is thus a “planetoid co-orbital”, OR
ii. orbits a planet, planetoid, comet, or cometoid, AND
1. is alone in its orbit and is thus a “planetoid moon”, OR
2. orbits with one or more co-orbital moons and is thus a “planetoid co-orbital moon”, OR
iii. is an interstellar or intergalactic traveler without a star, thus a “rogue planetoid”, OR
c. has sufficient mass for its own gravity to overcome rigidity so that it is necessarily near-spherical in shape, AND
i. orbits a planet or cometoid, AND
1. is alone in its orbit and is thus a “super planetoid moon”, OR
2. orbits with one or more co-orbital moons and is thus a “super planetoid co-orbital moon”, OR
ii. is an interstellar or intergalactic traveler without a star, thus a “rogue super planetoid”.
3. A comet is a celestial body that:
a. is composed of rock, dust, ice, and may have organic compounds on or in it, which is called the nucleus, AND
b. orbits a star in a highly elliptical orbit with an approach of the star sufficiently close to create:
i. a coma, or temporary atmosphere,
ii. a dust tail that points away from the star but also shows curvature according to the orbital path, AND
iii. a gaseous ion tail that points directly away from the star being more fully influenced by the stellar wind, AND
c. originated beyond the planetary disc or in its outermost region, AND
d. travels between regions of the solar system, AND
i. is alone in its orbit, OR
ii. orbits within a group of bodies as a cluster and is thus a “co-orbital comet”, OR
iii. orbits a larger member of a cometary cluster and is thus a “moon comet”.
4. A cometoid is a celestial body that:
a. is composed of rock and/or ice and may have organic compounds on or in it, but does not contain significant amounts of metal,
AND
b. does not have sufficient mass for its own gravity to overcome rigidity so that it is not necessarily near-spherical in shape, AND
i. orbits a star but does not orbit a planet, planetoid, comet, or cometoid, AND
1. is alone in its orbit, OR
2. orbits with one or more co-orbitals and is thus a “cometoid co-orbital”, OR
ii. orbits a planet, planetoid, comet, or cometoid, AND
1. is alone in its orbit and is thus a “cometoid moon”, OR
2. orbits with one or more co-orbital moons and is thus a “cometoid co-orbital moon”, OR
iii. is an interstellar or intergalactic traveler without a star, thus a “rogue cometoid”, OR
c. has sufficient mass for its own gravity to overcome rigidity so that it is necessarily near-spherical in shape and either:
i. orbits a planet or super cometoid, AND
1. is alone in its orbit and is thus a “super cometoid moon”, OR
2. orbits with one or more co-orbital moons and is thus a “super cometoid co-orbital moon”, OR
ii. is an interstellar or intergalactic traveler without a star, thus a “rogue super cometoid”.
5. A moon is a celestial body that:
a. is either a planetoid, comet, or cometoid, AND
b. is a special case of co-orbital, AND
c. orbits a planet, planetoid, comet, cometoid, another moon, or co-orbital, which primary has sufficient mass that the center of the
orbiting moon’s revolution lies within the mass of the primary.
6. A co-orbital is a celestial body that:
a. is either a planet, planetoid, comet, cometoid, or moon, AND
b. shares its orbit with another co-orbital of the same or similar size such that the two either:
i. alternate between nearly identical orbits, OR
ii. share a center of mutual revolution outside of either co-orbital’s mass, OR
iii. are clustered together with one or more co-orbitals as a group, OR
c. shares its orbit with a larger primary body and possibly other co-orbitals closer to its own size where it is not the moon of another
co-orbital or the primary, AND
i. lies alone in or near a liberation point of the primary, OR
ii. lies in or near a liberation point of the primary in a group of similarly sized co-orbitals.
7. A world is a celestial body that:
a. is either a planet, planetoid, cometoid, moon, or co-orbital, AND
b. has a diameter of at least 1 kilometer, AND
c. has an orbit within a single region of a star system.
In examining the size ranges and clusters of examples of various types of celestial bodies, it becomes apparent that diameter can be
used as a primary sorting criterion. This also gives us a chance to create simple universal definitions of “subworld”, “minor world”,
“middle world”, “major world”, and “ultra world”. Subworlds are too small to gravitationally attract one another. Though, they are not
entirely negligible.
Although this system classifies planets and planetoids based on diameter, there are additional factors in designating planethood that if
not present could make a large otherwise planetary object a planetoid, for instance, if it were a moon or a co-orbital. Also, if such an
object were no longer in the planetary disc of its (or any) star it would be relegated to being a planetoid or a rogue planetoid.
It should also be noted that once a body of the cometoid class attains major world status (and this includes ultra worlds, which are just
the largest of the major worlds) it is eligible for planethood. It need only have originated in the planetary disc and remain there without
being the moon of another large body and without crossing stellar regions to be considered a planet despite having the composition of
the comet and cometoid class.
There have already been names given to types of planets based on finer categories of composition and their states. Those designations
are continued here but also expanded. The term “gas dwarves” is coined to account for planets like Venus that have atmospheres so
significant that they create a different type of environment than we see on planets with lesser atmospheres. What are currently called “hot
Jupiters”, gas giants larger than Jupiter but very near their stars, must be even less dense than Saturn to exist as observed. These are
little understood at this time, so naming their type is difficult. However, to make at least a preliminary distinction between “hot Jupiters”
and Jupiter types the term “gas monsters” is used in this system. They mark the known limit of the size of a planet with Jupiter near the
small end of the class being at the previously thought maximum size.
Diameters (d) are in kilometers.
Note that Uranus and Neptune do not appear in the examples of the planetoid and planet class. This is because they have no metal
content and are ultra worlds of the cometoid class, but they are still considered planets here because of the previously stated caveat that
major world cometoids, including ultra world cometoids, are eligible for planethood in this system.
Again, the only differences between cometoids and comets are the orbital characteristics and the presence of tails. Thus, super comets
and even ultra comets are conceivably possible and may one day be observed, but their longevity as such may be of a different order than
the comets we observe in the Solar System.
The transition to ice giants seems likelier from “megaprotocometoids” than from terrestrial protoplanets. The Jovian system gives us
indication that even for jovian worlds when there are planetary materials in a protoplanetary system vast enough to form a gas giant the
denser material is relegated to making planetoid moons rather than a terrestrial planetary core of the gas giant. Beyond the inner
planetary region of the planetary disc of stars like the Sun lies a very interesting transitional zone that crosses regions. [For more on the
regions of the Solar System see Overt Science #2.]
The heavy planetary material that might have been a denser core to gas giants than we see in the Solar System instead was divided
amongst three of the four Galilean moons in the Jovian system. Thus, along with the Moon of Earth, Jupiter’s Io, Europa, and Ganymede
are super planetoid moons. Of them, only Ganymede is large enough to qualify as a planet if it were not already a moon. By size then,
Callisto and Titan join Ganymede as being large enough to be planets if they were not moons, but Callisto and Titan would be the
smallest of the super cometoid planets at the other extreme of Uranus and Neptune in that class.
The recent example of the periodic comet Holmes, which in late October had its coma increase to become larger than the planet Jupiter
shows that there is the ability for the smallest worlds in the Solar System to become, even if only briefly, the largest. During such events a
comet is reclassified here as an “ice ghost” with “super comas”. Perhaps in a star system in a different stage of its life ice ghosts might
be more common. What might it seem like to aliens on a distant world with comets flaring up to become short-lived “stars” again and
again all over the sky?
In the table of examples of moons "many, many" is used to indicate both the planetoid variety and that of the cometoid. Terrestrial planet-
sized moons have already been mentioned, but it is conceivable that other star systems might have even larger moons. And, although it
seems unlikely, it might be that there are found one day even gas giant or ice giant moons to even larger gas giants or gas monsters.
A number of new names for types within classes of celestial bodies can be found in these tables of worlds. They are helpful to
identification of these types and proposed here as well as the overall system, but I have let their definitions be deduced from their
locations in the tables and in respect to the previous definitions of classes due to space restrictions here.
While the demarcation of subworlds allows us to limit the number of moons and planetoids we need attempt to remember or name in
any family or system, the reasons for developing this system are not geocentric, but rather in anticipation of human interplanetary travel.
So, instead of ignoring objects barely viewable from Earth entirely, if at all, we can consider the implications of having them in the path of
future peopled exploration. Therefore “millimetric” refers to objects about a thousandth the size of the smallest major bodies, but still
worth noting as they could be problematic to travel as well as being valuable for research into the evolution of the solar system and as
resources and even small ferries for future explorers.
In creating the dividing lines between classes of moons and other celestial bodies by diameter, groupings of naturally occurring sets
were observed and used to guide the choices for these dividing lines. Further developments in the realms of celestial mechanics,
computer modeling, and telescopic observation, which are outside the realm of the work thus far undertaken here, will help to adjust
these numbers as needed or to establish them as sufficient and useful: good guesses.
Again, this work is not undertaken strictly as scientific speculation for the sake of advancing theory, but initially was begun to create a well-
developed backdrop of speculative science for use in science fiction. Then it was soon realized that this work could be helpful in
introductory astronomy education, then later as herald of a “new astronomy”.
In classifying types of moons as either planetoid or cometoid their composition is examined. Possessing a metallic core qualifies a body
as a planetoid in this system as it is a requirement of the class. The same is not true of the presence of a significant, lasting global
atmosphere, which does not include the comas of comets because they do not conform to the shape of the nucleus, except when ice
ghosts, and are not always present. Thus Titan is off the list of planetoid moons due to its lack of a metallic core, lesser density, and
significant quantities of ice. Entirely silicate, rocky worlds without a layer of ice more than a surface dusting qualify as planetoids. The
same is true of smaller bodies that are largely metallic in content but are not large enough for differentiation of components into specific
layers, thus Himalia is classified here as a planetoid while the other large minor moons listed all have significant ice content, making
them cometoids.
Density is an indicator, in a graduated sense, of the classes of non-stellar celestial bodies, but not an absolute factor. That is, for minor
worlds a density greater than about 2.5 suggests a planetoid, while about 2.5 or less suggests a cometoid. However, small major
worlds can have a density even less than 2.0 but qualify as a planetoid because of the presence of a metallic core, Ganymede being the
example here.
Very large planets, the gas giants, all have densities less than 2.0 (Saturn’s is even significantly less than 1.0) and they have non-metallic
cores, but Saturn and Jupiter are believed to have metallic hydrogen mantles where hydrogen has taken the lattice form of metal under
extreme pressure. The gas giants also do not have solid surfaces. Instead there is a graduation from dense gases to liquids which is a
subtle transition.
The ice giants, Uranus and Neptune, are not believed to have metallic hydrogen, but they do have hot ice, water under great pressure
turned solid despite the high temperature. We find that “gas monsters” (hot Jupiters) must be the least dense of all planets and their
composition is not yet understood. Perhaps they are plasma spheres, a kind of fractal offspring of the stars they are so very near. If so,
they would mark a new transition between planet and star and the debate between the classification of both would renew.
The exact dimensions of the TNOs are not yet known and Eris appears to be on the cusp of being a giant super cometoid. It will be
interesting to see if there are any notable differences between Eris and Pluto or any of the other simply large super cometoids. If so, it
would make an interesting point in favor of the division line where it is, but it is not absolutely necessary for significant differences to exist
between neighboring categories. Diametric differences are a sufficient starting point at least.
Now having classified the six major non-stellar celestial bodies that typically orbit a star or stars, we are better able to examine the
systems of other stars that are starting to emerge within our expanding galactic view. [For more speculation on co-orbitals see Overt
Science #2.]
This Classification of Worlds System, if adopted, would allow educators to make this emerging information better understandable in early
science and introductory astronomy classes. It also gives the space industry a metric system for its purposes as well as the scientific
community for its continuing research. It is hoped that all sectors will respond to the proposal of adopting this system with comments
and data that could help to improve this first model.
Also, there are some interesting theories on the very nature of the universe that if accepted would change our models of comets and
planetary formation. So it is possible that there will be a major breakthrough that will significantly alter the details of this classification
system and the definitions that it is based on. However, use of diameters as the primary criterion for differentiation of classes allows that
the system will maintain usefulness to a notable degree even if there were to come a major shake-up in our thinking about the
composition and history of celestial bodies.
(c) 2007 - Will Napoli, Cleveland, Ohio. First appearance in Overt Science #1, November 2007.
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Earth photo composites
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