About The Author
D. R. Prescott has written a novel, a collection of short stories, a nonfiction book, a collection of essays, planetarium show/display scripts, two family histories, technical articles and business plans as well as written for and edited several newsletters.
Awards and published work include Writers' Journal, Long Story Short, Taj Mahal Review literary journal, The Orange County Register, Writer's Digest, and Writing.com and four books among other challenges.
As a former aerospace executive and planetarium program director, Prescott currently writes and explores life in Orange, California.
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O R D E R   T O D A Y !
Stellar System Nomenclature
A Tessmann Proposal

By Dr. Stephen Eastmond and D. R. Prescott

On August 24, 2006, members of the International Astronomical Union (IAU) voted into existence definition changes for planets and other Solar System objects. Resolutions 5 and 6 approved in Prague appear to be riddled with ambiguities. As illustrated below, specific issues, highlighted in bold text and brackets, emerge under scrutiny of International Union Resolutions 5 and 6 language:


(1) A planet1 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, and

(c) has cleared the neighbourhood around its orbit.

[Issues with the IAU planet definitions 1(a)-(c):

(a) A criterion of orbiting the Sun is clear and concise except that it only seems to apply to one star, our Sun. Shouldn’t planet definitions apply to any stellar system?

(b) Requiring that a planet be nearly round creates a question of, “How round is round?” Virtually all rotating celestial bodies are not perfectly round or spherical. They are usually oblate spheroids. If an object is rotating fast enough, it may be large enough and massive enough to be called a planet but with an oblong shape about the axis of rotation. A possible example of this in our Solar System might be 2003EL61, a large oblong object, although there is a possibility that it might be two objects in very close proximity orbiting each other in only 4 hours.

(c) Clearing “the neighborhood around its orbit” is similar to the roundness issue but now we must ask, “How clear is clear?” Planets with Trojans, for and aft in their orbits like Jupiter, have not completely cleared their neighborhood. A bigger question, if an object has an elliptical orbit crossing another object’s orbit, i.e., the Neptune-Pluto-Eris situation, does that mean that none of these objects fulfils the criteria for ’clearing’ its neighborhood of interlopers? Does Neptune have some tidying up to do before we call it a planet? Worse, with an estimate of over 2000 Near Earth Objects accounted for at the moment, does that mean that Earth has not cleared its neighborhood?]

(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) shape2,

(c) has not cleared the neighbourhood around its orbit, and

(d) is not a satellite.

[Issues with IAU Dwarf Planet definitions 2(a)-(d):

(a) Again, restricting these criteria to the Sun limits their usefulness and ought to be broadened to cover all star systems.

(b) As with the nearly round criterion for planets, does this mean that anything that is nearly round qualifies? One can imagine things being nearly round that are relatively small and gravity had little to do with making it nearly spherical.

(c) Again, clearing or not clearing its neighborhood is vague. What exactly is meant by the term neighborhood?

(d) The Webster’s New World Dictionary of the American Language, Second College Edition, definition of a satellite offers little help here since it defines a satellite as “a small planet revolving around a larger one; a moon.” If we use that definition, we will have planets revolving about planets, which can be called moons! That brings us to the issue of what is or isn’t a satellite. Is Charon a satellite of Pluto or, since its barycenter lies outside of Pluto, should Pluto and Charon be dubbed as “binary dwarf planets”? In a few billion years or so, when our Moon moves far enough away so that the barycenter is no longer inside the Earth, does our system become a “binary planet?”]

(3) All other objects3, except satellites, orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”.

[Issues with IAU Small Solar System Bodies:
This is a “catch-all” for everything else. Seems every classification system needs one. However, this definition covers everything from subatomic particles to objects slightly smaller than Ceres. Could we not be a little more discerning?]

IAU footnotes:


1. The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

[Issue: Currently, most consider Pluto, Eris and Ceres as dwarf planets with at least 4 other objects in the wings awaiting labels. If we specify exactly what objects are planets, shouldn’t we be just as specific with dwarf planets?]


2. An IAU process will be established to assign borderline objects into either dwarf planet and other categories.

[Issue: As we have seen with the term satellite, currently used terms sometimes have multiple meanings leading to confusion. Hopefully, this process will be crisp and clear, unlike the definitions above.]


3. These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

[Issue: Trans-Neptunian Objects (TNOs) would not have meaning outside our solar system, unless we plan to use the name Neptune in other stellar systems that have planets in the right position.]



The IAU further resolves:
Pluto is a “dwarf planet” by the above definition and is recognized as the prototype of a new category of Trans-Neptunian Objects1.

1. An IAU process will be established to select a name for this category.

[Issue: Selecting a name for this new category further complicates the issue and demonstrates that this term is a testimony to the ambiguities inherent in these definitions.]

The Tessmann Proposal:

Assuming that crispness is the surest way to clarity, we suggest that there could be as few as four types of objects (besides the stars), in any stellar system. You can quickly separate out stars from other things by determining whether or not thermonuclear fusion reactions are occurring in their cores. We propose a Stellar System Nomenclature containing four types of objects, Planets, Dwarf Planets, Small Mass Objects and Ambient Stellar Materials.


1. It orbits around a star as its primary.
2. Its volume is equal to or greater than 5 x 1010 km3 (slightly smaller than Mercury).
3. Has a mass equal to or greater than 1023 kg.
4. It is not a satellite of another object.
5. Is not made of exotic materials, i.e., such as neutron star materials.
6. Does not have sufficient mass to initiate thermonuclear fusion in its core, e.g., is not a star or brown dwarf.

In this proposed Stellar System Nomenclature, the Minimum Planet volume and mass are established below Mercury and above Pluto recognizing the IAU position that Pluto-type objects may require special consideration. It should be noted that under this scheme, Earth’s Moon would have the dwarf planetary minimum mass and volume requirements. In this proposal, it would not qualify as a dwarf planet because the barycenter between the Earth-Moon system is inside of the Earth’s volume, a situation similar to other large moons, i.e., Jupiter’s Ganymede. Maximum planetary volumes or masses are not addressed, since more research is needed in this area. As proposed herein, any object that exceeds the minimum volume for a planet will be a planet until it is massive enough to have fusion occurring in its core.

Dwarf Planet:

1. Its volume is equal to or greater than 108 km3 (slightly larger than asteroids Vesta and Pallas).
2. Has a mass equal to or greater than 4x1020 kg.
3. Is not a planet but orbits about a star as its primary.
4. Is not a satellite of another object.
5. Does not have sufficient mass to initiate thermonuclear fusion in its core, e.g., is not a star or brown dwarf.

Dwarf Planets are given a minimum volume criterion of 108 km3 and a maximum volume of less than 5 x 1010 km3 in this scheme. Dwarfs also would have a minimum mass set at 4 x 1020 kg. Within these ranges, Ceres, Pluto, Charon (because the Pluto-Charon barycenter is outside of Pluto’s surface), Quaoar, Sedna, 2003EL61, Eris, and, possibly 2005 FY9, might be classified as Dwarf Planets.

Small Mass Objects:

1. Its volume is less than 108 km3.
2. Also, its volume is equal to or greater than 1 m3.
3. Is not made of exotic materials, i.e., such as neutron star materials.

Small Mass Objects (SMOs) would include most of the Kuiper Belt (except Eris, 2005 FY9 and 2003EL61), Asteroid Belt (except Ceres) and Oort Cloud objects (except Sedna).

Ambient Stellar Materials:

1. Has a volume less than 1 m3.
2. Is not made of exotic materials, i.e., such as neutron star materials.
Materials/objects with a volume of less than 1 cubic meter are considered as Ambient Stellar Materials. Below that value, everything else fits into this category including everything from dust and ions from comets to atoms caught in the star’s gravitational influence.


This discussion is likely a long way from over. With this proposal, our Solar System would currently have 8 planets, 7 dwarf planets, thousands of Small Mass Objects and virtually countless Ambient Stellar Materials (ASM’s).

Illustrating the relationship.

A crisp, more universal set of stellar system definitions is a necessary and attainable goal. This paper is offered by the Tessmann Planetarium to stimulate more thought on the subject because we think clarity is vital for understanding our universe and in teaching our students and the general public. Since science is about asking questions and getting answers then asking better questions and, hopefully, getting better answers, we hope we have asked a better question and invite your comments or suggestions.

The Tessmann Planetarium, 2005.

© Copyright 2010 D. R. Prescott (donprescott at Writing.Com). All rights reserved.
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