[meteorite-list] THE ALTERNATIVE DEFINITION OF PLANET

[Sterling K. Webb]

Hi, All,


    Here's the text of the alternative definition of planet,
containing in the following article:
http://www.space.com/scienceastronomy/060819_new_proposal.html

    Commentary first, but short. When the outline of this proposal
was put forward, I pointed out that it was probably based on a widely 
held view in the various theories of planetary formation first put forward 
by the late A. G. W. Cameron 45 years ago, of zonal sweeping that 
"outlawed" highly mixed sources of planetesimals. And sure enough,
the longest of the three paragraphs of justification attached to the
"definition" do just that. The ironic thing is that considerable evidence
has piled up in the last 5-8 years that this condition cannot be true,
so the theory on which this definition is based may be about to fall.
    Mostly, it's a bad idea to base a fundamental definition of the much
larger question of "what's a planet?" on a single theoretical possibility 
in cosmogony, particularly if it looks like it might be in trouble.
    How closely this definition is tied to airy theory is demonstrated in
the notes: "Pluto, Ceres and other large Trans-Neptunian objects... 
should be not considered as planets, since they never were the 
dominant bodies in their accretion zones." This implies that the signers
KNOW where and how Pluto and Company accreted. If so, they're
the only ones.
    Next, "local" is defined as orbits that cross or "close approach"
each other. Venus and Earth come within 0.255 AU of each other.
Mars and the Earth come nearly as close. Now, you know that if this
were in the other solar system, that would more than enough to disqualify
them as planets.
    There was no past formal definition of planet. We found'em, 
we named'em, until there were Nine. When we discovered that Pluto
was different and small, discontent grew. That is the origin of the
Gang of Eight (Planets): to rectify the lazy mental habits of the Nine
Worlds Organization and make Them pure in theory. Why stage their
coup now? They are threatened unexpectedly by the multiple discoveries
of objects they resolutely asserted (for a decade or so) could not exist
because Their Theory cannot account for them.
    Oh, this'll fix it...

    OK, I'll let them talk now.


Sterling K. Webb (text follows)
-----------------------------------------------------------

New proposal for Resolution 5: Definition of a Planet

(1) A planet is a celestial body that (a) is by far the largest 
object in its local population[1], (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) does not produce energy by any nuclear fusion 
mechanism [3]. 

(2) According to point (1) the eight classical planets discovered 
before 1900, which move in nearly circular orbits close to the 
ecliptic plane are the only planets of our Solar System. All the 
other objects in orbit around the Sun are smaller than Mercury. 
We recognize that there are objects that fulfill the criteria (b) 
and (c) but not criterion (a). Those objects are defined as 
"dwarf" planets. Ceres as well as Pluto and several other 
large Trans-Neptunian objects belong to this category. In 
contrast to the planets, these objects typically have highly 
inclined orbits and/or large eccentricities. 
(3) All the other natural objects orbiting the Sun that do not 
fulfill any of the previous criteria shall be referred to 
collectively as 'Small Solar System Bodies'.[4]

Notes:
[1] The local population is the collection of objects that 
cross or close approach the orbit of the body in consideration.
[2] This generally applies to objects with sizes above 
several hundreds km, depending on the material strength.
[3] This criterion allows the distinction between gas 
giant planets and brown dwarfs or stars.
[4] This class currently includes most of the Solar System 
asteroids, near-Earth objects (NEOs), Mars-, Jupiter- and 
Neptune-Trojan asteroids, most Centaurs, most 
Trans-Neptunian Objects (TNOs), and comets. 

Further Considerations:

There has been a long discussion about what a planet is. 
This problem appears at both ends: for the very massive 
bodies and for the smaller ones. At the large end, the limit 
seems to be clearer; it is now widely accepted that planets 
must not generate any energy from nuclear fusion, while 
brown dwarfs generate some nuclear energy from the 
fusion of deuterium. More problematic is the small end. 
We think that the definition should be kept as simple as 
possible and based on physical and cosmogonic reasons.

There is a wide consensus that planets formed by the 
accretion of small bodies  "the planetesimals." The accretion 
process led to the formation of embryo planets that, as 
they grew in size and acquired more powerful gravitational 
fields, went to a process of runaway accretion in which 
the size of a few of them detached from the rest of the 
bodies of their neighboring zones. Given the powerful 
gravitational fields of these massive bodies - that we can 
call at this stage protoplanets - they were able to clean 
the population that had close encounters with them. The 
bodies interacting with the protoplanets were finally 
incorporated to the planets or scattered to other regions.

>From a cosmogonic point of view, it therefore makes 
more sense to consider a planet as an object that acquired 
a mass large enough to clean a zone around its orbit. 
According to this definition, only eight planets, Mercury 
(perhaps marginally), Venus, Earth, Mars, Jupiter, Saturn, 
Uranus, and Neptune fulfill this condition. It is obvious 
that, at least for our solar system, this cosmogonic definition 
implicitly carries the condition of objects with a roundish 
shape determined by self-gravity.

>From our definition, Pluto, Ceres and other large 
Trans-Neptunian objects in quasi-hydrostatic equilibrium [1] 
should be not considered as planets, since they never 
were the dominant bodies in their accretion zones. It is 
suggested that Pluto be kept unnumbered by historical reasons.

Is may be possible that in the near future cases of objects 
not foreseen at present could appear beyond our solar 
system, as for instance free-floating planets, stray planets, 
or double planets. We think that we should not advance 
definitions at this point for these exotic cases and leave their 
discussion when if they became a part of the observed world. 
[1] From our present knowledge of the Solar System, 
we know that objects as small as Mimas (D~400km) are 
roundish. If this were the lower limit for an icy body to 
be in hydrostatic equilibrium, then we would already 
have several tens of bodies fulfilling this requirement. 

List of adherents to the above proposal:
Name/Country
Julio A. Fernandez Uruguay
Marcello Fulchignoni France
Daniela Lazzaro Brazil
Gonzalo Tancredi Uruguay
Alessandro Morbidelli France
Mario Di Martino Italy
Paolo Paolicchi Italy
Antonella Barucci France
Giovanni Gronchi Italy
David Vokrovhlicki Czech Rep.
David Nesvorny USA
Fernando Roig Brazil
Hugo Levato Argentina
Steven Chesley USA
Alsonso Sena Mexico
J. E. Arlot France
I. Shevchenko Russia
Patrick Michel France