this week looks to be no less crazy, the next vote is planned for Thursday 24th August,
and this is what they are voting on
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, (b) has sufficient
mass for its self-gravity to overcome rigid body forces so that it
assumes a hydrostatic equilibrium (nearly round) shape , (c) does
not produce energy by any nuclear fusion mechanism .
(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
(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?.
 The local population is the collection of objects that cross or close approach the orbit of the body in consideration.
 This generally applies to objects with sizes above several hundreds km, depending on the material strength.
 This criterion allows the distinction between gas giant planets and brown dwarfs or stars.
 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.
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
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  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
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.
 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:
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
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