Planet Counter Proposal Takes Shape

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Posted on the SkyMap site:

 

For those following the IAU discussions in Prague, this development

might seem interesting.

 

Widespread dissatisfaction on the IAU proposal has prompted a group of

astronomers to put forth a counter proposal to the IAU. Here are the

details of their proposal:

 

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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], © 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 © 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]

 

[1] The local population is the collection of objects that cross or

close approach the orbit of the body in consideration.

Full Coverage

 

* Pluto May Get Demoted After All

* Those Wild and Crazy Astronomers

* Earth's Moon Could Become a Planet

* Public Laughs and Shrugs at 12-Planet Proposal

* Astronomers Sharply Divided on New Planet Definition

* Adding Planets Means New Textbooks, Toys

* Nine Planets Become 12 with Controversial New Definition

* Image Gallery: The 12 " Planets "

 

[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

 

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Source:

http://www.space.com/scienceastronomy/060819_new_proposal.html