Details Emerge on Plan to Demote Pluto

This was a wild week for astronomers. At a meeting of the International Astronomical Union (IAU) in Prague, a proposal put forth to define the term "planet" would boost the tally from nine to 12 by adding asteroid Ceres, Pluto's moon Charon, and a distant icy object to the list.

Many astronomers criticized the proposal as being vague and setting arbitrary cutoff points. One researcher pointed out that with the definition, Earth's Moon might eventually become a planet when it drifts farther away.

Your 2 Cents

Join the dicussion on this whole affair in Robert Roy Britt's blog.

Your 2 Cents

Join the dicussion on this whole affair in Robert Roy Britt's blog.

Your 2 Cents

Join the dicussion on this whole affair in Robert Roy Britt's blog.

So a second proposal was put forth that would demote Pluto to a non-planet status, as first reported yesterday by

The next step is for the IAU Executive Committee to decide whether to stick with the original proposal, to modify it, or to go with the new proposal. A vote is slated for Thursday, Aug. 24.

Meanwhile, one of the adherents to the new proposal provided a copy to Here it is:

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]

[1] The local population is the collection of objects that cross or close approach the orbit of the body in consideration.

Full Coverage
Full Coverage
Full Coverage

[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:

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

More on the Planet Proposals

Defining Moments: The Saga's History

Have a news tip, correction or comment? Let us know at