Pluto: a matter of State?

The question of the planetary status of Pluto seems to persist beyond the reasonable. The state of New Mexico (U.S.A.) intends to declare Pluto as a planet and call March 13th the "Pluto Planet Day", apparently only during this year (see the links for this news and the legislative proposal ).

Pluto's discovery was announced on March 13th of 1930, by Clyde Tombaugh. The specific announcement date was chosen as it was Persival Lowell's anniversary. Lowell had proposed the existence of a planet beyond Neptune, calling it Planet-X. Despite his efforts to detect Planet-X, founding even the Lowell Observatory (Flagstaff, Arizona, U.S.A.), he died in 1916 without observing it.

This planet definition issue may be somewhat subjective and delicate. But, actually, from the study of Pluto point of view it doesn't matter how it is classified. It is there, it is interesting, hence it is studied. The New Mexico's proposal may be funny if seen as a memorial service. But if it catches on one day we will start seeing the "Flat Earth Day", the "Geocentrism Day", or even the "Creationism Day".

The planet definition is only for the Solar System

When changing e-mails with José Matos, I realized it is not immediately clear that IAU's definition of planet regards only to the Solar System objects. On an article in "Sky & Telescope" (November 2006 ), Owen Gingerich, from the Harvard-Smithsonian Center for Astrophysics (USA), member of IAU's committee for the planet definition, tells what happened. While trying to establish an expression for the gravitational domination of the orbital region of an object, they also restricted the definition to the Solar System only. The dynamical evolution of our Solar System is not necessarily applicable to the other planetary systems.

This issue is subject to some confusion. In 2003, IAU, through the "Working Group on Extrasolar Planets" (WGESP) emitted a declaration on the (extra-solar) planet's definition:

1) Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System.

2) Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed nor where they are located.

3) Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).

This declaration did not intend to be a canonic definition but an evolutive definition instead. It is, however, obvious that in the future a strict definition will have to be found. The story of the process involving the planet definition is quite in the Wikipedia (see: Planet; 2006 Redefinition of Planet; Definition of Planet). However, the entry "Planet" gives the illusion that IAU also created a strict definition for the extra-solar planets, when it is not the case yet.