Mathematics – Logic
Scientific paper
Dec 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufm.p11a0100d&link_type=abstract
American Geophysical Union, Fall Meeting 2005, abstract #P11A-0100
Mathematics
Logic
0399 General Or Miscellaneous
Scientific paper
It had proposed some definitions about what a planet is. It seems clear that the planet's mass superior limit should be lower than the threshold for deuterium thermonuclear fusion. However the inferior limit is more elusive. It had proposed either Pluto's mass or the minimum mass to produce a spherical form. The Working Group on Extrasolar Planets (WGESP) of the International Astronomical Union (IAU) gives the next definition of a planet: 'Object with the true mass below the limiting mass for thermonuclear fusion of deuterium (currently calculate to be 13 Jupiter masses for objects of solar metalicity) that orbit stars or stellar remanents (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' . The definition gave by Marcy and Butler (2000) is: 'A planet is an object that has a mass between that of Pluto and the Deuterium-burning threshold and that forms in orbit around an object that can generate energy by nuclear reactions' These definitions are not satisfactory because they have some statements that are not clear. By example, the minimum mass the IAU group defines in term of the minimum mass to the Solar System but this is not defined in the first instance, unless we take the mass of Pluto as defined by Marcy and Butler. But why the mass of Pluto should be the minimum mass? Which is the physical reason to choose this one as the minimum of mass? On the other hand the two definitions state that the planets should be orbiting around either a star or a stellar remanent. What with the satellites? Are they planets or not? The geological and geophysical similarity of the larger satellites with the terrestrial planets shows that both, planets and satellites, are specimens of the same class of objects. In this work it is proposed that the definition of a planet will be made taking into account its geophysical characteristics and no by its position or neighborhood. The threshold mass of nuclear reaction is a physical feature and then it is suitable for the definition. The spherical form is another physical feature that is determinate by the strength of the gravity field and it is not produced in smaller bodies where material cohesion is stronger than gravity and could maintain irregular forms, then the nearly spherical form it is proposed as a criterion to define the minimum mass of the planets. The bodies that until now had been recognized as planets are all differentiated in their interior and this feature is function of the mass since with very small bodies the process of differentiation could not be produced. By all these the definition that here it is proposed is as follows: Planet is a body, no matter how they formed, no matter which is the place or neighborhood it take up, that have a mass below the threshold mass for thermonuclear fusion of deuterium and a mass sufficiently larger to have a spherical form and a differentiated interior both induced gravitationally. This definition includes all the bodies of the Solar System that until now it had been considered as planets. Accepting this definition it could grown the number of planets in our system including some of the new uncovered trans-Neptunian objects like Xena, Sedna and Quaoar (if it is demonstrated that there are spherical and differentiated), and some asteroids like Vesta that have spherical form and seem to be differentiated.
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