User:Jeremysdelaney/sandbox

Definitions of a Planet, planetoid and asteroid. The currently accepted definition of a planet suffers from a fundamental shortcoming. The dominant definition of a planet is based on astronomical reasoning, but this is not the only perspective that is relevant. The geological properties of a body in space are at least as important as the location of that body in the consideration of its name. It is primarily a definition of the LOCATION of a planet. In other words it a view of a planet from the outside. An equally valid view of the term planet would be to consider those internal properties of a object in space that distinquish it from a star, an asteroid or a comet. So one may ask: 'What is a planet?' A planet is any body in space that has undergone whole-body differentiation and has not ignited internally under nuclear processes. The only size requirement is the implicit constraint that gravity be able to overcome material strength (a constraint that in itself does NOT limit size). This geological definition provides the context for all endogenous processes that occur in a planet without arbitrary constraints and permits the distinction of bodies that are planets from those that are not.

The current astronomical definition (IAU, 2006) of a planet is mostly one of location. It is an exogenous definition that overlaps the geological only at the upper size limit (the onset of nuclear fusion). The current (2013) prime example of a body that falls outside the purview of the astronomical definition but demonstrates the inclusiveness of the geological definition is 4 Vesta. Vesta appears to be a wholly differentiated body that shows the products of that process – basalts – on its surface and for which evidence from the Dawn mission confirms longstanding meteoritical evidence that it contains a core and mantle. Internally, Vesta is directly comparable with Earth, Mars,Venus, or even the Moon. The extent of any atmosphere (exosphere) on Vesta (the extreme outer limit of whole body differentiation) will become clearer when the Dawn mission arrives at that body. The exclusion of Vesta from the domain of the astronomical planets relies on the arbitrary lower size limit invoked by the astronomical definition. Vesta appears in every other respect to be planetoid. Corollary: asteroids and planetoids Within our local stellar system, the total number of 'planets' is indeterminate as the number of minor bodies that experienced whole body differentiation is unknown. There are objects for which the state of differentiation is unknown. Objects that have not differentiated have exposed surfaces with the infra-red spectral/compositional signatures comparable with that the most primitive chondritic meteorites, i.e. they are star-like or 'asteroid'. In contrast, differentiated bodies expose surfaces with an infra-red spectral/compositional signature of whole body differentiation, i.e. they are planet-like or 'planetoid'. Historically the minor bodies of the solar system were first called called 'planetoids'. The term 'asteroid' was used later to describe these bodies and, given the technological limits of the time became the commonly used descriptor. Asteroid and planetoid are both valid descriptors of different types of bodies. As spectral information becomes available for minor bodies they may be classified as either 'asteroidal' (primordial, undifferentiated surfaces) or 'planetoidal' (showing evidence of differentiation on their surfaces). Whether such a minor body should be termed a planet instead of planetoid or an asteroid will then be a subject for further study. Jeremysdelaney (talk) 13:59, 11 May 2014 (UTC)jsd