What constitutes a planet?

[hungrrrry1]

Certainly a topic on here before but I am new to the boards so I am curious about others views...<br /><br />I have my own as well...Though size is the main focus, it would seem, I would think the body in question whether locate within the inner solar system, the kuiper belt or where-have-you, should at least have an orbit more circular as the first eight. Comets have wide eliptical orbits but typicly are smallish bodies not rivaling even the smaller moons of the solar system, but what determines whether a comet is a comet? Size, chemical compounds, water ice, dust, something else? Planets have any or all of these but comets are not considered mini-planets. In fact they are referred to more as kuiper belt objects if not comets. So that brings us to pluto...called a planet, pluto has an elogated eliptical orbit like a comet, maybe not AS elongated but much more-so than the first eight planets. One might use the arguement that all the planets have an eliptical orbit but this is a very insignificant one. (I doubt there are many planets outside of Mercury's distance to the sun at many stars where an orbit is perfectly concentric). I can't understand why this couldn't be used as a method to determine what IS NOT a planet because I am sure that no one would want to demote Pluto to "Comet" status...or maybe they would. I am curious to know what others think...Also ideas on why it is so important that the comets, mini-worlds, etc., located in the kuiper belt be classified in different ways as opposed to just "KBO's" for instance. We could still name KBO's Xena, Pluto and so on...just for the record...names are so much better than numbers for identification.

 

[harmonicaman]

This argument is really all about <b>semantics, nomenclature and taxonomy!</b>. Obviously, more planetary like stuff will be discovered in the future and in Science, all things <i>must be</i> systematically classified with similar units in a process called <b><i>disambiguation.</i></b> <br /><br />The International Astronomical Union (IAU), which is a member of the International Council of Science (ICSU), is responsible for maintaining a unified system of astronomical classification. Within the IAU is the Working Group for Planetary System Nomenclature (WGPSN) which maintains the astronomical naming conventions and planetary nomenclature for orbiting bodies. <br /><br />This is the seminal authority which will decide on a system to <i>disambiguate</i> the Solar System.<br /><br /><i>"Though size is the main focus..."</i><br /><br /><i>"...should at least have an orbit more circular as the first eight."</i> <br /><br />Yes, size and orbit are important considerations; but they aren't the only criteria...

 

[Philotas]

When it comes to comets, I`m not sure about IAUs definition; but as I view it, a comet are irregular shaped objects that leaves trails of dust and/or gases as they swings past the Sun. They have to be irregular, otherwise Mars is a comet because of the gas trail it leaves behind when orbiting the Sun(lack of magnetic field make its atmospher "leak" into space). <br /><br />As for planets, I, in our solar system, consider every object that is roughly spherical in shape, and that orbits the Sun, a planet. <br />The maximum size of a planet should lie somewhere between 2x and 13x the mass of Jupiter. <div class="Discussion_UserSignature"> </div>

 

[silylene old]

Hello hungrrry1 and welcome to SDC.<br /><br />We have had this discussion of the definition of a planet in multiple times, in detail both before and after the SDC meltdown.<br /><br />Rather than start a new thread, I suggest searching for one of the old threads and adding to those. <div class="Discussion_UserSignature"> <div class="Discussion_UserSignature" align="center"><em><font color="#0000ff">- - - - - - - - - - - - - - - - - - - - - -</font></em> </div><div class="Discussion_UserSignature" align="center"><font color="#0000ff"><em>I really, really, really miss the "first unread post" function.</em></font> </div> </div>