Editing Definition of planet (section)

=== Double planets and moons ===
{{main|Double planet}}
[[File:Pluto-Charon system-new.gif|thumb|Simulated oblique view of the Pluto–Charon system, showing that Pluto orbits a point outside itself. The two bodies are mutually [[tidally locked]].]]

The definition specifically excludes [[natural satellite|satellites]] from the category of dwarf planet, though it does not directly define the term "satellite".<ref name=IAU0603/> In the original draft proposal, an exception was made for [[Pluto]] and its largest satellite, [[Charon (moon)|Charon]], which possess a [[barycenter]] outside the volume of either body. The initial proposal classified Pluto–Charon as a double planet, with the two objects orbiting the Sun in tandem. However, the final draft made clear that, even though they are similar in relative size, only Pluto would currently be classified as a dwarf planet.<ref name=IAU0603/>

[[File:Moon trajectory1.png|thumb|A diagram illustrating the [[Moon]]'s co-orbit with the Earth]]

However, some have suggested that the Moon nonetheless deserves to be called a planet. In 1975, [[Isaac Asimov]] noted that the timing of the Moon's orbit is in tandem with the Earth's own orbit around the Sun—looking down on the [[ecliptic]], the Moon never actually loops back on itself, and in essence it orbits the Sun in its own right.<ref>Asimov, Isaac (1975). ''Just Mooning Around'', In: Of time and space, and other things. Avon.</ref>

Also many moons, even those that do not orbit the Sun directly, often exhibit features in common with true planets. There are 20 moons in the Solar System that are massive enough to have achieved hydrostatic equilibrium (the so-called [[planetary-mass moon]]s); they would be considered planets if only the physical parameters are considered. Both Jupiter's moon [[Ganymede (moon)|Ganymede]] and Saturn's moon [[Titan (moon)|Titan]] are larger than Mercury, and Titan even has a substantial atmosphere, thicker than the Earth's. Moons such as [[Io (moon)|Io]] and [[Triton (moon)|Triton]] demonstrate obvious and ongoing geological activity, and Ganymede has a [[magnetic field]]. Just as [[star]]s in orbit around other stars are still referred to as stars, some astronomers argue that objects in orbit around planets that share all their characteristics could also be called planets.<ref name=Buie2005>{{cite web
 |date=March 2005
 |title=Definition of a Planet
 |publisher=Southwest Research Institute
 |author=Marc W. Buie
 |url=http://www.boulder.swri.edu/~buie/pluto/planetdefn.html
 |access-date=July 7, 2008|author-link=Marc W. Buie
 }}</ref><ref name=stern2008>{{cite web
  |date=June 15, 2008
|title=IAU Snobbery
  |publisher=NASA Watch (not a NASA Website)
  |url=http://nasawatch.com/archives/2008/06/iau-snobbery.html
  |access-date=July 5, 2008}}</ref><ref>{{cite book|author=Serge Brunier|title=Solar System Voyage|publisher=Cambridge University Press|year=2000|pages=160–165|isbn=978-0-521-80724-1}}</ref> Indeed, Mike Brown makes just such a claim in his dissection of the issue, saying:<ref name=Brown />

<blockquote>It is hard to make a consistent argument that a 400&nbsp;km iceball should count as a planet because it might have interesting geology, while a 5000&nbsp;km satellite with a massive atmosphere, methane lakes, and dramatic storms [Titan] shouldn't be put into the same category, whatever you call it.</blockquote>

However, he goes on to say that, "For most people, considering round satellites (including our Moon) 'planets' violates the idea of what a planet is."<ref name=Brown />

[[Alan Stern]] has argued that location should not matter and that only geophysical attributes should be taken into account in the definition of a planet, and proposes the term ''satellite planet'' for [[planetary-mass moon]]s.<ref name="satelliteplanet">{{cite web |url=http://news.discovery.com/space/should-large-moons-be-called-satellite-planets.html#post-a-comment |title=Should Large Moons Be Called 'Satellite Planets'? |publisher=News.discovery.com |date=May 14, 2010 |access-date=November 4, 2011 |archive-date=May 5, 2012 |archive-url=https://web.archive.org/web/20120505221146/http://news.discovery.com/space/should-large-moons-be-called-satellite-planets.html#post-a-comment |url-status=dead }}</ref>