Editing Definition of planet (section)

=== Clearing the neighbourhood ===
{{main|Clearing the neighbourhood}}
One of the main points at issue is the precise meaning of "cleared the neighbourhood around its [[orbit]]". [[Alan Stern]] objects that "it is impossible and contrived to put a dividing line between dwarf planets and planets",<ref>{{cite news|author=Paul Rincon|url=http://news.bbc.co.uk/2/hi/science/nature/5283956.stm| title=Pluto vote 'hijacked' in revolt|work=BBC News|access-date=February 28, 2007 | date=August 25, 2006}}</ref> and that since neither Earth, Mars, Jupiter, nor Neptune have entirely cleared their regions of debris, none could properly be considered planets under the [[International Astronomical Union|IAU]] definition.{{Ref label|C|c|none}}

[[File:InnerSolarSystem-en.png|thumb|The asteroids of the inner Solar System; note the [[Trojan asteroid]]s (green), trapped into Jupiter's orbit by its gravity]]
Mike Brown responds to these claims by saying that, far from not having cleared their orbits, the major planets completely control the orbits of the other bodies within their orbital zone. Jupiter may coexist with a large number of small bodies in its orbit (the [[Trojan asteroid]]s), but these bodies only exist in Jupiter's orbit because they are in the sway of the planet's huge gravity. Similarly, Pluto may cross the orbit of Neptune, but Neptune long ago locked Pluto and its attendant Kuiper belt objects, called [[plutino]]s, into a 3:2 resonance, i.e., they orbit the Sun twice for every three Neptune orbits. The orbits of these objects are entirely dictated by Neptune's gravity, and thus, Neptune is gravitationally dominant.<ref name=Brown />

In October 2015, astronomer [[Jean-Luc Margot]] of the [[University of California Los Angeles]] proposed a metric for orbital zone clearance derived from whether an object can clear an orbital zone of extent 2{{radic|3}} of its [[Hill radius]] in a specific time scale. This metric places a clear dividing line between the dwarf planets and the planets of the solar system.<ref name=Margot>{{Cite journal|date=2015|title=A Quantitative Criterion For Defining Planets|journal=The Astronomical Journal|volume=150|issue=6|pages=185|author=Jean-Luc Margot |arxiv=1507.06300|doi=10.1088/0004-6256/150/6/185|bibcode=2015AJ....150..185M|s2cid=51684830}}</ref> The calculation is based on the mass of the host star, the mass of the body, and the orbital period of the body. An Earth-mass body orbiting a solar-mass star clears its orbit at distances of up to 400 [[astronomical unit]]s from the star. A Mars-mass body at the orbit of Pluto clears its orbit. This metric, which leaves Pluto as a dwarf planet, applies to both the Solar System and to extrasolar systems.<ref name=Margot/>

Some opponents of the definition have claimed that "clearing the neighbourhood" is an ambiguous concept. Mark Sykes, director of the Planetary Science Institute in Tucson, Arizona, and organiser of the petition, expressed this opinion to [[National Public Radio]]. He believes that the definition does not categorize a planet by composition or formation, but, effectively, by its location. He believes that a Mars-sized or larger object beyond the orbit of Pluto would not be considered a planet, because he believes that it would not have time to clear its orbit.<ref>{{cite news| url=https://www.npr.org/templates/story/story.php?storyId=5788798| format=RealPlayer| title= Astronomers Prepare to Fight Pluto Demotion| first=Sykes| last=Mark| newspaper=NPR.org| date=September 8, 2006| access-date=October 4, 2006}}</ref>

Brown notes, however, that were the "clearing the neighbourhood" criterion to be abandoned, the number of planets in the Solar System could rise from eight to [[List of possible dwarf planets|more than 50]], with hundreds more potentially to be discovered.<ref name=browndwarf/>