the initial formation period, both failed to win the accretion competition. When the numbers for the winning objects are compared to those of the losers, the contrast is striking; if Soter's concept that each planet occupies an "orbital zone" is accepted, then the least orbitally dominant planet, Mars, is larger than all other collected material in its orbital zone by a factor of 5100. Ceres, the largest object in the asteroid belt, only accounts for one third of the material in its orbit; Pluto's ratio is even lower, at around 7 percent. Mike Brown asserts that this massive difference in orbital dominance leaves "absolutely no room for doubt about which objects do and do not belong." Ongoing controversies Despite the IAU's declaration, a number of critics remain unconvinced. The definition is seen by some as arbitrary and confusing. A number of Pluto-as-planet proponents, in particular Alan Stern, head of NASA's New Horizons mission to Pluto, have circulated a petition among astronomers to alter the definition. Stern's claim is that, since less than 5 percent of astronomers voted for it, the decision was not representative of the entire astronomical community. Even with this controversy excluded, however, there remain several ambiguities in the definition. 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", and that since neither Earth, Mars, Jupiter, nor Neptune have entirely cleared their regions of debris, none could properly be considered planets under the IAU definition. Mike Brown counters 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 asteroids), 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 plutinos, 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. In October 2015, astronomer Jean-Luc Margot of the University of California Los Angeles