When was pluto declared not a planet?
In the year 2006, was when pluto was changed to a dwarf planet.
Pluto is no longer a planet. The 2006 definition of planet defined Pluto not as planet, but as dwarf planet, because it have not cleared its neighbourhood. Pluto (unlike all other planets in our Solar system) does not orbit the Sun alone, but as part of a population of similarly sized objects.
In 2006 the International Astronomical Union (IAU) defined planets with Resolution 5A-a, Orbits the Sun, b, a nearly round shape, c, has cleared the neighborhood around its orbit. Resolution 6A defines Pluto as a Dwarf Planet, since it has not cleared its neighborhood (5A-c).
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