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Under the definition of a planet set by the International Astronomical Union in 2006, a body that orbits the Sun and has achieved hydrostatic equilibrium but has not cleared the neighborhood around its orbit is classified as what?
AA full planet under this definition
BA dwarf planet, not a full planet
CA moon orbiting the nearest planet
DAn ordinary asteroid despite its shape
Answer & Solution
Correct answer: B. A dwarf planet, not a full planet
1. The 2006 IAU definition of a planet lists three criteria: orbiting the Sun, having enough mass for hydrostatic equilibrium, and having cleared the neighborhood around its orbit.
2. A non-satellite body meeting only the first two of these criteria, without having cleared its orbital neighborhood, is classified as a dwarf planet instead of a full planet.
3. This is exactly the classification Pluto was given in 2006, after being demoted once this definition was adopted.
4. Such a body is not automatically a moon, and it is not merely labeled a shapeless asteroid either, since it still satisfies the hydrostatic equilibrium requirement that separates it from an ordinary asteroid.
_Source: OpenStax College Physics (CC BY 4.0), Ch 6 "Uniform Circular Motion and Gravitation", section 6.6 Satellites and Kepler's Laws: An Argument for Simplicity_
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