February 22, 2017

Proposed planet definition - round objects in space that are smaller than stars

A proposed planet definition - a planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters.

A simple paraphrase of our planet definition— especially suitable for elementary school students— could be, “round objects in space that are smaller than stars”. The definition clearly excludes stars or stellar objects such as white dwarfs, neutron stars, and black holes. We leave for the future the issue of brown dwarfs’ stellar versus planetary status so as to not force a premature definition on the larger end of planetary scales. In keeping with emphasizing intrinsic properties, our geophysical definition is directly based on the physics of the world itself rather than the physics of its interactions with external objects.

The planet definition adopted by the IAU in 2006 is technically flawed, for several reasons. First, it recognizes as planets only those objects orbiting our Sun, not those orbiting other stars or orbiting freely in the galaxy as “rogue planets.” Second, it requires zone clearing, which no planet in our solar system can satisfy since new small bodies are constantly injected into planet-crossing orbits, like NEOs near Earth. Finally, and most severely, by requiring zone clearing the mathematics of the definition are distance-dependent, requiring progressively larger objects in each successive zone. For example, even an Earth sized object in the Kuiper Belt would not clear its zone.

Do not judge them by their size

The eight planets recognized by the IAU are often modified by the adjectives “terrestrial,” “giant,” and “ice giant,” yet no one would state that a giant planet is not a planet. Yet, the IAU does not consider dwarf planets to be planets. We eschew this inconsistency. Thus, dwarf planets and moon planets such as Ceres, Pluto, Charon, and Earth’s Moon are “fullfledged” planets. This seems especially true in light of these planets’ complex geology and geophysics

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