Could “Planet Nine” be Considered a Planet?
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Planet Nine Location

Possible location for hypothesized Planet Nine. Credit: Universe Sandbox ²/ Bob Trembley

I got to wondering: given the International Astronomical Union's (IAU) current definition of a planet, if a hypothesized "Planet Nine" were to be found in the outer reaches of our solar system, could it (or anything in that region) be considered "a planet?"

The IAU definition of a planet is a celestial body that:
(a) is in orbit around the Sun.
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape.
(c) has cleared the neighborhood around its orbit.

An astronomical units (AU) is a unit of measurement equal to the mean distance from the center of the earth to the center of the sun - 149.6 million kilometers.

The Kuiper belt is a disc-shaped region of icy bodies in the solar system - including dwarf planets such as Pluto - and comets beyond the orbit of Neptune. It extends from about 30 to 55 AU.

A trans-Neptunian object (TNO) is any minor planet in the Solar System that orbits the Sun at a greater average distance (semi-major axis) than Neptune - 30 AU.

The Oort cloud as a hypothesized bubble of icy debris that surrounds our solar system. This distant cloud may extend a third of the way from our sun to the next star - between 5,000 and 100,000 AU.

The hypothesized location of "Planet Nine" is around 500 and 1200 AU; images I've seen all place it in a highly elliptical orbit, pretty near to the ecliptic plane. That puts it out in the trans-Neptunian region, and well into the Kuiper belt. The hypothesized mass of "Planet Nine" is ten times that of Earth, making it much larger than the typical size of trans-Neptunian objects.

The question is: with the existence of the Kuiper Best, and the pretty good chance that the Oort cloud exists, can any object out past Neptune ever be classified as a planet, given the requirement that a planet clear its neighborhood?

Oort Cloud Illustration. Credit: laurinemoreau.com

A related issue is that under the current IAU definition of a planet, exoplanets and satellites in orbit of brown dwarfs cannot be considered planets; I'd suggest a slight rewording to state that a planet must be "planetary mass object in orbit of a stellar or sub-stellar mass parent body."

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