Possible location for hypothesized Planet Nine. Credit: Universe Sandbox ²/ Bob TrembleyI 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?”
(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.comA 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.”
