The Moon Meets All Requirements of the IAU Definition for “Planet”

David G Russell

doi:10.4236/ijaa.2017.74024

Abstract

The Earth-Moon system has often been characterized as having some characteristics of a “double planet” system. It is demonstrated that while the orbital barycenter of the Earth-Moon pair lies inside the radius of the Earth, the Moon does meet all three requirements of the IAU definition for “planet” and therefore the Moon can correctly be identified as the Solar System’s 9th planet. In order to avoid confusion by this development it is necessary to add definitions for “double planet”, “double dwarf planet”, “satellite planet”, and “satellite” to complement the International Astronomical Union definitions for “planet” and “dwarf planet”. The Earth-Moon system meets the requirements of a “double planet” system while the Pluto-Charon system meets the requirements of a “double dwarf planet” system. In order to extrapolate sub-stellar taxonomy to exoplanetary systems, general formation mechanisms (star-like gas collapse in molecular clouds vs. proto-planetary disk formation) should be included in the definitions for the various classes of sub-stellar mass bodies.

Highlights

Due to the large radius of the Moon relative to the Earth, late 19th to late 20th century textbooks frequently suggested that the Earth-Moon system, in some respects, can be characterized as a double planet [1]-[7]
While the terms “satellite” and “double planet” have not been formally defined by the International Astronomical Union (IAU), it has been proposed that a pair be considered a “double planet” if the satellite is massive enough that the orbital barycenter lies between the pair and outside the body of the primary [8]
The following definitions are recommended to resolve these gaps in taxonomy: Double planet: a pair of bodies orbiting the Sun, with each having enough mass to dynamically clear the orbit and self-gravitate into a spherical shape, and meeting one or both of the following criteria: 1) the orbital barycenter lies outside the radius of the larger body in the pair; or 2) the Sun, rather than the larger planet in the pair, is the primary for the orbit of the smaller body in the pair

Summary

Introduction

Due to the large radius of the Moon relative to the Earth, late 19th to late 20th century textbooks frequently suggested that the Earth-Moon system, in some respects, can be characterized as a double planet [1]-[7]. The IAU definition for “planet” requires that a body meet three criteria to qualify as a planet. While the Earth-moon system does not meet the proposed orbital barycenter definition for a double planet, it can be verified that the Moon does meet all three requirements for planethood as defined by the IAU even though it has not been included on the IAU list of planets. This paper is organized as follows: Evidence that the Moon meets all three criteria of the IAU planet definition is discussed in Sections 2, 3, and 4.

The Shape of the Moon

The Moon’s Orbital Clearing Capacity

The Moon’s Orbital Primary

Dynamical Classes for Spherical Solar System Bodies

Exoplanets and Brown Dwarfs

Conclusions