“A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.”
In response to the question as to whether the Moon is a ‘Planet’, whilst some definitions might support this, in short the answer is a definitive NO according to current nomenclature.
19th – The century in which the word planet, as it was understood, was applied to a select few objects found in the solar system.
1992 – The year astronomer’s began to discover a host of celestial objects beyond the orbit of Neptune at the furthest reaches of the solar system. Crucially these discoveries raised significant questions as to how a planet might be defined owing to the variety in form and shape of these newly found objects.
136199 Eris – The trans-Neptunian’ object (TNO) that prompted a controversy over how a planet was to be defined.
05 January 2005 – The date that 136199 Eris was discovered (at 11:20AM PST), via Palomar Observatory’s Samuel Oschin telescope.
24 August 2006 – The date on which, in an attempt to resolve the debate over what a planet is (following the discovery of 136199 Eris, and how a planet might be defined in terms of its physical characteristics and behaviour in terms of nomenclature), the IAU (International Astronomical Union) provided the following definition that applies only to objects within this solar system:
“a planet is a body that orbits the Sun, is massive enough for its own gravity to make it round, and has “cleared its neighbourhood” of smaller objects around its orbit”.
The IAU also suggests that:
“a planet is any rocky body that has different layers, is geologically active, and most importantly orbits the Sun”.
While the Moon meets some of these criterion on the basis of its geological layers and activity (in terms of detectable quakes), current definitions also state that that the celestial object in question must not orbit a planet, nor must it be a star. On both counts, the Moon therefore remains just that…a moon.
4 – the distance, in centimetres, that the Moon is drifting away from the Earth. As a result, in the distant theoretical future, the moons barycentre will eventually lie beyond on the Earth. It is therefore suggested that in the distant future the Moon may finally become a planet according to the views outlined in the IAU’s 2006 draft proposal:
Contemporary observations are changing our understanding of the Solar System, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation “planets”. The word “planet” originally described “wanderers” that were known only as moving lights in the sky. Recent discoveries force us to create a new definition, which we can make using currently available scientific information. (Here we are not concerned with the upper boundary between “planet” and “star”.)
The IAU therefore resolves that planets and other Solar System bodies be defined in the following way:
A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.
We distinguish between the eight classical planets discovered before 1900, which move in nearly circular orbits close to the ecliptic plane, and other planetary objects in orbit around the Sun. All of these other objects are smaller than Mercury. We recognize that Ceres is a planet by the above scientific definition. For historical reasons, one may choose to distinguish Ceres from the classical planets by referring to it as a “dwarf planet.”3
We recognize Pluto to be a planet by the above scientific definition, as are one or more recently discovered large Trans-Neptunian Objects. In contrast to the classical planets, these objects typically have highly inclined orbits with large eccentricities and orbital periods in excess of 200 years. We designate this category of planetary objects, of which Pluto is the prototype, as a new class that we call “plutons”.
All non-planet objects orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”