Montage of "Not-planets" that would be considered "planets" under a geophysical definition. Image credit: Montage by Emily Lakdawalla. The Moon: Gari Arrillaga. Other data: NASA/JPL/JHUAPL/SwRI/UCLA/MPS/IDA. Processing by Ted Stryk, Gordan Ugarkovic, Emily Lakdawalla, and Jason Perry.

Montage of "Not-planets" that would be considered "planets" under a geophysical definition. Image credit: Montage by Emily Lakdawalla. The Moon: Gari Arrillaga. Other data: NASA/JPL/JHUAPL/SwRI/UCLA/MPS/IDA. Processing by Ted Stryk, Gordan Ugarkovic, Emily Lakdawalla, and Jason Perry.

The International Astronomical Union (IAU) defines a planet under these circumstances:

1-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, and (c) has cleared the neighbourhood around its orbit.

2-A "dwarf 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 not cleared the neighbourhood around its orbit, and (d) is not a satellite.

3-All other objects , except satellites, orbiting the sun shall be referred to collectively as "small solar-system bodies"

Because of these conditions, Pluto and Ceres, among other planetary bodies, have been deemed "dwarf planets", Trans-Neptunian Objects, or Kuiper Belt Objects. The reason for this definition change of "planets" derived from the discovery of too many small icy bodies outside of Pluto's orbit. No one should have to memorize over 100 planets!

Montage of the "major" dwarf planets! Image credit: Montage by Emily Lakdawalla/NASA.

Montage of the "major" dwarf planets! Image credit: Montage by Emily Lakdawalla/NASA.

However, the definition change is still buzzing around in the planetary community! Mainly due to the fact that the IAU definition change was completed by astronomers, not specifically planetary scientists. The definitions, as stated above, are only concerned by the orbital mechanics of the planetary body, not the building blocks of the planet itself. 

Earlier this year, planetary scientists banded together and formed a new concept of the planetary definition - defining planets by the geophysical sense! This would be inclusive to the definition of "exo-planets" or rogue planets off-set from their course. 

The suggested definition by planetary scientists: A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume spheroidal shape adequately…regardless of orbital parameters."

However, this definition would define moons, such as Ganymede or the Moon or Europa to be considered planets! The debate continues on...

Thank you for reading and next week- a look at the TRAPPIST-1 system!