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Will the IAU rework its definition of planetary status by Jan 1, 2025?

The International Astronomical Union defines a planet [1] as a celestial body that

  1. is in orbit around the Sun,
  2. is massive enough per material strength to be an ellipsoid (in hydrostatic equilibrium) and,
  3. has "cleared the neighborhood" around its orbit.

A debate has emerged in the planetary sciences over whether the community should instead embrace a purely geophysical definition of a planet (a substellar body in hydrostatic equilibrium), stated in more detail here: [2]. This point of view has been gaining some traction, e.g. in Metzger et al. 2018 [3].

The chief concerns with the IAU's definition are that it excludes exoplanets (they do not orbit the sun), small bodies in hydrostatic equilibrium (e.g. Pluto, Ceres, Titan, Quaoar), and that "clearing the neighborhood" is an imprecise definition that has many caveats (e.g. coorbital bodies/quasi-satellites). There have been attempts to rigorously define orbital clearing (e.g. Margot 2015 [4]), but they have not yet been adopted by the IAU.

The chief concerns with the geophysical definition are that it elides dynamical concerns (which are integral to planet formation), includes ellipsoidal satellites (e.g. Titan, Triton, Ganymede) as planets, and will result in having >50 planets, with that number growing as time goes on.

This debate conceals a difference in methodological approach - considering whether small, ellipsoidal (currently) subplanetary bodies* are more interesting in particular (as geophysical entities, like Earth) or in aggregate (as orbital populations, like sub-ellipsoidal asteroids). These concerns are, to first order, native to planetary geoscientists and planetary astronomers/dynamicists respectively. The geophysical and IAU definitions are both used in the literature, again employed ~along subdisciplinary lines.

This now brings us to the question: given the ongoing debate and reality of publishing differences the planetary sciences,

will the IAU revise its definition of a planet before 2025?


Resolution details

IAU's 2006 definition is:

A “planet” is defined as 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.

We'll refer to this as the "original definition". This question resolves positively if any of the following occurs:

  • Any of the Parts (a), (b) or (c) of the original definition are substantially revised; or
  • Any of the Parts (a), (b) or (c) of the original definition are removed; or
  • Another part not included in the original definition is included that requires a planet to have an additional property that is not implicit in parts (a), (b) or (c).

If more than one of these conditions occur, the question also resolves positively.

[1] IAU Definition

[2] Geophysical Definition

[3] Metzger et al. 2018

[4] Margot 2015

*currently characterized by the IAU as "dwarf planets"

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