formulating definitive estimations crowdsourcing predictive contingencies exploring intelligent understanding forecasting accurate understanding generating calibrated contingencies mapping the future assembling calibrated contingencies exploring definitive futures generating predictive wisdom exploring critical forecasts aggregating quantitative contingencies mapping quantitative futures formulating predictive insights delivering intelligent understanding


Metaculus Help: Spread the word

If you like Metaculus, tell your friends! Share this question via Facebook, Twitter, or Reddit.

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"


Metaculus help: Predicting

Predictions are the heart of Metaculus. Predicting is how you contribute to the wisdom of the crowd, and how you earn points and build up your personal Metaculus track record.

The basics of predicting are very simple: move the slider to best match the likelihood of the outcome, and click predict. You can predict as often as you want, and you're encouraged to change your mind when new information becomes available.

The displayed score is split into current points and total points. Current points show how much your prediction is worth now, whereas total points show the combined worth of all of your predictions over the lifetime of the question. The scoring details are available on the FAQ.

Note: this question resolved before its original close time. All of your predictions came after the resolution, so you did not gain (or lose) any points for it.

Note: this question resolved before its original close time. You earned points up until the question resolution, but not afterwards.

This question is not yet open for predictions.

Thanks for predicting!

Your prediction has been recorded anonymously.

Want to track your predictions, earn points, and hone your forecasting skills? Create an account today!

Track your predictions
Continue exploring the site

Community Stats

Metaculus help: Community Stats

Use the community stats to get a better sense of the community consensus (or lack thereof) for this question. Sometimes people have wildly different ideas about the likely outcomes, and sometimes people are in close agreement. There are even times when the community seems very certain of uncertainty, like when everyone agrees that event is only 50% likely to happen.

When you make a prediction, check the community stats to see where you land. If your prediction is an outlier, might there be something you're overlooking that others have seen? Or do you have special insight that others are lacking? Either way, it might be a good idea to join the discussion in the comments.

Embed this question

You can use the below code snippet to embed this question on your own webpage. Feel free to change the height and width to suit your needs.