Planet Definition Proposal

As published in the Astronomical Journal (2015)


The IAU definition of 'planet' needs improvement because it is neither quantitative nor general. I proposed a simple criterion that can be used to quantify, generalize, and simplify the definition. This criterion makes it possible to immediately classify 99% of all known exoplanets. All 8 planets and all classifiable exoplanets satisfy the criterion.


Planet test applied to solar system bodies

The figure below shows the criterion applied to the solar system (objects above the bold line are planets according to this criterion): Solar System

Planet test applied to objects in the solar system. All 8 planets have a mass that exceeds the mass required to clear the corresponding orbital zone.

Planet test applied to the putative Planet Nine

Would Planet Nine pass the planet test? The answer is yes.


  • Poster describing the proposal.
  • Peer-reviewed paper describing the proposal: J. L. Margot. A Quantitative Criterion for Defining Planets. Astronomical Journal 150, 185, 2015.