[DSRP Evidence](https://dsrpevidence.org/)

# What is a Planet?

## Details

**Authors** Soter, Steven

**Year** 2006

**Publisher** The Astronomical Journal

**Kind of work** article

**Discipline** Astronomy

**Secondary disciplines** Physics

**Applied** false

[Read it at the publisher](https://doi.org/10.1086/508861) 
10.1086/508861

## In authors' words

### What they set out to do (purpose)

To propose a definition of a planet based on formation and dynamics rather than on size or shape.

### Who or what was studied (sample)

The eight planets and selected minor bodies of the solar system, and 164 catalogued exoplanetary systems.

### How they did it (methods)

The author computes a planetary discriminant (a body's mass divided by the aggregate mass of other bodies sharing its orbital zone) and a scattering parameter for solar-system bodies, and examines exoplanet systems for orbital intersections and resonances.

### What they found (results)

The discriminant shows a gap of four to five orders of magnitude between the eight planets and the asteroids, comets and Kuiper belt objects (Mars about 5,100; Ceres about 0.33; Pluto about 0.07). A threshold of 100 separates the two groups, though any value from about 10 to 1,000 would serve. Mercury and Pluto differ in mass by about 25-fold but in the discriminant by about a million-fold. The definition needs no upper or lower mass limit. The author rejects hydrostatic equilibrium as a criterion because the thresholds are arbitrary.

## Commentary

### In short

The author defines a planet by how strongly it dominates the other bodies in its orbital zone, and shows a wide numerical gap between the eight planets and everything else in the solar system.

### Response

The paper is an explicit exercise in drawing a category boundary, and it rejects intrinsic properties such as mass or shape in favor of a measure taken against the surroundings (D). A body's status depends on its relation to the other bodies in its orbital zone and its ability to clear them (R), and the author proposes a scheme nesting tertiaries within secondaries within primaries, with planets as end products of disk evolution (S). Caveats: the author does not name these as structures. The classification is the central claim of the paper, while the relational and nesting readings are the reviewer's. This record rests on the arXiv text and not on the journal version.

**Patterns it shows** D, S, R

Shows the structure is there

**Added** 2026-10-12

**How to cite this** Soter, Steven (2006). What is a Planet?. The Astronomical Journal.