Dwarf Planets Explained: The Three IAU Rules and Why 2026 ED138 May Be the Sixth

A dwarf planet, under the International Astronomical Union's 2006 definition, is a celestial body that orbits the Sun, is massive enough to be pulled into a nearly round shape by its own gravity, but has not cleared its orbital neighborhood of other objects. Five bodies currently hold that status — Ceres, Pluto, Haumea, Makemake and Eris — and a newly discovered trans-Neptunian object, 2026 ED138, is now a strong candidate to join them. Here is how the rules work and how each body measures against them.

Understanding Dwarf Planets: The IAU's Defining Criteria

In August 2006, the International Astronomical Union — the world organization of astronomers that holds naming and classification authority — settled on a three-part definition of a planet. According to NASA, a planet must do three things: orbit its host star, be mostly round, and be big enough that its gravity cleared away any other objects of similar size near its orbit around the Sun.

A dwarf planet satisfies the first two requirements but fails the third. As the Laboratory for Atmospheric and Space Physics at the University of Colorado summarizes, drawing on NASA's formulation, a dwarf planet is a celestial body that "is in orbit around a star, but is not itself a satellite," "has sufficient mass so it is nearly spherical in shape," and "has not cleared the neighborhood around its orbit." The distinction between a planet and a dwarf planet therefore comes down entirely to the region surrounding each body: a planet has swept its orbital zone clean, while a dwarf planet shares its path with debris and neighbors.

Dwarf Planet Classification Checklist

The table below applies the three IAU criteria to each of the five recognized dwarf planets and to 2026 ED138. The pattern is consistent: every recognized dwarf planet passes the first two tests and fails the third, which is precisely what separates it from a full planet. For 2026 ED138, the first two criteria are supported by initial analysis and size estimates, while the third is inferred from its trans-Neptunian location, where orbital clearing is highly improbable.

The three IAU criteria applied to the five recognized dwarf planets and candidate 2026 ED138
Celestial Body	Orbits the Sun?	Sufficient Mass for Round Shape?	Has Cleared Its Orbital Neighborhood?
Ceres	Yes	Yes	No — shares the asteroid belt
Pluto	Yes	Yes	No — Kuiper Belt debris remains
Haumea	Yes	Yes	No
Makemake	Yes	Yes	No
Eris	Yes	Yes	No
2026 ED138 (candidate)	Yes — trans-Neptunian orbit at ~46 au	Expected, based on size estimates	No — expected, given its location beyond Pluto

Read the table row by row and the classification logic becomes visible: the five official bodies are dwarf planets not because they are small or irregular, but because each failed the single criterion — orbital clearing — that distinguishes a planet. The candidate row is necessarily provisional, resting on early measurements rather than confirmed characterization.