Space · Pluto's Status

Why is Pluto not a planet?

A digital illustration of a starry space scene featuring a bright star and two large, shadowed celestial bodies resembling planets or moons. — Lunar and Planetary Institute from Houston, TX, USA. CC BY 2.0. Original.

The short answer

Pluto is no longer classified as a planet because it does not meet all three criteria set by the International Astronomical Union (IAU)¹². While it orbits the sun and has enough mass to be spherical, it has not cleared the neighborhood around its orbit²³. The IAU voted to remove it from the official list based on this specific requirement¹. Instead, Pluto is now designated as a dwarf planet²³. This new classification was intended to create a distinct class of solar system objects².

What the evidence shows

The New Definition

In August 2006, the IAU wrote a new definition for a planet². A planet must orbit the sun, be massive enough to be spherical, and clear its orbital neighborhood¹².

Pluto meets the first two criteria but fails the third². Objects that meet the first two but not the third are called dwarf planets²³.

The Discovery of Eris

The discovery of Eris in 2005 appeared to be a 10th planet because it is larger than Pluto and has a moon¹. Eris has an orbit inclined 45 degrees relative to the orbital plane of Earth¹.

If both Eris and Pluto were planets, many other bodies would also have to be included¹. Many Kuiper Belt objects are larger than Pluto and have their own moons¹.

Orbital Dominance

Many astronomers interpret clearing the neighborhood to mean a planet is gravitationally dominant in its region³. A planet is a world with dramatically more mass than anything else orbiting near it³.

Some scientists argue the requirement to clear an orbit is irksome because it does not just depend on the body itself².

Our take

This part is our own interpretation, not a sourced finding.

Pluto's status depends on its neighbors rather than just its own physical properties². A looser definition that accepts Eris and Ceres would swell the number of planets to over 20¹.

What we can't say for sure

Some scientists argue that the requirement to clear an orbit is irksome because it does not just depend on the body itself².

Sources

Peter Zachar, Kenneth S Kendler. The removal of pluto from the class of planets and homosexuality from the class of psychiatric disorders: a comparison. Philosophy, Ethics, and Humanities in Medicine : PEHM, 2012. research paper, accessed 1 November 2026.
15 years ago, Pluto became a dwarf planet. Does that move still make sense?| Science News. Science News. other website, accessed 1 November 2026.
What Is A Planet? | The Planetary Society. The Planetary Society. other website, accessed 1 November 2026.

Published 1 November 2026. Sources last checked 1 November 2026. Prepared with automated tools and checked against the sources named here: how we work. Spotted a mistake? Tell us.

Surprise me