What the Planet Definition Changed
The 2006 IAU redefinition of planet reshaped solar system classification. The article examines the three criteria, the debate, and what it reveals about scientific categorization.
The word “planet” once meant “wandering star” — a label applied to seven objects visible to the naked eye: Mercury, Venus, Mars, Jupiter, Saturn, plus the Sun and the Moon. The count changed when telescopes revealed thousands of smaller objects orbiting the Sun. The asteroid Ceres was discovered in 1801. By the 1850s, hundreds of asteroids had been catalogued, and the word planet had been narrowed to the eight large bodies that dominate their orbital neighborhoods.
For most of the twentieth century, eight planets was the number taught in schools. That count changed in 2006 when the International Astronomical Union adopted a formal definition for the first time. An object must orbit the Sun. It must have enough mass for its self-gravity to pull it into a round shape. It must have cleared the neighbourhood around its orbit.
Pluto failed the third criterion. It was reclassified as a dwarf planet. The number of planets dropped to eight. The vote took place at the IAU General Assembly in Prague, where 424 astronomers participated.
The decision was controversial. It was not simply about Pluto. It was about what it means for a scientific body to draw a boundary around a word that has been used for centuries without a precise definition.
The three criteria
The definition was adopted as Resolution 5A at the 2006 General Assembly. A planet is a celestial body that satisfies all three of the following conditions:
It is in orbit around the Sun. This restricts the definition to the solar system. A separate category for exoplanets was developed later.
It has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium shape — essentially, a round shape. The criterion does not require perfect sphericity. A rotating body will bulge at the equator.
It has cleared the neighbourhood around its orbit. The object has become dominant in its orbital zone, either by absorbing smaller bodies, ejecting them, or gravitationally controlling them.
A dwarf planet satisfies the first two criteria but not the third. The resolution also introduces the category “small solar-system bodies,” which covers everything that does not qualify as a planet or a dwarf planet.
How the definition came to be
The definition was the result of growing pressure. The discovery of Eris in 2005 — a body in the Kuiper Belt more massive than Pluto — created an immediate crisis. If Pluto qualified as a planet, so did Eris. And a growing number of Kuiper Belt objects were approaching planetary size. Without a definition, the solar system could soon contain dozens of planets.
The IAU had long operated without a formal definition. The working group that drafted the resolution considered several proposals. One approach focused on geophysical properties: an object would be a planet if it was geologically active, regardless of its orbital environment. Another approach tied the definition to orbital dynamics. The final proposal combined both — a shape requirement for geophysical maturity and a clearing requirement for dynamical dominance.
The resolution was debated at the General Assembly in Prague. Only 424 astronomers voted on the final definition, out of a membership of over 1,800. This fact has been cited by critics as evidence that the vote did not represent the broader astronomical community.
The vote on Resolution 5A passed as part of a broader package of decisions. Some were approved by a separate vote of the full IAU membership, while others were adopted by the assembly attendees.
The controversy
The most heavily debated criterion is “cleared the neighbourhood.” Several astronomers argued that the phrase is vague and that the criterion would disqualify planets under different circumstances.
Alan Stern, the principal investigator of the New Horizons mission to Pluto, has argued that the definition is impossible to apply consistently, noting that “it is impossible and contrived to put a dividing line between dwarf planets and planets.” The clearing criterion depends on an object’s location. A body that has cleared its orbit in the inner solar system would not have done so in the densely populated Kuiper Belt, even if it had the same mass. This means the same object could be a planet in one context and a dwarf planet in another.
Stern and other critics have proposed defining a planet entirely by its physical properties. By that standard, Pluto, Eris, Ceres, and many other bodies would be planets because they are round. The number of planets would be large — potentially over a hundred.
Proponents of the clearing criterion argue that it distinguishes a planet from a large asteroid or Kuiper Belt object.
What the vote actually decided
Resolution 5A formally recognized five dwarf planets: Ceres, Pluto, Eris, Haumea, and Makemake. Several other objects — Sedna, Quaoar, Orcus, and Gonggong — were widely expected to qualify once their shapes could be measured.
The resolution also established that the term “planet” applies to eight objects in the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto, which had been classified as the ninth planet since its discovery in 1930, was moved to the new dwarf planet category.
A separate resolution defined exoplanets and sub-brown dwarfs. An object that orbits a different star is an exoplanet. Objects more massive than approximately 13 times the mass of Jupiter are classified as brown dwarfs, regardless of what they orbit. The mass threshold distinguishes bodies that can fuse deuterium from those that cannot.
What this reveals about scientific categories
Scientific categories are not discovered in nature. They are drawn by convention, often under pressure from discoveries that make existing categories inadequate. The word “planet” had no formal definition for most of its history. The discovery of new objects — asteroids, Kuiper Belt objects, exoplanets — forced the community to decide where the boundary lay.
Most scientific classification involves human judgment about what features matter most. The IAU definition prioritizes orbital dynamics over geophysical properties. Stern’s alternative prioritizes geophysical properties over orbital dynamics. Both are defensible. Neither is inevitable.
The people who work most closely with the objects being classified often have a different sense of where the boundary should fall than the people who define the category for everyone else. Stern’s mission to Pluto was designed to study a dwarf planet as if it were a planet. His objection to the definition is not purely semantic. It reflects a difference in how the objects are understood.
Why the definition matters
The definition affects how the International Astronomical Union catalogs objects, how new discoveries are named, and how mission priorities are allocated. It also illustrates a broader challenge in science communication. A definition debated in a specialized assembly is now taught in schools worldwide. The simplification that made it teachable — “a planet is an object that orbits the Sun, is round, and has cleared its orbit” — obscures the debate that produced it. Readers who learn the three criteria without learning why the third criterion was chosen may conclude that the classification is arbitrary. It is, in a sense. So is every classification that depends on human judgment about what features are most important.
What remains unresolved
The exoplanet resolution uses different criteria than the solar system definition. It defines an exoplanet as any object below the deuterium-fusion threshold that orbits a star. It does not require the object to be round or to have cleared its orbit. The inconsistency reflects the different ways astronomers encounter the two populations.
New objects are discovered in the Kuiper Belt and the scattered disk with increasing regularity. Several candidates are likely to be confirmed as dwarf planets once their shapes can be measured. The number of recognized dwarf planets will continue to grow.
The fundamental tension — physical properties versus orbital dynamics — has not been resolved. The IAU definition favors one side. Critics favor the other. The debate will continue as long as astronomers classify objects and as long as new discoveries challenge existing categories.
The planet definition is not a permanent statement about nature. It is a working agreement among astronomers at a particular moment in time. The agreement has held for two decades. It may hold longer. It may not. That is not a weakness. It is how scientific categories work.
Primary sources
- International Astronomical Union. (2006). Resolution 5A: Definition of a planet in the Solar System. IAU General Assembly, Prague.
- International Astronomical Union. (2006). Resolution C6: On the definition of a planet and the construction of the solar system. IAU General Assembly, Prague.
- Stern, S. A. (2007). On the inherent impossibility of classical planet classification. The Astrophysical Journal, 134(5), 1744-1747.