The International Astronomical Union (IAU) voted in August 2006 on a formal definition for the term “planet,” ending decades of informal classification. This decision resulted in Pluto’s reclassification as a “dwarf planet,” triggering controversy among planetary scientists and astronomers. The debate highlights a fundamental disagreement: should a planet be defined by its location and orbital dynamics (the IAU’s dynamical definition) or by its intrinsic physical characteristics (the geophysical definition)?
The Three Criteria Established in 2006
The formal definition adopted by the IAU established three specific conditions for an object to be considered a planet. The first criterion requires the celestial body to orbit the Sun and not be a satellite of another planet. The second condition states that the object must possess enough mass for its own gravity to overcome rigid body forces, pulling it into a shape of hydrostatic equilibrium (nearly round).
The third, and most contentious, criterion is that the object must have “cleared the neighborhood” around its orbit. This means the object must be gravitationally dominant enough to have either ejected or accumulated all other small bodies in its orbital path. Objects that satisfy the first two criteria but fail the third are classified as “dwarf planets,” a category that includes Pluto, Eris, and Ceres.
The Controversy Surrounding Orbital Dominance
The third criterion, often called orbital dominance, is the main objection for many astronomers. Critics argue this dynamical requirement is problematic because it makes classification dependent on an object’s location rather than its inherent nature. For instance, an Earth-sized object placed in the distant Kuiper Belt would likely not be massive enough to clear that vast, crowded region and would therefore fail the test.
The concept of “clearing the neighborhood” is considered vague and location-dependent. An object’s ability to clear its orbit decreases significantly the farther away it is from the Sun, making the definition non-uniform across the solar system. Pluto fails this test because it resides in the densely populated Kuiper Belt, sharing its orbital zone with many other objects. Critics point out that this effectively penalizes objects for residing in a crowded region of space.
The Argument for a Geophysical Definition
Many planetary scientists, including Alan Stern, advocate for an alternative, intrinsic definition. This viewpoint, often called the geophysical definition, is based purely on the object’s internal properties and physical state. Proponents argue that a planet should be defined solely by whether it is massive enough to be rounded by its own gravity, achieving hydrostatic equilibrium.
This definition focuses on the object’s physics and intrinsic nature. It argues that a world large enough to be spherical—and therefore likely to be geologically active and internally differentiated—should be called a planet, regardless of its orbital environment. Under this model, Pluto and all other dwarf planets would be considered planets because they meet the hydrostatic equilibrium threshold. This perspective is considered more useful as it classifies objects based on what they are, not where they are.
The Educational and Classification Impact
The ongoing debate has consequences for both scientific classification and public education. The IAU’s dynamical definition is difficult to apply to the thousands of exoplanets because observers cannot easily determine if a distant object has “cleared its orbit.” This limitation makes the current rule largely Sun-centric and less universal.
The controversy has also created confusion in educational settings, making it difficult to teach a definition based on complex orbital dynamics. The simpler, intrinsic definition—is it round and does it orbit a star?—is often seen as more intuitive and easier to grasp for students and the public. Ultimately, the disagreement matters because a planet’s definition shapes research priorities, influences how the solar system is understood, and affects the overall communication of science to a general audience.