What was the old definition of a planet?

The concept of a planet, a body orbiting our Sun, was understood for centuries through observation and shared consensus rather than a strict, universally agreed-upon scientific definition. ^[1][6] Before the famous 2006 meeting of the International Astronomical Union (IAU), what qualified as a planet often depended on who you asked or which historical precedent they followed. ^[6] This historical ambiguity meant the classification system was more descriptive than definitively technical, allowing astronomers to classify major solar system bodies based largely on their apparent nature as distinct, large, non-stellar objects. ^[5]

# Ancient Roots

The very term we use traces its origins back to the ancient Greeks, who called these moving lights asteres planetai, meaning "wandering stars". ^[5] This name perfectly captured their appearance to early sky-watchers: unlike the fixed background stars, these lights visibly moved across the celestial sphere relative to one another. ^[5] For millennia, the count settled on seven—Mercury, Venus, Mars, Jupiter, and Saturn, plus the Sun and the Moon, which were then also considered planets in a geocentric model. ^[5] When the heliocentric model took hold, the Sun and Moon dropped from the planet list, leaving the five known moving bodies. ^[5]

# Discovery Precedent

The establishment of what constituted a planet evolved slowly, often driven by new telescopic discoveries. ^[5] Once a new body was found orbiting the Sun and it appeared substantial, it was generally added to the roster of planets. ^[6] The discovery of Uranus in 1781 by William Herschel was a major expansion of this roster, followed by Neptune in 1846. ^[5] These additions demonstrated that the category could expand beyond the classical five, provided the object met the established, albeit unwritten, criteria: orbit the Sun and possess significant mass. ^[6]

This historical pattern reveals something important about the older understanding: it was primarily an observational classification. ^[6] If an object was clearly doing what the other acknowledged planets did—orbiting the Sun and appearing as a distinct point of light (even if magnified by a telescope)—it was accepted into the club. ^[5] There was no explicit discussion about its orbital neighborhood or its internal structure; its status was largely granted by its general size and its path around the central star. ^[6] For instance, early in the process of classification, objects like Ceres were initially called a planet when discovered in 1801, but later, as more similar, smaller bodies were found in the asteroid belt, they were reclassified as asteroids. ^[6] This earlier shift hints at the difficulty inherent in classification without firm rules: where does a "planet" end and a "very large asteroid" begin?^[6]

What is the best definition of planet?

# Missing Rigor

The core issue with the pre-2006 system was the lack of a formal, binding definition, especially as observational technology improved. ^[1][6] While most astronomers agreed on the eight "classical" planets, the boundary was fuzzy. ^[6] One way to implicitly define a planet was by what it wasn't: it wasn't a star, and it wasn't a satellite (moon). ^[6] The discovery of bodies like Pluto, which was added in 1930, further complicated things, as Pluto seemed smaller and less massive than the giant planets. ^[5] In fact, before 2006, there were discussions about whether Pluto should retain its status, even before the discovery of Eris. ^[6]

If we look at the situation from a modern perspective, the pre-2006 system operated on an implicit assumption of hydrostatic equilibrium—that the object was large enough for its own gravity to pull it into a round shape. ^[1][6] However, this was rarely the defining rule; it was more of a general characteristic shared by the accepted eight. ^[6] The fact that objects like Pluto were accepted while smaller, rounder Kuiper Belt Objects (KBOs) were not demonstrates a system reliant on an arbitrary threshold of size or distance, rather than dynamic dominance. This reliance on vague criteria meant that the classification was heavily influenced by the history of discovery rather than objective physical criteria. ^[6]

Consider the state of play before the critical 2006 decision:

This table highlights how classification was reactive. Ceres was demoted because more objects like it were found, suggesting a "belt" rather than a singular, unique entity. ^[6] Pluto’s status was debatable because it seemed more like a large KBO than a giant world like Jupiter, even if it was round. ^[1] The lack of the third, crucial criterion—clearing its neighborhood—left Pluto's status perpetually under review. ^[3]

# The Pluto Context

The impetus for formalizing the definition was the accelerating discovery rate of objects beyond Neptune, particularly in the Kuiper Belt. ^[1][3] When Eris was discovered in 2005, initial measurements suggested it was actually more massive than Pluto. ^[3] This discovery created an immediate crisis: if Pluto was a planet, Eris must be one too. Furthermore, if Eris was a planet, then dozens, perhaps hundreds, of other large KBOs would also need to be added to the planetary count. ^[1][3] Astronomers were facing the prospect of having a solar system with potentially dozens of planets, a scenario that didn't fit the traditional scientific concept. ^[1]

The very existence of a distinct, populous zone of icy bodies beyond Neptune made the old, observational grouping obsolete. ^[1] The scientific community needed a way to distinguish between the handful of large, dominant worlds that gravitationally sculpt their orbits, and the vast populations of smaller bodies that share those orbits. ^[3] The old definition—whatever one inferred it to be—could no longer manage the growing census of solar system objects. ^[6] The choice presented to the IAU in 2006 was stark: either expand the definition to include many new objects, or create a new, more restrictive definition for the "classic" eight. ^[3]

What planets have humans landed probes on?

# Classification Shift

To address this, the IAU created a working group to establish a precise, scientific definition, which was voted upon in Prague in August 2006. ^[3][1] The old framework, which relied on subjective size estimates and historical precedent, was officially replaced by a three-part test for a celestial body to be considered a planet in our solar system. ^[1][3]

The old understanding was effectively defined by failing to meet the new criteria, or by the absence of the criteria altogether. The new definition, which marks the end of the old way of thinking, requires a body to:

1. Be in orbit around the Sun. ^[1]
2. Have sufficient mass for its self-gravity to assume a nearly round shape (hydrostatic equilibrium). ^[1]
3. Have "cleared the neighborhood" around its orbit. ^[1][3]

It is the third point where Pluto and all smaller KBOs failed under the new rules. ^[3] The old definition simply did not contain this dynamic requirement—the need to be the gravitationally dominant object in its orbital path. ^[6] A planet, under the old system, was simply a major orbiter; under the new, it must be a master of its orbital zone. This shift signifies a move from a descriptive astronomical label to a dynamic dynamical definition, rooted in orbital mechanics rather than just visual appearance or size estimation. ^[3] While the discovery of large, round, Sun-orbiting objects continues, the requirement that they must have cleared their zone ensures that the eight classic bodies retain their unique status, while others are relegated to the "dwarf planet" category, a classification created precisely because the old system couldn't handle the abundance of large, non-dominant bodies. ^[1][3]