Why is Pluto no longer a planet?
Discovered in 1930, Pluto was counted as the Solar System's ninth planet for 76 years. In 2006 the International Astronomical Union adopted its Resolution B5, a formal definition of 'planet', and its Resolution B6 reclassified Pluto as a 'dwarf planet'.
5 established, 1 disputed, 2 debunked, 0 unresolved. We do not pick an answer; the evidence places each claim. How we decide
17 citations from 7 independent sources - 10 echo the same voices and count once. Graded A1-D1.
Added 3 Jul 2026 - updated 9 Jul 2026
What the four states mean
- Establishedsupported, uncontested
- Disputedcredible sources disagree
- Debunkedrefuted by the evidence
- Unresolvedno settled answer
Established 5supported, uncontested
Under the IAU definition Pluto fails only the third criterion: it has not cleared its orbital neighbourhood, because it shares the debris-rich Kuiper Belt with a vast population of other bodies.▰▰▰▱▱ likely2 points
Supports- A 'dwarf planet' is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.IAU Resolution B5: Definition of a Planet in the Solar System (2006) authoritative
- From a dynamical standpoint, our solar system clearly contains 8 überplanets and a far larger number of unterplanets, the largest of which are Pluto and Ceres.Stern & Levison 2002, Regarding the Criteria for Planethood and Proposed Planetary Classification Schemes primary
Pluto does meet the first two IAU criteria: it orbits the Sun and is massive enough for its own gravity to pull it into a nearly round shape (hydrostatic equilibrium).▰▰▰▱▱ likely2 points
Supports- (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shapeIAU Resolution B5: Definition of a Planet in the Solar System (2006) authoritative
- the planetary bodies in heliocentric orbit include the terrestrial planets, ice giants, giant planets, large asteroids, and (if KBO albedos are indeed near 4% as we assumed) large KBOs like PlutoStern & Levison 2002, Regarding the Criteria for Planethood and Proposed Planetary Classification Schemes primary
The 2005 discovery of Eris (then designated 2003 UB313), a distant body initially reported as certain to be larger than Pluto, triggered the debate that led to Pluto's reclassification.▰▰▰▱▱ likely2 points
Supports- The object is sufficiently bright that for all reasonable values of the albedo it is certain to be larger than Pluto.Brown, Trujillo & Rabinowitz 2005, Discovery of a planetary-sized object in the scattered Kuiper belt (ApJ 635:L97) primary
- The recent discovery of Eris (formerly known as UB313), a KBO larger than Pluto (Bertoldi et al. 2006), intensified the debate.Soter 2006, What is a Planet? (Astronomical Journal 132:2513) primary
Under the current IAU definition the Solar System officially has eight planets, Mercury through Neptune.▰▰▰▱▱ likely2 points
Supports- The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.IAU Resolution B5: Definition of a Planet in the Solar System (2006), footnote 1 authoritative
- Theoretical and observational measures of dynamical dominance reveal gaps of four to five orders of magnitude separating the eight planets of our solar system from the populations of asteroids and comets.Soter 2006, What is a Planet? (Astronomical Journal 132:2513) primary
In 2006 the International Astronomical Union adopted a formal three-part definition of 'planet' and reclassified Pluto as a 'dwarf planet', the prototype of a new category of trans-Neptunian objects.▰▰▱▱▱ likely2 points
Supports- A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.IAU Resolution B5: Definition of a Planet in the Solar System (2006) authoritative
- Pluto is a 'dwarf planet' by the above definition and is recognized as the prototype of a new category of Trans-Neptunian Objects.IAU Resolution B6: Pluto (2006) authoritative
Disputed 1credible sources disagree
Whether the IAU's orbit-clearing criterion is the correct basis for defining a planet is disputed: the IAU and some researchers hold that dynamical dominance is the meaningful divide, while a faction of planetary scientists (including Stern) reject it and propose a geophysical definition that would restore Pluto.▰▰▰▱▱ unlikely2 against3 points
Supports- A planet is an end product of disk accretion around a primary star or substar. I quantify this definition by the degree to which a body dominates the other masses that share its orbital zone.Soter 2006, What is a Planet? (Astronomical Journal 132:2513) primary
Against- The planet definition adopted by the IAU in 2006 [1] is technically flawed, for several reasons.Runyon, Stern, Lauer, Grundy, Summers & Singer 2017, A Geophysical Planet Definition (LPSC XLVIII, 1448) analysis
- these purely dynamical criteria cannot be an intrinsic aspect of a body, but is necessarily dependent on the environment and dynamical context in which a body finds itself.Stern & Levison 2002, Regarding the Criteria for Planethood and Proposed Planetary Classification Schemes primary
Debunked 2refuted by the evidence
Eris is larger than Pluto. Early measurements after Eris's discovery indicated it was the bigger body, but NASA's 2015 New Horizons flyby found Pluto to be larger in diameter, though Eris remains slightly the more massive of the two.Refuted by
Pluto is larger than all other known solar system objects beyond the orbit of Neptune.
refuted1 against2 pointsThe claim rested on- The object is sufficiently bright that for all reasonable values of the albedo it is certain to be larger than Pluto.Brown, Trujillo & Rabinowitz 2005, Discovery of a planetary-sized object in the scattered Kuiper belt (ApJ 635:L97) primary
Refuted by- Pluto is larger than all other known solar system objects beyond the orbit of Neptune.How Big Is Pluto? New Horizons Settles Decades-Long Debate - NASA authoritative
Pluto lost its planet status because it is too small. In fact size is not one of the IAU criteria; Pluto was reclassified for failing the orbit-clearing test, which even a much larger body in the Kuiper Belt would fail.Refuted by
even an Earth sized object in the Kuiper Belt would not clear its zone.
refuted1 against2 pointsThe claim rested on- In a nutshell, Pluto is considered to not be a planet because of its size, its shape, its orbit and the discovery of many other bodies just like it.Why Pluto is not a planet - BBC Sky at Night Magazine summary
Refuted by- even an Earth sized object in the Kuiper Belt would not clear its zone.Runyon, Stern, Lauer, Grundy, Summers & Singer 2017, A Geophysical Planet Definition (LPSC XLVIII, 1448) analysis
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