For 76 years, Pluto was the ninth planet of our solar system. Schoolchildren learned it as the cold, distant world at the edge. Mnemonics like "My Very Excellent Mother Just Served Us Nine Pizzas" taught the planets. Then in 2006, the International Astronomical Union (IAU) voted to demote Pluto to "dwarf planet" status. The decision was controversial and remains so today — but it was based on consistent scientific reasoning about what makes a planet a planet.
The Short Answer
Pluto was reclassified as a dwarf planet in 2006 because it doesn't meet the IAU's third criterion for planetary status: clearing its orbital neighborhood of other debris. Pluto shares its orbital region with many other objects in the Kuiper Belt. Astronomers established three criteria: (1) orbits the Sun, (2) has enough mass to be roughly spherical, (3) has cleared its orbital path. Pluto meets the first two but not the third.
The Three Criteria
The 2006 IAU definition of a planet:
- Orbits the Sun (not another body)
- Has sufficient mass to assume hydrostatic equilibrium (basically, is large enough that gravity makes it nearly round)
- Has cleared the neighborhood around its orbit (its gravity dominates the local orbital region)
Pluto meets criteria 1 and 2 but fails 3. The eight other planets in our solar system meet all three.
Why Pluto Doesn't Clear Its Orbit
Pluto orbits in the Kuiper Belt — a ring of icy bodies beyond Neptune. The region contains thousands of objects, including some comparable in size to Pluto itself:
- Eris: about Pluto's size (initially thought larger)
- Haumea: about half Pluto's size, with rapid rotation and rings
- Makemake: about 3/4 Pluto's size
- Quaoar, Orcus, Sedna, and many others
Pluto's gravity hasn't cleared this neighborhood — it's one object among many. By contrast, Earth's gravity has cleared its orbital path of essentially all other significant objects (Earth's mass is millions of times that of all other objects in its orbit combined).
The Discovery of Eris
The immediate trigger for the IAU's 2006 vote was the 2005 discovery of Eris. Eris is essentially the same size as Pluto (slightly smaller in diameter but more massive). This created a dilemma: should astronomers add Eris as a 10th planet? Or perhaps Sedna and others? Or was the very concept of "planet" overdue for revision?
The IAU chose to redefine "planet" rather than continually add more planets as similar-sized objects were discovered.
What Makes Pluto Different From the Eight Planets
Several characteristics distinguish Pluto from the main planets:
- Size: Pluto is smaller than Earth's Moon (2,377 km diameter vs Moon's 3,474 km).
- Mass: Only 0.2% of Earth's mass.
- Orbit: Highly elliptical (sometimes closer to the Sun than Neptune) and significantly tilted relative to the main solar system plane.
- Composition: Ice-rich like other Kuiper Belt objects; main planets are rocky or gaseous.
- Location: Embedded in the Kuiper Belt, surrounded by similar objects.
The Discovery of Pluto
Pluto was discovered in 1930 by Clyde Tombaugh at Lowell Observatory in Arizona. The search was inspired by perceived irregularities in Neptune's orbit, suggesting a "Planet X" affecting it. Tombaugh systematically compared photographs of the same sky region looking for moving objects, eventually spotting Pluto. (The orbital irregularities turned out to be measurement errors, not Pluto's influence — Pluto is far too small to perturb Neptune.)
Pluto's Naming
Pluto was named after the Roman god of the underworld. The name was proposed by 11-year-old Venetia Burney from Oxford, England. Her grandfather forwarded the suggestion to astronomers, who chose it. The first two letters (P-L) honor Percival Lowell, who started the Planet X search but died before Pluto's discovery.
What Is a Dwarf Planet?
The "dwarf planet" category was created in 2006. A dwarf planet:
- Orbits the Sun
- Has enough mass to be roughly spherical
- Has NOT cleared its orbital neighborhood
- Is not a satellite (moon) of another body
Currently recognized dwarf planets: Pluto, Eris, Haumea, Makemake, Ceres. Many other candidates exist but haven't been formally recognized.
The 2015 Flyby
NASA's New Horizons spacecraft flew past Pluto in July 2015, providing the first close-up images. Discoveries surprised scientists:
- Geologically diverse surface with mountains, glaciers, and possible cryovolcanoes
- The famous "heart" (Tombaugh Regio) — a bright nitrogen ice plain
- Mountains made of water ice — like Earth's mountains but harder
- Five moons (Charon, Nix, Hydra, Kerberos, Styx)
- Atmosphere of nitrogen with traces of methane and CO
- Evidence of geological activity surprisingly recent
The flyby revealed Pluto to be far more complex than expected — leading some to argue it should be considered a planet on its own merits.
Pluto's Moons
Pluto has five known moons:
- Charon: Half Pluto's diameter — Pluto-Charon is sometimes called a "double dwarf planet" because they orbit a common point.
- Styx, Nix, Kerberos, Hydra: Small irregular moons.
The Pluto-Charon system is unique because Charon is so massive compared to Pluto that the center of mass is between them, outside Pluto.
The Controversy
The reclassification remains controversial:
- Only about 4% of IAU members voted; many astronomers were absent
- The definition's "cleared the neighborhood" criterion is debated
- Many scientists (especially planetary scientists, vs astronomers) disagree with the decision
- Schools have to update textbooks (now showing 8 planets)
- Public sentiment often remains pro-Pluto-as-planet
Alan Stern, principal investigator of the New Horizons mission, is among the most vocal critics. He argues the definition is arbitrary and that Pluto should be considered a planet by other criteria.
Alternative Definitions Proposed
Some have proposed different definitions:
- Geophysical definition: "Any rounded object orbiting any star, brown dwarf, or stellar remnant" — would make many objects planets.
- Composition-based: Focus on physical structure rather than orbital position.
- Mixed criteria: Various blended approaches.
The IAU's definition remains official, but the scientific community continues debating it.
Are There Really Just 8 Planets?
By the IAU definition, yes:
- Inner rocky planets: Mercury, Venus, Earth, Mars.
- Outer gas/ice giants: Jupiter, Saturn, Uranus, Neptune.
Beyond these eight, several dwarf planets are recognized, and many more objects await categorization in the outer solar system.
The Search for Planet Nine
Some scientists have proposed a "Planet Nine" — a hypothetical large planet in the far outer solar system, suggested by unusual orbital patterns of distant Kuiper Belt objects. If found, it would presumably meet all three planet criteria. Searches are ongoing but no Planet Nine has been confirmed.
Pluto in Popular Culture
Pluto's demotion provoked strong public reaction:
- Bumper stickers: "Pluto is a planet"
- State legislatures (Illinois, New Mexico) passed pro-Pluto resolutions
- The 2014 Stephen Hawking quote: "Pluto is not even officially a planet"
- Many continue to consider Pluto a "real" planet despite the IAU
Key Facts
- Pluto was demoted in 2006 because it shares its orbital region with many similar objects.
- The IAU planet definition has three criteria; Pluto meets two of three.
- The discovery of Eris (similar to Pluto) triggered the reclassification.
- Pluto is now classified as a dwarf planet along with Eris, Haumea, Makemake, and Ceres.
- The 2015 New Horizons flyby revealed surprising geological complexity.
Fun Facts
- Pluto was named by an 11-year-old British girl named Venetia Burney.
- Pluto's moon Charon is so large that Pluto-Charon orbits a common point in space between them.
- Only about 4% of IAU astronomers voted on Pluto's reclassification.
- Pluto is smaller than Earth's Moon.
- The "Pluto is not a planet" decision remains controversial among scientists.
New Horizons Discoveries
The 2015 New Horizons flyby of Pluto revolutionized our understanding. The spacecraft revealed Pluto's "heart" (Tombaugh Regio) with its bright nitrogen ice plain. Mountains made of water ice rise 3,500 m high. Apparent recent geological activity surprised researchers — Pluto was expected to be a frozen, dead world. The atmosphere extends much further than predicted. Hazes layer above the surface. Some features may indicate active cryovolcanism (ice volcanoes). The dwarf planet has 5 moons, with Charon being almost half Pluto's diameter — making Pluto-Charon a "double dwarf planet" system. Each discovery added complexity to Pluto's story.
The Bottom Line
Pluto was reclassified as a dwarf planet in 2006 because it doesn't meet the IAU's criterion of clearing its orbital neighborhood. It shares its region with thousands of other Kuiper Belt objects, including some comparable in size. The decision was controversial and remains contested, but the fundamental scientific issue — that Pluto is one of many similar objects rather than the dominant body in its orbit — is real. Whether you call it a planet or a dwarf planet, Pluto remains a fascinating world deserving of exploration and wonder.