Why is Pluto not a planet anymore? The 2006 decision

Pluto is not a planet because in August 2006 the International Astronomical Union (IAU), the body that names things in space, defined a planet for the first time, and Pluto failed one of the three tests. That is why astronomers now call it a dwarf planet. It is round and it orbits the Sun, but it has not cleared its orbit: it shares its region with thousands of other icy bodies.

In short

• Since 2006, a planet must orbit the Sun, be massive enough to pull itself round, and have cleared the neighborhood around its orbit. Pluto passes the first two and fails the third.
• The discovery of Eris, a world beyond Pluto with more mass, forced the question: either it became the tenth planet, or Pluto was something else.
• Pluto didn't change, only the definition did, and some planetary scientists still argue the definition is wrong.

What are the rules for being a planet?

Until 2006 there was no official definition. The IAU's resolution, passed on 24 August 2006 at its meeting in Prague, set three conditions, as NASA summarizes them. A planet:

1. orbits the Sun;
2. has enough mass for its own gravity to pull it into a nearly round shape, called hydrostatic equilibrium;
3. has cleared the neighborhood around its orbit of other objects of similar size.

A body that meets the first two but not the third, and isn't a moon, is a dwarf planet. A second resolution passed the same day named Pluto the prototype of a new class of icy dwarf planets beyond Neptune, and the first dwarf planets were Pluto, Eris and Ceres, the largest body in the asteroid belt.

What does "clearing its orbit" mean?

Clearing the neighborhood means a planet's gravity dominates its zone. Over billions of years it has swept up, flung away or captured most other material near its path, so that it is by far the biggest thing there.

Pluto is not. It orbits in the Kuiper Belt, a ring of icy bodies beyond Neptune, and it crosses Neptune's orbit. Its orbit is set by Neptune, not the other way round: Pluto goes around the Sun twice for every three of Neptune's orbits, a pattern that keeps the two from ever colliding.

How lopsided the difference is

The astronomer Steven Soter put a number on clearing in a 2006 paper in the Astronomical Journal. He divided each body's mass by the total mass of everything else sharing its orbital zone. A planet should come out far above 1, and anything else at or below it.

Body	Its mass compared with everything else in its zone
Earth	about 1,700,000 times
Neptune	about 24,000 times
Mars	about 5,100 times
Ceres	about 0.33 times
Eris	about 0.10 times
Pluto	about 0.07 times

Read the last row: everything else in Pluto's zone weighs about 14 times as much as Pluto. The smallest planet on this measure, Mars, still outweighs its zone more than 5,000 times over. There is no borderline case: the weakest planet scores about 15,000 times higher than the strongest dwarf planet, Ceres.

Why did astronomers decide this in 2006?

Because they kept finding Plutos. The timeline:

• 1801: Ceres is discovered between Mars and Jupiter and counted as a planet. As dozens of similar bodies turn up, it is reclassified as an asteroid in the decades after.
• 1930: Clyde Tombaugh discovers Pluto at Lowell Observatory in Arizona. It is named the ninth planet.
• 1978: Pluto's large moon Charon is found, which shows that Pluto is far smaller than first thought, with about one-sixth the mass of Earth's Moon.
• 1992: The first Kuiper Belt object after Pluto and Charon is found. Over the next years, hundreds more follow.
• 2005: A team led by Mike Brown announces Eris, found in images from 2003. It is about the same size as Pluto and, later measurements showed, about 27% more massive.
• 16 August 2006: The IAU publishes a first draft definition that would have made 12 planets, adding Ceres, Charon and Eris. Astronomers object, since many more round Kuiper Belt objects would follow.
• 24 August 2006: The final resolution adds the clearing rule. Pluto becomes a dwarf planet.

Eris was the turning point. If Pluto was a planet, Eris had to be one too, and so, probably, would dozens more as surveys found them. Astronomers had to choose between a solar system with eight planets and one with an open-ended, growing list.

What did New Horizons find at Pluto?

NASA's New Horizons spacecraft flew past Pluto on 14 July 2015, the first close look at it. According to NASA's Pluto facts, Pluto is about 2,377 kilometers wide, about two-thirds the width of Earth's Moon, and takes about 248 years to orbit the Sun.

It turned out to be far more active than a dead ice ball. A bright, heart-shaped region holds a vast plain of frozen nitrogen with no craters, which means its surface is young and still renewing itself as the ice slowly churns. Mountains made of blocks of water ice rise kilometers high. It has a thin, hazy atmosphere and five known moons. For some planetary scientists, this complexity strengthened the case that Pluto deserves the name planet.

Still argued

Some planetary scientists reject the IAU definition. In 2017, Kirby Runyon, Alan Stern, who leads New Horizons, and colleagues proposed a geophysical definition: a planet is any body smaller than a star, never powered by fusion, that is massive enough to be round, wherever it orbits. That would count about 110 planets in the solar system, including Pluto and large moons such as Earth's. They argue a planet should be defined by what it is, not where it is. Defenders of the 2006 rule answer that dynamical dominance is what divides the solar system's bodies so sharply, as Soter's numbers show, and that eight is a clearer category for teaching. A 2019 study in Icarus by Philip Metzger and colleagues, Stern among them, argued that history supports the geophysical side: asteroids lost planet status over their physical differences, not their shared orbits. The IAU definition remains the official one.

Is Pluto's demotion a loss?

Not for science. A category is a tool, and Pluto is the same world either way. Being the prototype of the dwarf planets puts it at the head of a large family of icy worlds beyond Neptune, the outer edge of the solar system, most of them still barely studied. The debate itself shows how science works: definitions follow discoveries, and they get revised when new evidence makes an old word fit badly.

Pluto is also a reminder of scale. All eight planets and every dwarf planet together hold a tiny fraction of the mass of the solar system; the Sun holds more than 99% of it. And the solar system is one of billions of planetary systems in the universe.

Check yourself

• Without looking back: what are the three tests for a planet, and which one does Pluto fail?
• Why did the discovery of Eris force astronomers to make a decision?
• What is the main argument of the scientists who want Pluto counted as a planet again?

Sources

• What is a planet? — NASA Science
• Pluto facts — NASA Science
• Eris — NASA Science
• The IAU draft definition of planet and plutons — International Astronomical Union, 2006
• What is a planet? — Steven Soter, The Astronomical Journal, 2006
• A geophysical planet definition — Runyon and others, Lunar and Planetary Science Conference, 2017