Why Pluto isn't a planet (the actual argument)

The three criteria

Failing the third puts Pluto in the category of dwarf planet, alongside Eris, Makemake, Haumea and Ceres. Note that Ceres, in the asteroid belt, was itself briefly called a planet after its discovery in 1801 before being reclassified as an asteroid, so this has happened before.

What forced the decision

Not Pluto's size, though it's small. Smaller than our Moon. The trigger was Eris, found in 2005 and at first thought larger than Pluto. Later measurements found it slightly smaller across but about 27% heavier.

That created an unavoidable problem. If Pluto is a planet for being round and orbiting the Sun, so is Eris. And so, probably, are a great many other Kuiper Belt objects the surveys were starting to find.

Astronomers had a choice. Accept a solar system with a growing, open-ended list of planets, or write a definition that kept them out. They chose the definition, and Pluto landed on the wrong side of it.

So the clear framing isn't that Pluto was demoted for being inadequate. Pluto turned out to be the first-found member of a large population, and 'planet' was redefined to mean the dominant bodies, not every round one.

Pluto should stay demoted. The alternative is a hundred planets and a definition that means nothing.

The objection astronomers still raise

The 'cleared the neighbourhood' criterion is contested, and the objections aren't merely sentimental.

It isn't a property of the body alone. Whether an object clears its orbit depends on where it is. Earth placed in the Kuiper Belt wouldn't clear that region, so under a strict reading Earth's planethood depends on its location instead of its nature, which many find philosophically unsatisfactory.

No orbit is fully cleared. Earth shares its orbit with near-Earth asteroids and Jupiter has thousands of Trojan asteroids in its orbital path. The criterion is really about gravitational dominance, which is a matter of degree instead of a clean yes or no.

It only applies to our solar system. The 2006 definition specifies orbiting the Sun, which technically leaves thousands of confirmed exoplanets outside the definition of 'planet' entirely.

Some planetary scientists advocate a geophysical definition instead: anything massive enough to be round is a planet, regardless of what else shares its orbit. That would restore Pluto and add well over a hundred bodies, including several moons, our own Moon among them.

What Pluto is

Far more interesting than the classification argument, and the flyby of 2015 transformed our picture of it.

It has a thin nitrogen atmosphere that expands and freezes onto the surface as its highly elliptical orbit has it nearer and further from the Sun. It has mountains of water ice several kilometres high, and a vast plain of nitrogen ice showing convection cells: evidence of ongoing geological activity on a body nobody expected to be active.

Its largest moon, Charon, is so large relative to Pluto that the two orbit a common centre of mass lying outside Pluto's surface. They're arguably a binary system instead of a planet and satellite. Pluto has four smaller moons besides.

It also has an orbital resonance with Neptune, completing two orbits for every three of Neptune's, which is what prevents the two from ever colliding despite their crossing paths.

What astrology did about it

Very little, which is itself informative. Pluto was discovered in 1930 and modern astrologers assigned it rulership of Scorpio and a set of themes around power and transformation.

The 2006 reclassification didn't prompt astrologers to drop it. That's a reasonable position (astrological meaning isn't derived from the IAU's taxonomy) but it does highlight that the assignment was never dependent on Pluto's physical status in the first place.

The rulership question it created is covered in modern versus traditional rulers, and AsterNym computes Pluto's position while using traditional rulerships and labelling modern associations as associations.

Why is Pluto not a planet?

Under the 2006 IAU definition, a planet must orbit the Sun, be round under its own gravity, and have cleared the neighbourhood around its orbit. Pluto meets the first two but shares its region with many other Kuiper Belt objects, so it fails the third.

What caused Pluto to be reclassified?

The discovery of Eris in 2005, a body of comparable size and about 27% more massive. Accepting Pluto as a planet would have meant accepting Eris and potentially many other Kuiper Belt objects, so astronomers wrote a definition that excluded them instead.

Do astronomers disagree about the Pluto decision?

Yes. The 'cleared the neighbourhood' criterion depends on location instead of the body itself, no orbit is truly cleared, and the definition technically excludes all exoplanets. Some planetary scientists favour a geophysical definition where any body round under its own gravity counts.

Is Pluto a dwarf planet?

Yes, alongside Eris, Makemake, Haumea and Ceres. Ceres has its own history here. It was classed as a planet for around fifty years after its 1801 discovery before being reclassified as more asteroids were found in the same region.