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 actually forced the decision

Not Pluto's size, though it is small — smaller than our Moon. The trigger was Eris, discovered in 2005 and initially thought to be larger than Pluto. Later measurements found it slightly smaller in diameter but about 27% more massive.

That created an unavoidable problem. If Pluto is a planet on the basis of being round and orbiting the Sun, so is Eris. And so, probably, are a great many other Kuiper Belt objects that surveys were beginning to turn up.

Astronomers therefore faced a choice: accept a solar system with a growing and open-ended list of planets, or write a definition that excluded them. They chose the definition, and Pluto fell on the wrong side of it.

So the honest framing is not that Pluto was demoted for being inadequate. It is that Pluto turned out to be the first-discovered member of a large population, and 'planet' was redefined to mean the dominant bodies rather than every round one.

The objection astronomers still raise

The 'cleared the neighbourhood' criterion is genuinely contested, and the objections are not merely sentimental.

It is not a property of the body alone. Whether an object clears its orbit depends on where it is. Earth placed in the Kuiper Belt would not clear that region, so under a strict reading Earth's planethood depends on its location rather than 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 rather than 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 actually 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 carries 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 are arguably a binary system rather than 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 did not prompt astrologers to drop it. That is a reasonable position — astrological meaning is not 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.