Is the Moon Actually a Planet?

Believe it or not, according to the International Astronomical Union's own definition of a planet — the same definition that famously demoted Pluto in 2006 — there's a serious, legitimate argument that Earth's Moon qualifies as a planet. Not a metaphor. Not a joke. A real, criteria-by-criteria case that the Moon checks every scientific box the IAU laid out. And once you see it, you can't unsee it.

Let's walk through the IAU's three official criteria one by one, apply them to the Moon, and then dig into the hidden fine print that reveals something uncomfortable about how modern planetary science actually works.

Why Did the IAU Demote Pluto?

Before we defend the Moon's planethood, it helps to understand what triggered this whole debate. In August 2006, the IAU passed its now-infamous resolution formally defining what a planet is — and in doing so, knocked Pluto off the list it had been on since 1930.

The motivation was practical: astronomers had discovered that Pluto wasn't alone. The Kuiper Belt is packed with similarly sized icy objects, and if Pluto was a planet, so were dozens of other things. Rather than expand the planet club, the IAU chose to shrink it — creating a new category called "dwarf planets" to catch the leftovers, including Pluto, Eris, and Ceres.

The resolution established three criteria that a celestial body must satisfy to earn the title of planet. And here's where things get interesting.

What Are the 3 IAU Criteria for a Planet?

The IAU definition states that a planet is a celestial body that:

  • Criterion A: Is in orbit around the Sun.
  • Criterion B: Has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape.
  • Criterion C: Has cleared the neighbourhood around its orbit.

Criterion C is the one that kills Pluto — it shares its orbital neighbourhood with a swarm of Kuiper Belt objects and doesn't gravitationally dominate them. But here's the twist: when you apply all three criteria to the Moon, it passes every single one.

Does the Moon Orbit the Earth or the Sun?

Criterion A requires orbit around the Sun. We instinctively think of the Moon as orbiting Earth — and from Earth's perspective, it does. But zoom out to the Sun's perspective, and the picture changes dramatically.

The Moon and Earth together orbit the Sun, with the Moon tracing a wavy, always-sunward-curving path as it goes. In fact, the gravitational pull from the Sun on the Moon is twice as strong as the gravitational pull from the Earth on the Moon. This means the Moon's orbit always curves toward the Sun — never away from it. It never loops out around Earth in a way that would truly put Earth at its gravitational center.

By this logic, the Moon satisfies Criterion A: it is a celestial body in orbit around the Sun.

What Does 'Cleared Its Neighbourhood' Actually Mean?

Criterion B — the roundness test — is easy. The Moon is obviously a sphere. It's massive enough that its own gravity crushed it into a rounded shape billions of years ago. In fact, the Moon is significantly more massive than Pluto, Ceres, and virtually every other object that sits near the boundary of this criterion. The Moon clears Criterion B by a massive margin.

Now for the big one: Criterion C — clearing the neighbourhood. This is asking whether an object is gravitationally dominant within its orbital zone. Does it pull in nearby objects, kick them out, capture them as moons, or absorb them through collisions?

Astronomers actually plot this on a graph: object mass on one axis, distance from the Sun on the other. A diagonal line separates "big enough to clear its orbit" from "not big enough." Pluto sits below the line — it's too small and too far out to dominate its neighbourhood, and is itself overshadowed by Neptune.

The Moon? It sits clearly above the line. If Earth vanished tomorrow, the Moon would have no problem gravitationally dominating its orbital zone and earning full IAU planet status. It has more than enough mass relative to its distance from the Sun to pass Criterion C.

So the Moon passes all three. Criterion A: orbiting the Sun. Criterion B: gravitationally rounded. Criterion C: capable of clearing its neighbourhood. By the IAU's own science, the Moon is a planet.

What's Wrong With the IAU's Planet Definition?

Here's where the IAU quietly reveals its hand. Buried in the resolution — before the three criteria are even listed — is a clause that excludes "satellites" from consideration. Call it Criterion X: the object must not be a moon of another object. Since the Moon is, by name and by nature, a moon, this quietly disqualifies it before the science even gets a chance to speak.

But it gets worse. At the bottom of the resolution, there's a footnote — an actual footnote — that reads: "The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune."

Let that sink in. They wrote a rigorous-sounding scientific definition with three testable criteria… and then listed the answer at the bottom anyway.

This completely exposes the process behind the definition: the IAU started with a predetermined list of what they wanted to call planets, and then reverse-engineered scientific-sounding justifications to defend that list. That's not how science is supposed to work. Science is descriptive — you observe the universe, notice how things naturally cluster, and build categories from evidence. The IAU definition is prescriptive: here's our list, here's the definition we crafted to match it.

The more honest approach would have been to simply say: "These eight objects are culturally and historically significant to human civilization, so we're going to call them planets." That would at least be transparent. Instead, we got a definition with hidden criteria and a footnote that does the real work.

Could a Star Be Called a Planet Under IAU Rules?

As if all that weren't enough, the IAU definition has one more embarrassing flaw. Imagine a small red dwarf star — a dim, failed star — that wanders into our solar system and gets gravitationally captured into an orbit around the Sun. Under the IAU's three criteria, that star would technically qualify as a planet. It orbits the Sun. It's massive enough to be round. It would almost certainly clear its neighbourhood.

A star. Classified as a planet. That's not a hypothetical gotcha — it's a recognized problem, which is why definitions for exoplanets often include a separate "substellar" mass requirement that the basic IAU planet definition entirely lacks.

So What Should We Call a Planet?

At the end of the day, "planet" is a human concept that serves human needs. There's nothing wrong with acknowledging that. What is wrong is pretending an arbitrary, culturally motivated list has rigorous scientific backing when the definition can't even consistently exclude stars.

According to the best contemporary planetary science — science that focuses on geophysics, internal structure, and the natural properties of objects — the universe looks quite different. The Moon is a planet. Ceres is a planet. The large, rounded moons of Jupiter, Saturn, Uranus, and Neptune are planets. And yes, Pluto is a planet.

That's not a sentimental argument for bringing Pluto back. It's what a scientifically consistent, observation-first definition actually produces when you apply it honestly. The IAU gave us a definition shaped by a conclusion they'd already reached — and as the case of the Moon shows, it doesn't hold up to scrutiny.