The Short Answer
No, the Moon is not a planet. It’s a natural satellite, which is really just the technical name for a moon. The simplest reason comes down to one thing, what it orbits. Planets go around the Sun. The Moon goes around us. And that single fact is enough to keep it out of the planet club, even though in almost every other way it’s a surprisingly planet-like world.
It’s a fair question to ask, though. The Moon is round, it’s rocky, it has mountains and plains and a whole geological history of its own. Stand under a bright full Moon and it certainly feels like a world in its own right. So why doesn’t it count? The answer is a fun little tour through how astronomers actually decide what a planet is.
What Makes Something a Planet
Back in 2006, the International Astronomical Union sat down and wrote an official definition of the word planet. To qualify, an object has to tick three boxes. It has to orbit the Sun. It has to be big enough that its own gravity has pulled it into a round shape. And it has to have cleared its orbital neighbourhood of other debris, meaning it’s the dominant object in its lane around the Sun.
The Moon nails the second box easily. It’s plenty big enough to be round. But it falls at the very first hurdle, because it doesn’t orbit the Sun. It orbits Earth. That makes it a moon by definition, no matter how impressive it is otherwise. Technically the Moon does loop around the Sun too, since it’s tagging along with Earth, but its primary relationship is with our planet, and that’s what counts.
The Strange Part: The Moon Is Bigger Than a Dwarf Planet
Here’s where it gets genuinely interesting. The Moon isn’t some tiny pebble that obviously belongs in a lower category. It’s about 3,475 kilometres across, which makes it bigger than Pluto, the most famous dwarf planet, which measures only around 2,377 kilometres.
Sit with that for a second. Pluto gets its own official designation as a dwarf planet, while the Moon, which is larger, gets classed as a mere satellite. It’s not about size at all. It’s entirely about what an object orbits and whether it dominates its own path. If you plucked the Moon out of Earth’s grip and set it in its own orbit around the Sun, it would very likely qualify as a dwarf planet, maybe even more. Pluto’s dramatic reclassification came out of that same 2006 meeting, and the story of how it was found and later demoted is a great one, told in our piece on the farm boy who found a planet.
The Moon Used to Be Called a Planet
Now for a twist that surprises a lot of people. For most of human history, the Moon actually was counted as a planet.
The word planet comes from the Greek for wanderer. To ancient skywatchers, the planets were simply the points of light that wandered against the fixed background of stars, and there were seven of them, the Sun, the Moon, Mercury, Venus, Mars, Jupiter, and Saturn. The Moon made the list without question. It was only when Copernicus and those who followed him worked out that Earth orbits the Sun, and the Moon orbits Earth, that the whole picture reshuffled. Once we understood the actual arrangement, the Moon slid neatly into its new role as Earth’s satellite. So the Moon didn’t fail to become a planet. It got demoted, centuries before Pluto ever did.
Are Earth and the Moon a Double Planet?
There’s one more idea worth touching on, because it’s a genuinely close call. Our Moon is unusually large compared to the planet it orbits. It’s about a quarter of Earth’s diameter, which is a much bigger ratio than most moons manage next to their host planets. That has led some people to wonder whether Earth and the Moon should really be thought of as a double planet.
The deciding factor is a point called the barycentre, the shared centre of mass that the two bodies actually swing around. For Earth and the Moon, that point sits about 4,670 kilometres from Earth’s centre, which is still inside the Earth, since our planet’s radius is around 6,371 kilometres. Because the balance point is buried within Earth, the official verdict is that the Moon orbits Earth rather than the two of them orbiting each other as equals. So no double planet, at least not by the current rules. It’s closer than you might think, though, and that’s exactly what makes the Moon such a special case. For more on just how remarkable our companion is, our look at what makes our Moon so special digs into its size, origin, and slow drift away from us.
So Where Does That Leave Us
The Moon is a world in almost every sense that matters to the eye. Round, rocky, ancient, larger than an entire category of dwarf planets. What it isn’t is a planet, and the reason is simply the company it keeps. It threw its lot in with Earth a long time ago, and by the definitions we use today, that makes it a moon and nothing more. If you want to see where the real planets sit in the pecking order, our beginner’s guide to the planets lays them all out, and NASA’s own Moon overview is a great next stop for anyone who’s fallen a little in love with our nearest neighbour.
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