The question “Is the Moon a planet?” is surprisingly often asked. Especially by children, but also by adults who are just starting with astronomy. And it’s understandable: the Moon is round, has a surface full of craters, and moves across the sky like a planet would. Yet the answer is clear: no, the Moon is not a planet. In this blog, we’ll explain why not and why that answer is more interesting than you might think.
What Makes Something a Planet?
According to the International Astronomical Union (IAU), an object must meet three conditions to be considered a planet:
- It orbits directly around a star (like the sun).
- It is spherical due to its own gravity.
- It has cleared its orbit of other objects.
Earth meets all these conditions. The Moon doesn’t, mainly because it orbits Earth, and not directly around the sun. This makes it not a planet, but a natural satellite.
What is a Natural Satellite?
A natural satellite is an object that orbits a planet. In our solar system, more than 200 moons have been discovered. Some examples:
- Ganymede (Jupiter’s moon) is the largest moon in the solar system, even larger than Mercury and possibly has an underground ocean.
- Titan (Saturn’s moon) is the only moon with a dense atmosphere and has lakes of liquid methane. In 2027, NASA will launch the Dragonfly mission: a flying drone that will explore Titan in 2034 for traces of life and organic chemistry.
- Europa (Jupiter’s moon) has an ice crust under which an ocean potentially suitable for life lies hidden.
- Enceladus (Saturn’s moon) spews water and organic material through active geysers. Under the icy surface, there is almost certainly a liquid ocean with the right ingredients for life.
Earth’s Moon is not the largest, but is unique: it is by far the largest moon in proportion to the planet it orbits.
But Isn’t the Moon Large?
Yes, that’s correct. The Moon has:
- A diameter of 3,474 km (more than a quarter of Earth’s)
- A mass 80 times smaller than Earth’s
- A surface with craters, seas, and mountains
This makes it large enough to be spherical – one of the planet criteria – but that’s not sufficient. Its orbit determines its classification.
The Center of Mass: why the Moon Isn’t a Double Planet
Technically, Earth and the Moon don’t exactly orbit each other, but orbit around a common center of mass: the barycenter. This lies approximately 1,000 km below Earth’s surface.
If the barycenter had fallen outside Earth, we could speak of a double planet system. But because it’s still inside Earth, the Moon remains a satellite.
Does the Moon Have Planet-Like Properties?
Absolutely. The Moon:
- Has a core, mantle, and crust
- Has moonquakes, similar to earthquakes
- Formed from a collision between young Earth and a planet-like object (Theia)
- Has a stable orbit and predictable rotation
But despite these properties, it lacks an independent orbit around the sun, and that’s decisive.
Is the Moon Moving away from Us?
Yes. The Moon moves away from Earth by approximately 3.8 centimeters each year. Once it was only 14,000 km away from us – now it’s an average of 384,000 km.
Over billions of years, this effect will:
- Make eclipses rarer
- Make tides less strong
- Lengthen our days
However, this process will likely be overtaken by another cosmic fate: the Sun will engulf Earth and the Moon in about 5 billion years.
How Did the Moon Form?
The most accepted theory is the Theia impact hypothesis. About 4.5 billion years ago, shortly after Earth’s formation, an object the size of Mars that we now call Theia collided with young Earth.
Was Earth Fully Formed then?
No, Earth was still in full development at that time:
- Not yet cooled down
- No stable atmosphere yet
- Much smaller than now
- And with lower gravity than the current planet
Yet Earth already had sufficient mass to pull enormous amounts of material into orbit after the impact. This debris, mainly from Theia and Earth itself, formed a disk around our planet over the course of several thousand years.
Through gravity, this material clumped together into one large object: the Moon.
Why Does the Moon Now Orbit Earth?
Earth’s rotation and the speed at which the debris was flung into space after the collision ensured that the clumped object came into a stable orbit. Compare it to a satellite launched with exactly the right speed to orbit Earth.
What’s important is:
- The formed object had an orbit and speed that didn’t lead to falling back or escape.
- Earth’s mass was even then sufficient to bind this object through gravity.
Since then, the Moon has continuously orbited Earth, while tidal forces and rotational acceleration have further influenced our systems.
Why could We, as a Young Planet, Still Hold onto a Moon?
Although this is a valid point, there are a few things to know:
- Even a smaller planet has a gravitational field, especially during a collision of this scale. Objects in space are really huge, even when they’re called “small”.
- The debris was already relatively close to Earth after the impact.
- It wasn’t necessary to attract an external moon; the Moon formed from local material, within Earth’s gravitational field.
This makes our Moon different from most moons in the solar system, which are often captured (like Mars’ moons Phobos and Deimos).
What Makes our Moon Special?
- The Moon has been actively studied: through missions like Apollo, Lunar Reconnaissance Orbiter, and Artemis.
- It is the only celestial body outside Earth that humans have ever walked on.
- Its presence stabilizes Earth’s rotation axis, making it essential for a stable climate.
- Without the Moon, there would be no tides, and our nights would be much darker.
What Can You See with a Telescope?
Even with an entry-level model you can see many details on the lunar surface:
- Craters like Tycho and Copernicus
- Lunar seas like Mare Tranquillitatis
- Shadow areas along the terminator (the boundary between light and dark)
Preferably use a moon filter to increase contrast and prevent glare.
🔗 Also check out our eyepieces and filters for lunar observation
Conclusion: is the Moon a Planet?
No. The Moon is a natural satellite of Earth. It doesn’t orbit the sun, and therefore doesn’t meet the official planet definition. Yet it is planet-like in properties and invaluable for life on Earth.
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