What makes an object a moon rather than a planet?
The basic distinction is orbital: a moon circles a planet or dwarf planet, while a planet circles the Sun directly. But the line is not as sharp as it sounds. A 2025 paper proposes a geophysical definition in which planets are defined by their physical properties rather than by what they orbit, and under that definition moons, dwarf planets, and even rogue planets all count as planets [2]. In other words, whether a body is called a moon depends partly on which definition you use.
Size also matters for how moon-like a body seems. Earth's Moon is roughly one-quarter of Earth's radius, a larger satellite-to-planet ratio than all known satellites other than Pluto's Charon [1]. That makes our Moon unusual: most moons in the solar system are much smaller relative to their planets.
Where do moons come from?
Moons can form in different ways, and Earth's Moon is a well-studied example. The leading ideas for lunar origin involve a giant impact early in Earth's history, but the exact process is still debated [1]. The early Moon was hot and at least partially molten, with a deep magma ocean—a sign that it formed through a violent, high-energy event [1].
The Moon's interior is also unusual: it has a tiny core, perhaps only about 1% of its mass, compared with nearly 30% for Earth [1]. That difference is a clue about how the Moon assembled and what materials it was made from.
Why does the definition of a moon keep changing?
Definitions in astronomy are human tools, not natural laws. The International Astronomical Union's definition of a planet has been criticized as fuzzy because it does not clearly define what 'clearing the neighborhood' means [2]. That ambiguity pushes some researchers to propose alternatives, such as a geophysical definition that classifies bodies by their own properties—shape, interior, and roundness—rather than by their orbital neighborhood [2].
Under such a definition, a moon is not a separate category from a planet; it is a planet that happens to orbit another planet [2]. This does not mean the everyday word 'moon' is wrong—it just shows that the scientific category is still being negotiated.
Sources used in this answer
Origin of the Moon
Earth's Moon is roughly one-quarter of Earth's radius, has a tiny core of about 1% of its mass, and formed hot and at least partially molten, with a deep magma ocean.
Dwarf Planets and Moons are Planets: Geophysical Definition of Planet
A proposed geophysical definition of planets classifies moons, dwarf planets, and rogue planets as planets, because the current IAU definition of a planet is ambiguous.