Is the Moon a planet?

By the most widely accepted definition, the Moon is not a planet, although there is much controversy surrounding such classification.

The Moon is Earth's natural satellite. Photo: CNN

Like Earth, the Moon is round, has a solid, rocky surface. But is the Moon a planet? In theory, the answer is no, according to NASA and the International Astronomical Union (IAU), a global organization for astronomy. The IAU says an object must meet four criteria. It must be large enough that its gravity pulls it into a perfect circle and can clear any asteroids and space debris from its orbit. It must also orbit the Sun and not be a satellite of another body.

The IAU definition of a planet was finalized in 2006, but there has been disagreement among researchers. The ancient Greeks, for example, considered the Moon a planet. Even today, the IAU definition remains controversial.

“Whether the Moon is a planet or not really depends on who you ask,” says Skylar Grayson, an astronomer at the University of Arizona. “Some planetary scientists like to consider different definitions of planet, some including Pluto and dwarf planets, others even including the Moon.”

An alternative definition comes from geophysicists. According to them, a planet is anything that has enough mass to be circular but is small enough that it cannot produce energy from thermonuclear fusion at any point in its life. The first half of the definition distinguishes a planet from a smaller object like an asteroid, while the second half distinguishes a planet from a star. By this definition, the Earth is a planet, as are the Moon and Pluto.

Most people think of the Moon as a body smaller than a planet, especially one that orbits it. But that’s a bit complicated. The Moon is pretty big, about a quarter the size of Earth. Meanwhile, Pluto’s largest moon, Charon, is about half its size, big enough to be considered a binary planet. Outside the solar system, astronomers have found a moon as big as Neptune orbiting a Jupiter-sized planet 8,000 light years away.

The IAU definition is the most widely accepted and indicates that the Moon is not a planet. However, the ambiguity surrounding what constitutes a planet makes it difficult to define the term in astronomy, according to Grayson.