Is the Moon a planet?

Similar to the Earth, the Moon is round, has a solid and 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 needs to meet four criteria. The object must be large enough that gravity pulls it into a perfect circle and can erase all asteroids and space junk from its orbit. It also needs to orbit the Sun and not be a satellite of another object.

The IAU’s definition of a planet was finalized in 2006, but there is not always agreement among researchers. For example, the ancient Greeks believed that the Moon was a planet. Even today, the IAU definition remains controversial.

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

Another alternative definition comes from geophysicists. Accordingly, a planet is anything that has enough mass to form a circle but is also small enough that it cannot produce energy from fusion reactions at any time in its life. The first half of the definition distinguishes planets from smaller objects such as asteroids, while the second half distinguishes planets from stars. According to this definition, Earth is a planet, as are the Moon and Pluto.

Most people think of the Moon as a celestial body smaller than a planet, especially an orbiting planet. But that’s also quite complicated. The Moon is quite large, 1/4 the size of the Earth. Meanwhile, Pluto’s largest moon, Charon, is half its size, large enough to be considered a binary planet. Outside the solar system, astronomers also found a moon as large as Neptune orbiting a planet as large as Jupiter 8,000 light years from Earth.

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