Just like Earth, our moon is round and has a solid, rocky surface. Technically, it’s not a planet (at least according to NASA and the International Astronomical Union), but some astronomers aren’t convinced.
According to the International Astronomical Union (IAU) – the most important world authority on astronomy – an object must meet four criteria to be a planet.
It has to be in orbit around a star (like the Sun); It must have enough mass to assume a shape almost spherical; must have eliminated from its orbit all asteroids and other debris; and It can’t be a satellite from another body. Now, our moon fails two of these criteria.
The IAU’s definition of a planet was finalized in 2006. But it did not always meet consensus.
“The 2006 IAU definition draws a rather arbitrary line between planets and dwarf planets that led to Pluto’s demotion. Some planetary scientists choose to consider different definitions of planets, some of which include Pluto and the other dwarf planets, and others that even include the moon,” Skylar Grayson, an astronomer at Arizona State University, told .
An alternative definition comes from geophysics which suggests that a planet (or planetary body) is anything massive enough to be roundbut small enough that it cannot generate energy from nuclear fusion at any point in its life.
The first half of this definition divides the planets from the smaller parts of the solar system; and the second draws a line between the planets and the stars. By this definition, the Moon would be a planet – just like Earth.
Most people think of a moon as something smaller than a planet, especially the planet it orbits. But this also becomes complex. Our moon is surprisingly big compared to our planet – a quarter the size of Earth.
Another example is that Pluto’s largest “moon,” Charon, is actually half the size of Pluto – large enough to be often referred to as a binary planet rather than a moon.
Looking beyond the solar system, astronomers once again moon as big as Neptune orbiting a Jupiter-sized planet about 8,000 light-years from Earth.
As Live Science writes, the IAU definition is the most widely accepted for planets and clearly states that our moon is not a planet.
However, “the vagueness surrounding what is and is not a planet shows how difficult it can be to define terms in astronomy,” said Skylar Grayson.
“Most things in the universe do not exist in neat boxes, but rather on a spectrum, and We have to decide where we want to draw the lines between categories. Sometimes these lines are consensual, sometimes they are not”, he concluded.
