The universe is home to a wide diversity of different objects. In our solar system alone, there are many different objects such as the Sun, planets, asteroids, comets, meteorites, moons, dwarf planets, etc. When we talk about some of these objects, most of us likely have good knowledge of what these objects are, but do we know what makes them different? In terms of the moon, why isn’t it a planet or a star?
What Is A Planet?
The following criteria define a planet:
- It must orbit the Sun.
- Its gravity must be strong enough to make the object spherical.
- The object must have a strong gravitational pull to clear its orbit of debris.
If an object follows these three criteria, it is defined as a planet. In the case of Earth’s moon, it follows criteria number two and three, but it fails to meet the first criteria. The moon revolves around the Earth only and not the Sun. Therefore the moon is not defined as a planet.
Is The Moon A Star?
A star is defined as an object whose mass is sufficient enough for the process of nuclear fusion to occur and be sustained. Nuclear fusion is a process by which hydrogen atoms are fused together to form helium, and it only happens when an object has enough mass. Stars are different from planets in that they are mostly more massive, allowing atoms to fuse together and release energy. The moon is nowhere near being massive enough for nuclear fusion to occur. Therefore the moon is definitely not a star.
The Moon Is A Natural Satellite
Rather than being a planet or a star, the moon is a natural satellite that orbits the Earth. The moon formed around 4.5 billion years ago due to a collision between the Earth and another planet. The debris from this collision entered into Earth’s orbit and eventually coalesced to form the moon. In terms of composition, the moon is similar to the Earth in some ways. Interestingly, if the moon were not orbiting the Earth, it would likely be defined as a planet. The moon is more closely related to the planets than it is to the Sun, and since it only orbits the Earth, it is neither considered a star nor a planet.