Our moon’s diameter is 3476 km (Link), which is larger than that of Pluto, which stands at 2370 km (Link). Hence, can our moon be a planet? Answer is yes, if our moon orbits in a circular or slightly elliptical orbit around a star. It is our moon because it orbits around our planet, Earth.
Hence, size is not the sole criterion for defining a planet or a moon (also known as a satellite). Specifically, what matters is the orbital characteristics of the astronomical body. For example, if the body orbits around a star in a circular or slightly elliptical orbit, it is a planet. On the other hand, if the astronomical body orbits around a planet in a circular or slightly elliptical orbit, it is a moon of the planet.
More importantly, the relative sizes of the astronomical bodies determine their orbital characteristics. Naturally, gravitational attraction determines that the larger astronomical body would be stationary with respect to the smaller body orbiting around it; thus, the star is stationary with respect to the planet.
Category: space exploration, astronomy, exoplanets,
Tags: size of astronomical object, satellite, planet, star, circular orbit, elliptical orbit,