The eight planets of the Solar System with size to scale (up to down, left to right): Saturn, Jupiter, Uranus, Neptune (outer planets), Earth, Venus, Mars, and Mercury (inner planets).

A planet is a large, rounded cosmic body that spends its entire existence orbiting something much hotter and more important, while insisting very strongly that it has its own identity. According to modern astronomical rules, a planet must be large enough for its own gravity to pull it into a spherical shape, but not large enough to accidentally turn into a star. In other words, it is an object that “almost became something magnificent” but stopped at the stage of cosmic middle management. In the Solar System, eight planets are officially recognized, although history shows that this list is occasionally revised with a level of drama worthy of an international political conflict. There are terrestrial planets — compact rocky spheres that look as though they were designed for walking on — and giant planets, which resemble atmospheric experiments that accidentally accumulated far too much mass.

The most popular explanation for the origin of planets is the nebular hypothesis. It states that there was once a vast interstellar cloud of gas and dust that began collapsing under gravity, as if the Universe suddenly decided to begin a “cosmic renovation.” A young star formed at the center, while around it spun a protoplanetary disk — a giant ring of matter in which dust, debris, and gas spent millions of years chaotically colliding with one another until planets gradually emerged from the disorder. This process is called accretion, although in essence it is simply a very long cosmic “everything sticking to everything else.” The term “planet” itself comes from the Greek word for “wanderers,” because ancient people noticed that some bright points in the sky behaved suspiciously and refused to stay fixed in place like normal stars. Because of this, Mercury, Venus, Mars, Jupiter, and Saturn were recognized as special celestial objects, while the Sun and the Moon also belonged for some time to this club of “moving strange things.”

In ancient cultures, planets were often granted the status of gods or at least highly influential cosmic characters. Humanity looked at these objects and concluded: “if it moves across the sky in an incomprehensible way, it probably possesses divine authority.” Even today, newly discovered celestial bodies continue to receive names from mythology, as if astronomy still partially functions as an ancient fan club. The greatest irony is that Earth itself was not considered a planet for a very long time. Humanity was so convinced that everything revolved around it that it took centuries of observation and scientific revolutions to officially recognize that Earth is also just one of the planets — traveling through space alongside all the others and possessing no privileged status in the Universe except for its local importance to its inhabitants.