How many planets were known to Copernicus?

Copernicus knew of six planets: Mercury, Venus, Earth, Mars, Jupiter, and Saturn. This was the complete planetary lineup visible to astronomers before the invention of the telescope.

In Copernicus’s heliocentric model, the Sun stood at the center while Earth became one of the planets orbiting it. The Moon was treated separately because it orbited Earth, not the Sun directly. Copernicus published this revolutionary arrangement in De revolutionibus orbium coelestium in 1543.

The word “known” is important here. Uranus, Neptune, and Pluto were not part of Copernicus’s planetary system: Uranus was discovered in 1781, Neptune in 1846, and Pluto in 1930. Copernicus also did not have a modern understanding of planets as worlds with physical properties; his count referred to the recognized wandering bodies that moved against the background stars.

A common mistake is to answer five by leaving Earth out, as older geocentric traditions sometimes treated Earth as stationary and distinct from the planets. In the Copernican system, Earth’s new status as a planet is precisely what makes the total six.