![]() ![]() Pluto was also named a planet when it was discovered in 1930. The planet Neptune was discovered in 1846, expanding the size of the known solar system again, this time to 30 AU. ![]() By the time a fifth world, Astraea, was discovered in 1845, astronomers referred to them as “asteroids,” “small planets,” or “minor planets,” and considered them to be a subset of planets, just like rodents are a subset of mammals. Between 18, astronomers found 4 new worlds much closer to home: Ceres, Pallas, Juno, and Vesta, all orbiting the Sun between Mars and Jupiter, at distances ranging from 2 to 4 AU. While Mercury, Venus, Mars, Jupiter, and Saturn have been known since antiquity, Uranus wasn’t discovered until 1781, orbiting 20 astronomical units (AU) from the Sun, doubling the size of the then-known solar system. Over time, philosophers and scientists from Copernicus to Kant to Hubble modified this perspective until Earth was viewed as just one of many planets, orbiting an average star that, itself, orbited the distant center of the Milky Way, which was one of many galaxies. The ancient Greeks believed that Earth was at the center of the universe, and the planets-which included the Sun and Moon-revolved around us on fixed, concentric spheres. ![]() We’ll never know when humans first noticed that some stars moved while most did not, nor what name they first called those wandering stars. The word "planet" comes from the Greek word "planetes," which means "wanderer," and likely has more ancient origins. “Planet” is a word used by the ancient Greeks to describe stars, visible to the naked eye, that moved in relation to the fixed, background stars. ![]()
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