What planet was once thought to be two planets?

Answer 1

Both Mercury and Venus have been known as the "morning star" or "evening star" in some form for various cultures and civilizations. Sometimes, these evening and morning appearances were thought to represent different "stars." (Of course, they are not stars at all and are rather the two planets closest to the Sun.)

In ancient Greece, Mercury was called Apollo when it appeared as a morning star with sunrise and Hermes (the Greek equivalent of the Roman god Mercury) when it appeared as an evening star near sunset.

Venus was also known to ancient civilizations both as the "morning star" and as the "evening star". The Greeks thought of the two as separate stars, Phosphoros and Hesperos, until the sixth century BC.

The Romans designated the morning aspect of Venus as Lucifer (meaning Light-Bringer) and the evening aspect as Vesper.

Venus is dramatic in the morning and evening sky while Mercury is smaller, dimmer and hard to see. (Most people have never seen Mercury and everyone has seen Venus whether they know it or not.) As a consequence, references to Venus are more common when the term morning star or evening star are made. (It is also true that sometimes any star that is the first visible start of the morning or evening is given such a name for that particular day.)

Answer 2

The planet Uranus was originally thought to be two separate stars due to its slow movement and dim appearance. It wasn't until 1781 when Sir William Herschel observed it through a telescope that Uranus was confirmed as a single planet.