One of the earliest astronomical discoveries appears to have been done by the Sumerians, some time around 3 and a half millennia ago, who realized that the Morning Star and the Evening Star are the same body, which disappears, then reappears.

This was long before the discovery of the other planets. By discovery I mean when the Babylonians have discovered which is the movement on the sky of the planets and that it is predictable. Before that, people believed that besides fixed stars there are stars that appear and disappear, but they did not know that they see the same bodies that have a periodic movement on the sky. The Babylonians discovered the planets because they have made written records with the positions observed on the sky of the stars, for many years. After enough old written records had accumulated, they were able to see that stars do not appear and disappear randomly, but there are only a few wandering stars whose positions repeat periodically, and some of them have long periods of many years.

While the Sumerians already knew that the Morning Star and the Evening Star are the same, and they conceived a myth about how the Goddess Inanna, associated with the planet Venus, was descending to an underground realm (of death) and then she was coming back, reflecting this planetary movement, in most other parts of the World, people were not aware of this even millennia later.

For instance, Homer talks very frequently about the Morning Star and about the Evening Star, but there exists not even the slightest hint that he was aware that these 2 are not distinct stars. The same was true for later Greek authors, until the Babylonian astrology became widely known in Greece, bringing with it the knowledge about planets, which were renamed with names of Greek gods replacing the names of Babylonian gods (later replaced with the names of Roman gods, e.g. Sumerian Inanna => Babylonian Ishtar => Greek Aphrodite => Roman Venus or Babylonian Marduk => Greek Zeus => Roman Jupiter).