For a long time, the theory R+L=J has been well-known among fans of Game of Thrones. According to this theory, Jon Snow is not Ned Stark’s son but the offspring of Rhaegar Targaryen and Lyanna Stark.
Rhaegar was originally the son of King Aerys, the brother of Viserys and Daenerys Targaryen. Unlike his father, who was despised by the people, Rhaegar was admired, seen as a talented and beloved figure with a passion for music.
Meanwhile, Lyanna was a notable beauty of the North and the younger sister of Ned Stark. She was betrothed to Lord Robert Baratheon of Storm’s End. During a battle at Harrenhal, Rhaegar had passed her by for Elia Martell but offered Lyanna a rose, symbolizing love and beauty. This act was seen as a slight because Rhaegar had previously been with Lyanna, who was already engaged to another.
Lyanna was later abducted by Rhaegar, leading her to the South and hiding in Dorne. When King’s Landing was besieged, Lyanna’s father and brother, Rickard and Brandon Stark, were executed by King Aerys. Robert Baratheon and Ned Stark led a rebellion that ultimately resulted in the downfall of the Targaryen dynasty. Rhaegar himself was slain by Robert during the Battle of the Trident.