After "Game of Thrones" season 7 episode 5, "Eastwatch," we know that Rhaegar Targaryen annulled his marriage to Elia Martell and remarried somewhere in Dorne. This is huge: Jon Snow might just have a legitimate claim to the Iron Throne. He's not a bastard after all! He's a true and legitimate Targaryen, as Drogon sniffed out during the episode in his epic first meeting with the King in the North.
What happened
In "Eastwatch," during a Samwell scene, Gilly uncovered information from an old book, a story about how High Septon Maynard had issued an annulment for Prince Rhaegar Targaryen and remarried him to someone who is clearly Lyanna Stark.
Sam was so concerned about other matters that he didn't notice the precious information, but it's very likely that Gilly took the book with her when she left Oldtown with Samwell and little Sam.
Why this changes everything
The secret marriage between Rhaegar and Lyanna means that Jon is not only the legitimate son of Rhaegar Targaryen, he's also his only living son. Which means that Jon has a more legitimate claim to the Iron Throne than his aunt, Daenerys!
What about Dany and Gendry?
In medieval times, the legitimate heirs to the rulers were in most cases the closest male lineage, which means that the firstborn son comes before the former ruler's brothers and sisters. Daenerys' claim to the throne is being next in line to Jon.
Gendry, on the other side, is a bastard. Fans love him, and he's actually a nice guy, but no one legitimized him, therefore his claim is quite weak.
Being the male heir of Aerys II Targaryen (we should point out that before Robert's Rebellion the throne belonged to the Targaryens for centuries), Jon has the best claim to the Iron Throne.
Not that he wants that kind of responsibility, but sooner or later he's going to find out the truth about his parentage, and it will be very interesting to see how Jon and Daenerys will react to the news.
Who is going to tell Jon about his parents?
Two persons in Westeros could tell Jon the truth about Rhaegar and Lyanna, and they are Samwell Tarly (who owns the book in which High Septon Maynard registered the secret wedding) and Bran Stark, who may have already discovered the truth about Jon during one of his time travels.
Don't forget! The sixth episode of "Game of Thrones" season 7 airs on August 20, on HBO. At the moment, there is no title or synopsis available for the next episode of the show.
