There's a misunderstanding in the question. Planets don't "land" on anything. They are massive celestial bodies that orbit stars.

Perhaps you are thinking about:

* The first spacecraft to land on another planet: This was the Soviet Union's Luna 2, which landed on the Moon on September 13, 1959.

* The first human-made object to orbit another planet: This was Mariner 9, which entered orbit around Mars on November 14, 1971.

Please clarify your question if you meant something else!