It's tricky to say definitively which planet was "first explored" because the definition of "exploration" can be broad. However, here are some key points to consider:

* Earliest Observations: Humans have been observing the planets for thousands of years, with early civilizations tracking their movements across the sky.

* Telescopic Observations: With the invention of the telescope in the 17th century, astronomers like Galileo Galilei made detailed observations of planets like Venus, Mars, and Jupiter, discovering features like moons and phases. This could be considered the first "scientific exploration" of planets.

* Spacecraft Missions: The first spacecraft to visit another planet was Venus, with the Soviet Union's Venera 1 in 1961. However, it didn't actually land on the surface.

* First Successful Landing: The first successful landing on another planet was achieved by the Soviet Union's Venera 3 in 1966, which crash-landed on Venus.

* First Soft Landing: The first soft landing on another planet was achieved by the Soviet Union's Venera 7 in 1970, which landed on Venus and sent back data for 23 minutes.

Therefore, depending on your definition of "exploration," the answer could be Venus, Mars, or Jupiter, as all three have been extensively observed and studied throughout history.