Currently, all major planets in our solar system have been successfully visited by at least one spacecraft. Therefore, in the context of the eight officially recognized major planets, there is no planet that has never been visited.
Current Status of Planetary Exploration
All eight major planets in our solar system have been subject to various space missions, ranging from flybys to orbiters and even landers, which have significantly advanced our understanding of these worlds:
- Inner Planets:
- Mercury: Visited by Mariner 10 and MESSENGER.
- Venus: Visited by numerous missions, including Venera series, Magellan, and BepiColombo.
- Earth: Continuously observed by countless satellites and probes.
- Mars: Explored extensively by orbiters, landers, and rovers, such as Viking, Mars Rovers (Spirit, Opportunity, Curiosity, Perseverance), and InSight.
- Outer Planets:
- Jupiter: Visited by Pioneer, Voyager, Galileo, and Juno missions.
- Saturn: Visited by Pioneer, Voyager, and Cassini missions.
- Uranus: Visited by Voyager 2.
- Neptune: Visited by Voyager 2.
Historical Context: Pluto's Past Status and Exploration
While all major planets have now been visited, the answer to this question would have been different historically, especially concerning Pluto's status before its reclassification and dedicated mission.
As of the early 2000s, Pluto was widely considered the ninth planet in our solar system. At that time, Pluto was the only planet that had not been visited by an explorer spacecraft. Our scientific knowledge about Pluto was notably limited, with the best available images of its surface being hazy pictures taken by the Hubble Space Telescope in 1996. This limited understanding spurred the development of ambitious missions aimed at exploring this distant world.
- Pluto's Reclassification: In August 2006, the International Astronomical Union (IAU) reclassified Pluto as a dwarf planet, a new category of celestial body.
- New Horizons Mission: Despite its reclassification, Pluto was eventually visited by the New Horizons spacecraft, which performed a historic flyby in July 2015. This mission provided the first detailed images and extensive data about Pluto, its moons, and its surrounding Kuiper Belt, a vast region beyond Neptune composed of thousands of icy, rocky objects.
Therefore, while historically Pluto was the last major body to be visited, all officially recognized planets, and even the dwarf planet Pluto, have now been explored by spacecraft.
