Jupiter
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Jupiter
Voyager did not discover any new planets. By the time Voyager was launched we already knew of all the planets in our solar system that we know of today. There were also two Voyager probes, not one. The first planet that either probe studied was Jupiter, which we had known for millennia. Voyager 1 flew by Jupiter in March 1979 while Voyager 2 flew by in July of the same year.
The first spacecraft to visit Jupiter was pioneer 10 in 1973, followed a few months later by Pioneer 11. Aside from taking the first close-up pictures of the planet, the probes discovered its magnetosphere and its largely fluid interior. The voyager 1 and Voyager 2 probes visited the planet in 1979, and studied its moon and the ring system, discovering the volcanic activity of Io and the presence of water ice on the surface of Europa. Ulysses further studied Jupiter's magnetosphere in 1992 and then again in 2000. The Cassiniprobe approached the planet in 2000 and took very detailed images of its atmosphere. The New Horizons spacecraft passed by Jupiter in 2007 and made improved measurements of its and its satellites' parameters.
The Voyager probes were deep space probes, sent to scout out the outer planets and deep space, so they never really "landed" on any of the planets although voyager I was sent first voyager II overtook it and encountered Saturn on august 1981.
Only one spacecraft has visited Neptune: NASA's Voyager 2 probe, which made a flyby of the planet in August 1989. Voyager 2 provided our first close-up images and scientific data of Neptune and its moons.
Jupiter was first.
Because Cinderella's fairy godmother tolled the probes to go to another planet first.
No space probes have visited Uranus to date. The only spacecraft that has conducted a close flyby of Uranus was NASA's Voyager 2 probe in 1986. There are proposed missions in development that aim to send probes to Uranus in the future.
The two Voyager space probes are robotic spacecraft launched in 1977 as the first spacecraft to examine the outer planets. Both are still operating, having passed the orbits of the outer planets, and drifting toward the boundary of the solar system.
Well, Voyager is now well beyond much useful gravitational influence of any of the solar system's planets. What we have learned is that planetary gravity wells can be used to provide a boost to passing spacecraft. Since Voyager most spacecraft to the outer planets spend some time in the inner system first, performing slingshots from one planet to the next to increase velocity. Each of these maneuvers robs a wee bit of the planet's angular momentum.
Voyager 1 and Voyager 2. There is a matter of some debate as to whether the two Voyager probes have actually left the solar system, an where the "edge" of the solar system actually is. Both are beyond the orbit of Pluto, but have not passed beyond the vaguely-defined Kuiper Belt, and the two probes are just approaching the heliopause, the boundary layer between the solar wind and the broader currents of interstellar space. But it seems likely that however that boundary is defined, the two Voyager probes either were or will be the first man-made objects to pass it.