We have seen Pluto. We have not seen Planet Nine.
Pluto has been discovered (sorry, truthers). We have photographic evidence of Pluto's existence. Here it is:
Planet Nine actually hasn't been discovered. It hasn't been seen through a telescope. At this point, Planet Nine's existence is theoretical, in part based upon how dwarf planets and other small objects in the neighborhood move. It seems like there's a "massive perturber" lurking about, and the gravity of this potential new planet could be influencing the orbits of nearby bodies.
Now, here's a helpful diagram that clearly demonstrates how Pluto is not Planet Nine:
There's a lot of stuff in the outer solar system that isn't Pluto
Space may seem empty and vast and whatnot, but astronomers previously discovered more than 30 dwarf planets and other objects in the outer reaches of our solar system. One such dwarf planet, Eris, is actually bigger than Pluto. Eris ≠ Pluto, Eris ≠ Planet Nine, Planet Nine ≠ Pluto, etc. (Also, see above for the equation "Planet Nine > Pluto.")
Being salty about Pluto getting the boot does not make "Planet Nine" Pluto.
Look, I get it: You're still upset about Pluto's demotion from the solar system, the makeup of which you committed to memory as a child. Just think about how Pluto must feel. But it's time to let it go.
A decade ago, when the International Astronomical Union defined what constitutes a planet, Pluto and Eris got the boot and were classified as dwarf planets. That same set of criteria would be used to usher in "Planet Nine" into our solar system. If/when it is discovered, researchers say, it will fit the planet criteria.
So, here's the bright side for nine-planet solar system proponents: You may get your ninth planet after all!
Which, by the way, will not be called "Planet Nine." That's right, Planet Nine will not be (named) Planet Nine. It won't be Pluto, either — you can't have a dwarf planet and a planet planet with the same name. You're just going to have to memorize a new one.