Wikimedia CommonsCool Facts
A Hidden World. Planet Nine is a planet we have never seen, but scientists think it exists somewhere way out past Neptune. It would be so far away that it takes thousands of years to orbit the Sun.
The Gravity Clue. Astronomers noticed that icy objects far beyond Neptune bunch up in strange ways, as if something invisible is pushing and pulling them. Planet Nine's gravity could explain why these distant objects all cluster together instead of spreading out randomly.
Super Distant Objects. These distant objects, called extreme trans-Neptunian objects, orbit at distances more than 250 times farther than Earth is from the Sun. That is so impossibly far that light from the Sun looks like just another star to them.
The Big Debate. Not all scientists agree Planet Nine is real. Some say the clustering we see is just because our telescopes find these distant objects more easily in certain places, not because a hidden planet is herding them.
Still a Mystery. Planet Nine remains one of astronomy's biggest unsolved puzzles. Scientists are searching for it, but finding such a distant, dark object is like hunting for a needle in the cosmic haystack.
Omega Centauri
Enceladus
Galileo (spacecraft)
Comet NEOWISE