Does our solar system contain a giant, unnamed planet? It does, according to the Planet X theory. This theory says that our solar system has a 10th planet (if you count Pluto as a planet, otherwise the mystery planet is no. 9). The planet is supposed to be enormous and on an orbital path that will bring it close to Earth soon. Proponents of this theory cite earthquake and weather data as evidence of this planet's growing influence on Earth. They also claim that the government is forcing observatories to close to keep the planet's approach secret and prevent panic. Several astronomers have said that there's no evidence for the Planet X theory, and that if the planet did exist, humans would be able to see it even without a telescope.
Even if you did need a telescope to see this planet, very little would stop amateur astronomers from seeing it. Keeping countless amateur stargazers quiet about such a massive discovery would be impossible. Unless the world's governments have collaborated in secret planet-sized stealth technology, this theory seems pretty flimsy.