Is there a ninth planet hiding beyond Neptune?

Maybe, but nobody knows, and there is a serious chance the answer is no. Planet Nine is a hypothetical planet, thought to be five to ten times the mass of Earth on a long, stretched orbit far beyond Neptune. It has never been seen. It is suspected because a group of distant icy objects, extreme trans-Neptunian objects, seem to travel in oddly similar orbits: they make their closest approach to the Sun in one sector and their orbits are similarly tilted. An unseen planet's gravity could be shepherding them.

The skeptics' reply is just as simple. Some astronomers say the clustering is an illusion produced by observational biases: these far-off objects are hard to find and track during much of the year, so the apparent pattern may only reflect when and where they are easiest to spot.

The idea has a history of false alarms. After Neptune's discovery in 1846 there was speculation about yet another planet. Percival Lowell predicted one, called Planet X, and began searching for it in 1906. Clyde Tombaugh found Pluto in 1930, but it was too small to be Lowell's planet. In 1989, Voyager 2 showed the Uranus discrepancy had been caused by an inaccurate mass for Neptune.

The modern case came in early 2016, when Konstantin Batygin and Mike Brown of Caltech argued that the orbits of six distant objects could be explained by Planet Nine. Brown put the odds at about 90 percent, and Batygin was blunt: until Planet Nine is caught on camera it does not count as being real. It remains purely conjectural unless it is observed.