There are eight known planets in the Solar System (ever since Pluto was booted from the club), but for a while, there has been some evidence that there might be one more.

A hypothetical Planet 9 lurking on the outer edge of our Solar System. So far this world has eluded discovery, but a new study has pinned down where it should be. The evidence for Planet 9 comes from its gravitational pull on other bodies. If the planet exists, its gravity will affect the orbits of other planets.

So if something seems to be tugging on a planet, just do a bit of math to find the source. This is how Neptune was discovered when John Couch Adams and Urbain Le Verrier noticed independently that Uranus seemed to be tugged by an unseen planet.