Small changes in the movement of Neptune led several astronomers to believe that an undiscovered planet was affecting Neptune's orbit.

VOA: special.2010.07.11

The planet's orbit lies about four billion five hundred million kilometers from the sun.

VOA: special.2011.07.27

"We will use the tremendous capabilities of this ion propulsion system to climb out of orbit around Vesta, travel for almost another three years through the asteroid belt to dwarf planet Ceres and then go into orbit around Ceres and undertake the same kind of measurements there that we'll be doing at Vesta.

VOA: special.2011.07.27

But if the planet's orbit is nearly edge-on, as viewed from Earth, then the planet will transit its parent star.

ECONOMIST: Transits of Venus

He and later astronomers spent decades looking for the missing planet within the orbit of Mercury which, they were convinced, explained what was going on.

ECONOMIST: Physics

The second piece of evidence pointing to is the eccentric orbit of an existing planet around the star, which is in an orbit not normally expected in this type of system.

FORBES: Astronomers Discover A Star That May Have Destroyed Its Own Planet