Not 'hidden'!
Secluded!
They like their privacy.
With the discovery of Neptune in 1846, some wondered that there might be any number of planets two far and dim to ever be discovered. Pluto was a chance outlier, discovered almost by accident crossing the ecliptic in 1930. As more out solar system objects were found, the definition of planet was changed to exclude Pluto, and similar objects.
There was precedent. Ceres was discovered on January 1st, 1801, the first day of the 19th Century. 400 km in diameter, much smaller than the moon, or several of Jupiter’s moons, it was still relatively bright, and easily sighted. At first Ceres was designated a new planet, but when other asteroids in similar orbits turned up, starting with Vesta, a decision was made not to call these objects not visible to the naked eye “Planets”. (Galilieo recordered an observation of Neptune in 1612 and 161e, but he did not recognize it to be a planet.)
I suppose “planet y” will have to observed, and found not to be in resonance with Neptune, or any other planet to promoted to the rank of “planet”.
Gated community.