They must like it very, very COOOOOLD!

@Coffeo It's one reason they seeded Earth. They like it warmer.

Where did they do that?

Is this what Zechariah Sitchin proposed? I have to ask: if they learned of that planet, why didn't they learn of other things? Like the Earth being approximately spherical, it moving around the Sun along with the other planets, eclipses being shadow effects, and the "fixed" stars being distant suns.

@lpetrich Sitchin's books said all of that. There was something about there being 12 planets, including the moon. Lots of things that weren't known, or rediscovered until modern times. Sitchin's books are hard to read. Like reading a book of facts, that you have to decide if they're true or not. And decide the significance. His books are full of believe it or nots.

It's a question of spotting it so far out there in such a vast area. We've found exoplanets much further away because of the dampening of the light from their sun when they eclipse it, but there's no such benefit in finding a planet out on the edge of our own solar system.

I vaguely remember reading an article some years back where an Astronomer suggested that this mysterious 'planet 9' could be either, a) a 'dark planet' due to I) its orbit is so far distant from the sun that any reflected light is lost BEFORE it reaches Earth, ii) it is made up of materials that are completely non-reflective, OR, b) that is a 'dark star, the burned out cinder of a much smaller solar body that 'failed' to make the grade, so to speak, as our own sun did and merely shrunk to a body the size of Mercury. Mars or Venus for example and its orbit is not synchronous with those of the other 8 known planets.

Being very far from the Sun should do it. [en.wikipedia.org] has some numbers for absolute magnitudes of the planets, and one can use them to estimate how bright such a planet would be. A good reference case is Uranus and Neptune, at about -7. At 100 AU, its apparent magnitude would be -7 + 5log10(100100) = +13. Likewise, at 320 AU, it would be +18, and at 1000 AU, +23 -- very difficult to see.