@syz “How could we have missed it all this time (erratic orbit or not)?”
Because it’s really, really far out there. For comparison Neptune is (roughly) 30 astronomical units (AU) from the Sun. An AU is roughly equivalent to the distance from the Sun to the Earth (93,000,000 miles). So Neptune is 30x farther from the Sun than the Earth is and this new hypothesized planet (if, indeed, it does exist) is out 20x farther than Neptune.
At that point there’s not a lot of light hitting it and reflecting back, thus the odds of directly spotting it would be terribly slim (you wouldn’t see it at all from the surface of the Earth, not even with large, stationary telescopes). This might be a poor metaphor, but sighting that thing would be a bit like spotting a needle in an extremely large and spread out haystack (with a lot of debris between you and the stack) from a few blocks away, while relying on the light from a flashlight a little bit behind you to reflect off it some time.