It's hard to detect dark, cold things that are far away.
Planet Nine is very cold, likely to be quite dark and also very far away (anywhere between 200 AU and 1200 AU depending on where it currently is in its orbit). Its estimated apparent magnitude is greater than 22.5. That means you'll need a large telescope (I guess around 5m), a dark sky and sensitive cameras to be able to see it.
Stars are neither dark nor cold, however brown dwarves that are pretty cold are much harder to detect.
If you look at the list¹ of the sixty nearest stars to our sun, you'll notice that many close brown dwarves have only been found fairly recently (e.g. Luhman 16a/b, 6.5ly away were found in 2013).
Stars are neither dark nor cold, however brown dwarves that are pretty cold are much harder to detect.
If you look at the list¹ of the sixty nearest stars to our sun, you'll notice that many close brown dwarves have only been found fairly recently (e.g. Luhman 16a/b, 6.5ly away were found in 2013).
¹ https://en.wikipedia.org/wiki/List_of_nearest_stars_and_brow...