[Someone]

Earth-sized in mass, maybe, but earth-like?

They argue there may be a ∼ 1.5–3 M⊕ object out there, but don’t say anything about its size or surface temperature.

At ∼ 200–500 au, chances are it would be mostly ice. (https://en.wikipedia.org/wiki/Kuiper_belt: “most Kuiper belt objects are composed largely of frozen volatiles (termed "ices"), such as methane, ammonia, and water”)

https://en.m.wikipedia.org/wiki/Kuiper_belt#Composition: “The temperature of the belt is only about 50 K”

“The densities range from less than 0.4 to 2.6 g/cm³”

⇒ a bit heavier and likely way less dense than earth (at 5.5 g/cm³, the densest object in the solar system. See https://www.forbes.com/sites/startswithabang/2020/03/31/this...), so likely at least double its diameter, and very cold, so not earth-like.

That title is click-bait.