Possibility of discovering Earth-sized planets out in the Oort Cloud

Space.com had an article on this subject yesterday, containing some speculation on just what might lie out in the Oort Cloud where we still have little to no idea of what it contains. The most famous object originally thought to belong to the Oort Cloud (now considered to be a detached object) is Sedna, larger than Pluto, and a planetoid that is much less well known than one might expect considering its size.

One interesting point to mention: the article says than an Earth-sized object that far out would be a kind of frozen time capsule, and unsuitable for life. But is that true? Because there is a lot of evidence to support the idea of rogue planets being able to maintain heat (especially one able to keep hold on a moon of its own), and if this is true then it should apply to Earth-sized bodies way out in the Oort Cloud as well.