“Jokers, worlds that don't fit the pattern. And there is a pattern, generally speaking. But the broad patterns are broken up by these exceptions: Jokers in the pack, as scholars of the Long Earth call them...”
USS Neil A. Armstrong and the USS Eugene A. Cernan flying side by side over a Joker - Concept art by DSonla
is a world that is out of the ordinary in its chain of planets, such as a
world that is as smooth as a bowling ball
in the middle of
the Corn Belt
Long Earth, many Jokers are free of life because of some calamity they have suffered. For humans, they are difficult to survive in or completely uninhabitable.
Jokers are the opposite of
Diamonds - worlds that are also out of ordinary, but instead provide an unique attraction.
In the Long Mars, most worlds cannot host life, so the Jokers are the Marses that can.
↑ The Long Earth - Chapter 18
↑ The Long Mars - Chapter 20