[1] They are terrestrial planets where liquids may be present, in which tidal locking will induce a spatially dependent temperature gradient (the planet will be hotter on the side facing the star and colder on the other side).
This temperature gradient may therefore limit the places in which liquid may exist on the surface of the planet to ring- or disk-shaped areas.
A "cold" eyeball planet, usually farther from the star, will have liquid on the side facing the host star while the rest of its surface is made of ice and rocks.
[citation needed] Because most planetary bodies have a natural tendency toward becoming tidally locked to their host body for a long enough timeline, eyeball planets may be common and could host life, particularly in planetary systems orbiting red and brown dwarf stars which have lifespans much longer than other main-sequence stars.
[4] According to the observations of the James Webb Space Telescope in 2024, the super-Earth[a] planet LHS 1140b might either have a thin ice shell with a subsurface ocean or an icy surface covered partially in liquid water, the latter of which is an attribute of "cold" eyeball planet.