When a planet is in a sufficiently close orbit around its parent star, the planet can become tidally 'locked', so that it always presents the same face to its star. The Moon is tidally locked to the Earth, for example. Astronomers have discovered an entire slew of such planets around nearby stars, and have dubbed them 'eyeball planets'. For example, on a world rich in water, the side that permanently faces away from its star is likely frozen, while the other hemisphere remains liquid - creating a circular 'iris' of liquid water surrounded by ice. This is called a cold eyeball planet.