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, when a rocky world is so close to its parent star, the side that permanently faces towards its star is likely molten and volcanic, while the other hemisphere remains solid - creating a circular 'iris' of lava surrounded by solid rock. This is called a hot eyeball planet.