An eyeball planet is a hypothetical type of

tidally locked Tidal locking between a pair of co-orbiting astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. In the case where a tidally locked bo ...

planet A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...

, for which tidal locking induces spatial features (for example in the geography or composition of the planet) resembling an

eyeball Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and con ...

.They are

terrestrial planet A terrestrial planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to the Sun: Mercury, ...

s 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. Such planets are further divided into "hot" and "cold" eyeball planets, depending on which side of the planet the liquid is present. A "hot" eyeball planet is usually closer to its host star, and the centre of the "eye", facing the star (day side), is made of rock while liquid is present on the opposite side (night side). 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. Because most planetary bodies have a natural tendency toward becoming tidally locked to their host body on a long enough timeline, it is thought that eyeball planets may be common and could host life, particularly in planetary systems orbiting

red Red is the color at the long wavelength end of the visible spectrum of light, next to orange and opposite violet. It has a dominant wavelength of approximately 625–740 nanometres. It is a primary color in the RGB color model and a secondar ...

and

brown dwarf Brown dwarfs (also called failed stars) are substellar objects that are not massive enough to sustain nuclear fusion of ordinary hydrogen ( 1H) into helium in their cores, unlike a main-sequence star. Instead, they have a mass between the most ...

stars which have lifespans much longer than other main sequence stars.

Potential candidates

Kepler-1652b Kepler-1652b (also known by its Kepler Objects of Interest designation ''KOI-2626.01'') is a super-Earth exoplanet, orbiting within the habitable zone of the red dwarf Kepler-1652 about 822 light-years away in the Cygnus constellation. Discovered ...

is potentially an eyeball planet. The TRAPPIST-1 system may contain several such planets.

References

{{Exoplanet Tidal forces Hypothetical planet types