HIGHLIGHTS

Not an actual eyeball!
It’s a metaphor.
One side of the planet always
faces its star.
Two very different hemispheres.
Possible habitable zone at
the day/night divide.
Link to: ATLAS – WATER for image ideas

Imagine a planet-sized eyeball floating in space, its giant pupil endlessly staring at its host star. Sounds gross and weird? Sure. But these planets do exist, and they may even host life.

Eyeball planets are created when a terrestrial planet orbits so close to its star that it becomes “tidally locked,” which means that one side of the planet always faces the star, and the other side faces the darkness of space. Our Moon, for example, is tidally locked: you can only see one side of it from Earth, the “far side” always points away from us. 

The tidal locking creates two very different environments on the eyeball planet’s surface, because the “day” side of the planet receives sunlight and warmth, whereas the “night” side is permanently dark and cold. Depending on how far away from its star the planet is orbiting, astronomers distinguish between “hot eyeballs” and “cold eyeballs.”

On a “hot eyeball,” the dayside would be a desert; all the water would have been boiled away into space. The night side, however, would be covered in ice. That creates the intriguing possibility that where the day and night sides meet conditions could be perfect for life to develop. In this perpetual dawn, there would be a ring of liquid water and vegetation circling the entire planet.

A “cold eyeball” would orbit further away from its star, so the entire planet would be covered in a sheet of ice. On the dayside of the planet, it could become warm enough for that ice to melt, creating an ocean surrounded by sea ice. This could be another promising environment for life to develop.

Right now, eyeball planets are theoretical models, none have actually been confirmed. But, the idea of concentric circles of different types of habitable zones is certainly an intriguing idea!

VIDEO: Eyeball Explainer

Here’s a fantastic video by SciShow Space’s Reid Reimers explaining the main characteristics of eyeball planets – and what dark and stormy creatures may inhabit them:
Life on an Eyeball Planet?

Hot Eyeballs / Ice Eyeballs

This in-depth article explains the different “flavours” of eyeball planets, how “hot” and “icy” eyeballs form, and what it may be like to actually live there.
We’ll Find the First Aliens on an Eyeball

Kepler-22b: Tipped on its Side

This planet orbits its Sun-like star in the habitable zone. It is tipped on its side and rotates around its axis like a rotisserie chicken, a bit like our planet Uranus.  Because of its radical tilt, its north and south poles would be alternately bathed in sunlight and darkness, for half a year each, as the planet circled its star. It’s a very Earth-like 15°C on Kepler 22b, just like another day in Vancouver.
Find out more about Kepler 22b

Link to: ATLAS – DESERT for image ideas
Link to: ATLAS – ICE for image ideas

HABITABILITY

Ideally, an eyeball planet would orbit a red dwarf star, which would be cool enough not to scorch the entire planet. In that case, it could provide a fantastic diversity of different terrains, especially the “hot eyeball” variety, where deserts, swamps, seas, and ice fields could sit side-by-side in fairly narrow strips.

As described in the Life on an Eyeball Planet? video, life would likely develop in the area of perpetual dawn. Reid Reimers suggests that lifeforms would probably be black, in order to absorb as much starlight as possible. Since the temperature difference between the two hemispheres would cause strong winds to develop, any organism living on the planet would have to either figure out how to hunker down or how to take advantage of the storms to get around.

STORY IDEAS

The concentric rings of different types of habitable zones would be fairly narrow, which makes them vulnerable to the slightest change in temperature. What if the planet got a couple of degrees hotter or colder? How would the lifeforms have to adapt or move? There is also the phenomenon of “libration.” Even a tidally locked planet may not hold perfectly still but wobble a bit, exposing some of the “dawn” areas to more or less starlight. This happens with Earth’s Moon; sometimes we can see craters on the edges that disappear a few weeks later. How would that impact the climate of the habitable rings?

The strong winds on the planet’s surface could also be explored as a convenient mode of transport or a hazardous menace. Also, winds make sounds – what songs may be created as the winds blow across the terrain?