Welcome to Real-life Sci-fi worlds. We use science to explore life-bearing worlds that are good settings for science fiction. Up today: the hot Eyeball planet.
Planets very close to their stars are too hot for life, right? Well, not always!
Take the Earth and move it closer and closer to the Sun. It gets hotter and hotter and … then it gets fried. What I mean by fried is that the greenhouse effect in Earth’s atmosphere crosses a point of no return. At this point, Earth gets so hot that the oceans evaporate. I guess boiled might be a better description than fried. Over time, Earth’s water is lost to space. Earth eventually turns into a hellhole like Venus. Not a happy story.
But it doesn’t have to be like that! There exists another solution: the hot Eyeball planet.
Imagine an Earth-like planet orbiting close to its star. It doesn’t matter exactly how close, let’s just say that it is close enough that it should be fried.
Before we talk about the planet’s climate, there is something important about this planet that is different than Earth: how the planet spins. Earth spins spins pretty fast: once a day. But its spin axis — its “obliquity” — is tilted.
A planet close to its star feels strong tides from its star. Like the tides Earth feels from the Moon, but much stronger. Strong tides change how a planet spins. Tides drive the planet’s obliquity to zero, so the planet’s equator is perfectly lined up with its orbit. Tides also force the planet to always show the same side to the star. It looks like this, except with the star in the middle and the planet orbiting the star:
So our possibly-fried planet always shows the same face to the star. The planet is hot on its permanent day side and cold on its permanent night side. We’re talking blazing hot on the day side and deathly cold on the night side. Frying pan and deep freezer.
What happens to the planet’s water? It is heated up and boiled on the day side, and frozen on the night side. But winds transport the water vapor from the day side to the night side. Water that boils away on the day side can end up as ice on the night side. This can create a cold trap: all of the planet’s water can be locked up in a giant layer of ice on the permanent night side. Dry day side, ice-covered night side.
But the story doesn’t end there. When a layer of ice gets thick enough, its bottom layer melts. This causes the ice to flow. This is how glaciers behave.
So our planet’s thick night side ice cap should spread out and slowly flow toward the day side. There may be a trickle of water that flows into starlight to be evaporated all over again. There are characteristic wind patterns that pile clouds up in a specific region on the night side (to the East of the anti-stellar point). Here is a cartoon of what this looks like:
The name Eyeball planet comes from the planet’s non-uniform appearance. The night side is icy, the day side is rocky. The sub-stellar point, the place on the planet where the star is always directly overhead, is really hot. If the planet is close enough to the star, rocks could even melt at the sub-stellar point. That would be the pupil. Rivers that flow from the night side to eventually evaporate on the day side might even look like veins.
The hot part of the name is a hint that there is another kind of eyeball planet (the cold kind of course). We’ll get to that one later.
Where on a hot Eyeball planet would you want to live? It’s a classic Goldilocks story. The day side is roasting and dry. The night side is frigid and icy. In between, it’s just right! The sweet spot — what I call the “ring of life” — is at the “terminator” (not the movie), the boundary between night and day.
Here is a nice artist’s view of a hot Eyeball planet:
The ring of life is bounded by deserts on one side and ice on the other. There is a constant flow of water from the night to the day side. In other words, a series of rivers, all flowing in the same direction. The Sun is fixed in the sky right at the horizon, and the area is in permanent light. Conditions are pretty much the same across the ring of life, from the equator to the poles.
To speculate, I can imagine vegetation following the rivers onto the day side until they dry up. Different ecosystems interspersed along the way. I also wonder if there would be mountains at the edge of the ice sheets, since the ice-covered continents would be heavily weighed down (this is called isostasy).
How many hot Eyeball planets exist in our Galaxy? Let’s see. About half of all stars like the Sun have a planet that might fit the bill! These planets are usually called hot super-Earths. These planets — at least the ones that have been found so far — tend to be a little big larger than Earth.
But not all hot super-Earths are likely to be hot Eyeball planets. The hot Eyeball can only exist for a limited range of planetary conditions. A planet with too thick of an atmosphere has too strong a greenhouse effect, melts its ice and gets fried. There are a couple of other conditions that need to be met to be able to properly hot-Eyeball-it up (see here for the gory details).
I can’t realistically estimate what fraction of hot super-Earths have the right conditions to be hot Eyeballs. Still, there are a few hundred billion hot super-Earths in our Galaxy. Let’s be pessimistic and say that only 1 in 1000 of these has the right conditions. There would still be a couple hundred million hot Eyeball planets in the Milky Way! Not too shabby. Statistically-speaking, there should be one in our immediate Galactic neighborhood (within about 100 light years). People are actively looking for Hot Eyeball planets as we speak (see here), so stay tuned.
Life abounds in the “ring of life” on a hot Eyeball planet. This includes an intelligent species (say, like us). The story centers around the rituals that adolescents on this planet must experience. The rites of passage.
There are rivers flowing from the night side of the planet, through the ring of life, onto the day side. Each river flows across the day side until it becomes so hot that it evaporates. Vegetation grows on the banks of each river, narrow green fingers threaded across the barren rocky landscape.
The first rite of passage is to take a trip down the river and make it back to the ring of life. The rivers only flow in one direction, so the way back has to be by foot. And you have to stay close to the river to have a chance to bear the heat.
The second rite involves an excursion onto the icy night side of the hot Eyeball. The teenagers must find a sacred thermal spring in the vast icy plain and return with a sample of its mineral-infused water. They must cross mountains and navigate the ice, all in the dark.
The final rite of passage is less dark (pun intended). The ring of life provides an easy path for an “around the world” trip. That’s a much more fun ritual!
There you have it: the hot Eyeball planet! I have seen a couple of posts about hot Eyeball planets on sci-fi websites (see here and here), so I imagine there are sci-fi stories set on hot Eyeballs. I would love to hear about any stories you know of.