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Tidally locked planets could be more common than Earth-like planets! And these "eyeball planets" might even be a promising place to look for unique lifeforms!

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[♪ INTRO].

What do you picture when you think of an exoplanet? It's a surprisingly important question, because it probably shapes how you think about other star systems, maybe even how you think about their habitability, or how they evolved.

So take a second to consider it. Off the top of your head, you might imagine icy, rocky worlds, or scorching gas giants, but what about something a little stranger? Like, eyeball planets.

These are planets that, well, look kind of like eyeballs, with one hemisphere totally different than the other. Because of how they orbit, they would have all of their ice on one side, and all of their deserts on the other. They could even have concentric rings with different climates, like the pupil, iris, and white of an eye.

And while they sound like something out of science fiction, astronomers think they're probably not all that rare. They could be more common than Earth-like planets, and according to some researchers, they might even be able to support life. Despite their unusual appearance, eyeball planets form thanks to a pretty standard phenomenon: gravity.

Gravity is what pulls a star and a planet together, and it's also mostly what keeps a planet in orbit. But strange things start to happen when a world orbits its star too closely. The closer a planet gets, the stronger the pull of its star's gravity is.

And over time, that pull can actually slow down a planet's rotation. Eventually, the world becomes tidally locked, meaning it rotates in exactly the same amount of time it takes to orbit its star. In other words, it's day is as long as its year, and the planet always keeps the same face turned toward its star.

If you want an example of this, just check out the Moon. It's tidally locked to the Earth, which is why you always see the same pattern of craters on it. The difference is, the Earth is the Earth and not a giant scorching nuclear furnace.

Which as it turns out, does make quite a big difference! Around a star, tidal locking means that one of a planet's hemispheres would bake and experience constant day, reaching temperatures up to 100°C or more. Meanwhile, the back side would be in an icy, perpetual night, at less than -100°.

Within the last few years, scientists have suggested that worlds like this may be even more common than we thought. But that doesn't mean you need to abandon your dream of finding life off-Earth. Because eyeball planets could potentially be habitable, at least, under some specific conditions.

For one, your planet would likely need to orbit a red dwarf star. These stars are smaller and cooler than the Sun, so even if your planet was close enough to be tidally locked, it could still be in the habitable zone. That's the area around a star where conditions are just right for liquid water.

Admittedly, red dwarfs are also more active than the Sun, producing lots of flares. But some researchers think that if a planet had a magnetic field, it would be okay for a while. Besides a red dwarf and a magnetic field, your planet would also need to actually have liquid water, along with an atmosphere.

This is partly because we're pretty confident lifeforms need water and air, and partly because those things would help with the temperature differences. Wind and weather could even out the temperatures on an eyeball planet to something more like we see on Earth, between 50 and -50°C. It would still be unpleasantly hot or cold on some parts of the planet, but life could enjoy more temperate conditions in the narrow ring where the two hemispheres meet.

Still, even if the eyeball planets are habitable, living there would be almost nothing like what we're used to. In that narrow ring, the sun wouldn't move in the sky, and life would witness a perpetual sunset. This, combined with the dimmer red dwarf star, would mean that any photosynthesizing creatures would get much less light than they would on Earth.

Plants would have to take advantage of any light they could get, so they might evolve to be totally black, since black absorbs all colors of visible light. Also, because of the temperature differences, winds would blow constantly around the planet, up to a few thousand kilometers per hour. So organisms would either have to be streamlined to withstand this gale, or could take advantage of it to move around.

Finally, there would be no day or night cycle on an eyeball world, either. That might not sound like a big deal, but pretty much every lifeform we know of on Earth has some kind of circadian rhythm, driven by the planet's rotation. Maybe the rhythms of life would evolve differently on an eyeball world, but with no examples around here, scientists really don't know how that would work.

So, maybe there's an eyeball planet out there with a perfectly streamlined, black creature that's adapted to a world of twilight. Unfortunately, it will probably be a while before our technology can figure that out. Right now, though, scientists are actively looking into the potential habitability of eyeball planets.

Because these worlds aren't science fiction, or a distant theoretical possibility. Red dwarf stars actually make up around 70% of all the stars in our galaxy, and statistics suggest that all stars have at least one planet around them. So there could be a hundred billion or so eyeball planets out there.

Scientists have already found probable candidates, too. The TRAPPIST-1 system, which is nearly 40 light-years away, consists of seven tidally-locked planets around a small red dwarf. Whether they have conditions right for life is still unknown, but with the next generation of telescopes coming online, we're certainly getting closer to finding out.

Thanks for watching this episode of SciShow, especially to our patrons on Patreon! You're the best, and we're thankful for your support, creativity, and curiosity about the universe. If you want to help us keep making episodes like this one, you can go to [♪ OUTRO].