A tidally locked planet can only see its "sunward" side, which has an effect on its climate. In one scene, the planet appears as a giant eyeball, as shown here. The light side of the icy eye planet is a liquid ocean, while the dark side is covered with an icy crust.

On some worlds, the sun never sets. If a planet takes the same amount of time to orbit its axis once as it does to orbit its parent star once, the star will appear stationary in the sky. Such planets have only one star facing them, a condition known as tidal locking.

A tidally locked planet is a bifurcated world. There is always day on one side and eternal night on the other. On the boundary between the two is a thin spiral Eternal Loop, where the sun sets forever. In our own solar system, many worlds are tidally locked (including our Moon). Exoplanets in other solar systems orbiting their own stars may also be tidally locked.

• When gravity slows or speeds up a body's rotation, it becomes tidally locked to its parent body. In this case, the orbiting body always presents the same side as its parent body. Our moon is tidally locked to Earth (so we can only see one side of the moon), and scientists suspect that many exoplanets orbiting their stars may also be tidally locked.

Many of these planets would be uninhabitable, baked on one side and frozen on the other. Yet, like science fiction buffs, astronomers have wondered whether such worlds might harbor life. Now, sophisticated computer models and new data are revealing the atmospheres of these tidally locked distant worlds. This study further supports such a view.

spin down

For a nearby example of tidal locking, we just have to look up. Our moon is tidally locked to Earth. This is due to the moon's gravitational interaction with the earth. Over millions of years, the moon's rotation gradually result in slowing down. Just as the moon's gravity flings our oceans around, causing tides (hence "tidal locking"), the star's gravity pulls on the rocky bodies of orbiting planets, causing bulges.

If a planet is spinning faster or slower than it orbits its star, the bulge will be slightly off-center. Over time, the star's gravitational tug on the misaligned bulge will gradually slow or push the planet's rotation until they reach equilibrium. Every time a planet orbits its star, it rotates on its axis exactly once.

This effect is most pronounced on planets that are very close to their orbital partners, which is why the Moon is tidally locked to the Earth, but the Earth is not tidally locked to the Sun. Pluto and its moon Charon also have only one side, and they orbit so closely and are so similar in size that tidal forces synchronize their rotations.

While astronomers don't have an exact number of tidally locked worlds, the "basic expectation" is that many exoplanets are gravitationally linked to their stars in this way, said Daniel Cole, a planetary scientist at the Massachusetts Institute of Technology.

• Pluto and its moon Charon, pictured here from profile, are tidally locked to each other. Normally, one orbiting body is tidally connected to another larger body, but Pluto and Charon are very similar in size and distance, so they are tidally connected to each other.

Many of these stars are extreme places that are both "grilled" and "frozen". Take, for example, a large, rocky exoplanet called 55 Cancri e, which orbits its star (Sun-like G-type) roughly every 18 hours.

In a recent Annual Review of Fluid Dynamics, Pierre Ambert described the atmospheres of tidally locked worlds, saying: "It gets so hot that during the day it creates permanent oceans of magma." "Then you get rocks The vapor evaporates into the atmosphere and then condenses closer to night. So the 'rain' is made of things like silicon monoxide."

Planets with tidally locked atmospheres like Earth might redistribute heat fairly efficiently. The result could be an environment suitable for life.

air and water

At the turn of the last century, astronomers incorrectly believed that Venus might be tidally locked. In 1903, Xie Haihua wrote: "Between these two independent areas of eternal day and night, there must be a vast rose-colored hazy zone, where the climatic conditions may be very suitable for the existence of intelligent life."

Optimism about the habitability of the "transition zone" has also emerged in science fiction, says Dorian Abbott, a geophysicist at the University of Chicago. The thinking was that the region between the light and dark sides of the planet would be mild enough for water to exist in liquid form, and perhaps life to survive.

Research has cast doubt on this theory. The transition zone, which astronomers call the "Terminator," is a barren place for organisms that need sunlight to survive.

• Dusk, the gradual transition from day to night captured from above Earth, has captured the imagination as a place that might support exoplanet life. But if an exoplanet has a suitable atmosphere, the obscurity zone might not be the only habitable part of it.

But with a suitable atmosphere, a transition zone would not be needed to support life. The atmosphere transports heat around the planet, creating the conditions for the widespread spread of liquid water (and perhaps life). There's a balance here, the atmosphere has to be dense enough to transport heat, but not so dense that it's suffocating. Conversely, if the dark side gets too cold, it could freeze gases, disrupting atmospheric circulation.

A 2016 simulation developed by Abbott and Cole showed that the right balance is achievable, whereby some tidally locked exoplanets could have "just right" atmospheres that efficiently move heat , can even keep that side warm at night.

"It's weird because it's going to be perpetual night, but you might still have conditions where life like ours could exist," Cole said. Take Earth's poles, for example, where there's little sunlight but life would still exist. "It's not too cold, largely because the wind or ocean movement actually redistributes the heat."

On Earth, the ocean is a key player in the global thermal cycle. Water stores more heat than air and transfers it more efficiently. Thus, oceans, like atmospheres, play an important role in maintaining temperature on the day and night sides of tidally locked exoplanets.

Oceans also cause evaporation, which stimulates the formation of clouds, which also play an important role in regulating planetary environments, said Ding Feng, a Harvard researcher who studies exoplanet atmospheres. Clouds of water accumulating in the atmosphere act as a kind of reflective blanket that bounces incoming star radiation back, helping to cool the planet.

Some computer simulations suggest that clouds can keep temperatures low enough to form oceans even on exoplanets that would otherwise be scalding hot. These clouds form rain. During Earth's daytime, strong updrafts that create where the sun is most intense can move warm, moist air upward, causing heavy rain, Pierre Humbert said.

land and life

Rainfall could help regulate the temperature of tidally locked exoplanets, especially if they have land. On Earth, rainwater reacts with exposed rocks to trap some carbon in the form of minerals and remove it from the atmosphere, which helps cool the planet. This chemical weathering could also keep carbon dioxide levels manageable on tidally locked planets over time, Pierre Humbert said.

He speculates that other atmospheric gases may also make exoplanets more habitable. Nitrogen, for example, can help prevent water loss by trapping moisture lower in the atmosphere. In the atmosphere, nitrogen is exposed to less ultraviolet light, which splits water vapor into oxygen and hydrogen. A nitrogenous atmosphere would help maintain liquid oceans that are key regulators of temperature.

• An exoplanet orbiting the red dwarf star TRAPPIST-1. All seven planets orbiting the dim red dwarf are thought to be tidally locked to the star. It's in the star's habitable zone, making it the most likely "TRAPPIST-1" planet to host liquid water.

Scientists can input many of these variables into computer simulations, but exoplanet atmosphere studies are all speculative. The study of tidally locked objects is no exception. First, there are few data that can prove the reality of all these variables. Atmospheric astronomy of most exoplanets has been limited to planets that don't look much like Earth. They are much larger and are often covered by thick atmospheres, like Neptune and Uranus.

But new instruments, such as NASA's Transiting Exoplanet Study Satellite (TESS), will greatly increase the number of exoplanets that have been detected. When NASA's James Webb Space Telescope launches in 2021 (though it may be delayed because of the coronavirus pandemic), it will allow for more detailed observations of exoplanets, including data on the composition of their atmospheres.

The data will help astronomers refine their models and better understand the conditions for habitability on any planet, including those that are distant and tidally locked. There are trillions of exoplanets that could potentially exist, with shallow oceans where life could form, warm continents where creatures could grow or crawl, and atmospheres that would allow them to take to the skies. There, life can thrive under the eternal light of an ever-present alien sun.