The universe is teeming with all sorts of extrasolar planets. Some of them are wandering through space alone, on the outskirts of the galaxy, with no Sun of their own to call "home." Others belong to planetary systems that are extremely different from our own. Some orbit multiple stars; others are in binary systems, and some aren't planets at all, but moons that may be habitable for a number of reasons.
One factor astronomers must consider before deeming a planet potentially "habitable" or "Earth-like" is through analyzing its characteristics, the characteristics of its star, and any other information that's possible to glean from the planet as it transits its sun. Some of this information can be used to determine if the planet lies in a goldilocks-zone, where life is theoretically possible, and whether it orbits its star from such a close distance that the two are tidally-locked.
What is Tidal Locking Exactly?
Have you ever noticed that no matter how long, or at what time of day or night, if you look up at the moon, you always see the same features? You might find this strange, since we know the Earth rotates around the Sun, and the moon, in turn, orbits the Earth, but that doesn't quite paint a full picture. Yes, the Moon rotates on its axis around Earth, and the Earth rotates on its axis. However, because the Moon and Earth are only separated by 238,900 miles (384,472 km), the pull of gravity over time has slowed the rotation of the Moon—a phenomenon that can happen when two celestial objects are in close proximity to one another.
The rotation of the smaller object slows until the two objects are in sync, and because of the gravitational interactions of both celestial bodies, the smaller body rotates on its axis at the same pace at which it rotates around the other object — in other words, the length of a day (when a planet makes a full rotation on its axis) and a year (when a planet or moon makes one full rotation around its sun) are the same. Another word for this is synchronous rotation.
Over a long enough period of time, the process can continue until a planet and its moon (or a planet and its sun) are permanently tidally locked together, with each showing only one face to the other. In our solar system, Pluto and Charon are in this position. In fact, in about 50 billion years, the Earth and the Moon will also be tidally locked to each other, and the Moon will stop moving in the sky, and hang motionless, visible from only half the Earth.
Beyond Our Solar System
The phenomenon of tidal locking is also seen when we venture beyond our solar system, looking at exoplanets that ordinarily orbit stars that are smaller and less massive than our Sun. The problem is, planets or exomoons caught in this cycle are bipolar — one side is always warm and the sun never sets, while the other hemisphere is very cold and eternally in night. This means there are extreme temperature variations on these tidally-locked celestial bodies.
Just by virtue of how exoplanets are found, a lot of the ones we've discovered that could possibly support life may be tidally locked. The reason why is that as many as three-quarters of suns in our galaxy are red dwarfs, or “M-dwarfs". These are smaller and cooler than our sun. A planet orbiting an M-dwarf would need to be much closer to its star to support life—as close as Mercury is to our sun. And at that distance, the star’s gravity would pull it into a tidally locked orbit.
This presents somewhat of a problem when it comes to deeming a planet "habitable" or "Earth-like" No matter how similar the exoplanet or exomoon is to Earth in terms of distance from its sun, size, mass, and chemical composition, tidally locked planets present a lot of challenges: They're just not very similar to the environment we've grown accustomed to.
Meet the Terminator Zone
Hypothetically, there is a sliver of land in some tidally locked planets that may be more conducive to life: This is known as the so-called "terminator zone." Tidally locked exoplanets mostly appear static, even when they are definitively rotating on their axis and around their parent star, However, they just appear that way to outsiders because of their tidally locked nature.
Planets and moons of this nature experience night and day much differently than Earth. Half of the planet is extremely hot and inhospitable, while the other half is below freezing and also inhospitable to life. For example, the sunlit side of the moon is 260 degrees Fahrenheit (127 degrees Celsius), while the "dark side of the moon" has temperatures that reach minus 280 degrees Fahrenheit (-173 degrees Celsius). However, there's a portion of these planets that experience perpetual twilight. This is the terminator zone, the area between the day side and the night side, which is in a sort of perpetual twilight. In this area, there is the possibility that the temperature and conditions may be more conducive to human life.
As mentioned before, tidal locking is the result of gravitational interactions between a larger body and a smaller one. The larger one tends to pull the "near" side (or the day-facing side) much more strongly than the night-time (or the far) side. As a result, the smaller tidally locked body tends to be slightly flattened out, with a "tidal bulge" — meaning the longer axis is pointing toward the larger planet. Given a couple billion years, the larger planet's gravity will pull the tidal-bulge off-center, toward the larger planet's alignment. Inevitably, the pull of the planet will cause the smaller bodies to spin to slow down to match the larger body's orbital period.
Rocky planets trapped in tidally locked orbits will look pretty unusual. In fact, they may eerily resemble an eyeball. The star-facing side would likely be pretty barren, given that the land would receive more light and radiation. The opposite hemisphere might be perpetually frozen over. In between the night and day side is a portion of the planet where the 'Sun' never sets-where it may neither be extremely hot or cold, but just right for admittedly what would probably be pretty weird lifeforms.
A study in astrobiology argues that if one of these planets had an atmosphere similar to Earth's, the planet may be able to transfer heat from the star-facing side to the night-side, which would potentially prevent liquid or the atmosphere from becoming tied up in ice on the nightside. It could make the temperatures stable enough planet-wide for liquid water, and thus life, to exist.
Humans settling in the terminator zone on these worlds might be able to generate geothermal energy, using cold water from the nightside and hot water from the dayside in a sort of thermal reaction.
The nature of the star the tidally-locked planet orbits is also an important factor. Since these planets must orbit their stars pretty closely, the planet and its atmosphere remain at the mercy of solar flare-ups, which are pretty common in red dwarfs, not to mention lots of radiation. The planet would need to generate enough atmosphere to build a protective shield against the harmful effects of solar radiation, or human settlers wouldn't stand a chance. With enough of an atmosphere, however, cloud cover and air currents could make the terminator zone habitable, and maybe even areas beyond.
Of course, living on a tidally locked planet would be an unusual experience. Imagine looking up into the sky and seeing nothing ever change. Any intelligent life forms that may have evolved on one of these planets would have a much different understanding of the universe than we do. Even the passage of time would be different, as they might lack a concept of day and night. However, research shows we shouldn't be counting tidally locked planets out for habitability.