Based on our experience here on Earth, we know life here to be tenacious and ubiquitous. Even in the harshest climates, if we look hard enough, we can usually find some sort of life there. We are hoping the same is true on worlds around other stars, and recent research has begun looking into the question of whether life on other worlds could exist in the most inhospitable environs: the terminator zone of tidally locked planets.
All planets that orbit a star have a terminator zone. This is the band of shadow that marks the night side of a planet and the day side. On most planets, the terminator moves along the surface as it rotates, but one a tidally locked planet, the terminator stays over the same spot all the time.
Tidal locking is a phenomenon that occurs when a planet rotates at the same speed as it orbits its star, so it always shows the same face to it. This happens when the gravitational pull of the star is stronger than the rotational force of the planet, and it causes the planet to slow down until it reaches a stable state. You might be familiar with this effect from our own moon, which is tidally locked to Earth and always shows us the same side.
But what if a planet is tidally locked to its star? Well, that would create a huge contrast in temperatures between the two sides of the planet. The day side would be constantly exposed to the star’s heat and light, while the night side would be in perpetual darkness and cold. Depending on how close the planet is to its star, the day side could be hot enough to melt rocks, while the night side could be cold enough to freeze water.
But there is a region in between, called the terminator zone, where things get more interesting. The terminator zone is a band that separates the day from the night, where the temperatures are more moderate and could allow liquid water to exist. And where there is water, there could be life, right? Well, maybe. It depends on a lot of factors, such as the type and size of the star, the amount of water on the planet, and the presence of an atmosphere.
Some scientists have been studying these terminator zones on exoplanets, especially those that orbit dim red dwarf stars, which are very common in our galaxy. Red dwarf stars are smaller and cooler than our sun, but they can still emit powerful flares and radiation that could harm life on nearby planets. However, some of these planets could have habitable climates in their terminator zones, if they have enough water but not too much, and if they have rocky surfaces instead of global oceans.
Why does water and surface matter? Well, water is essential for life as we know it, but too much water could create a runaway greenhouse effect that would make the planet too hot for life. On the other hand, too little water could make the planet too dry and barren for life. A rocky surface could help regulate the temperature and provide nutrients for life, while an ocean-covered surface could make the planet more uniform and boring for life.
While this is an interesting idea, there are still a lot of unknowns and challenges for life to survive on these extreme worlds. For example, how would life cope with the constant light or darkness? How would it deal with the strong winds and storms that could occur in the terminator zone? How would it adapt to the possible volcanic activity on the day side or the ice formation on the night side? How would it communicate and evolve in such diverse environments?
These are some of the questions that astronomers hope to answer in the future, as they search for signs of life on these exotic planets. Using telescopes like the James Webb Space Telescope and the future Habitable Worlds Telescope, maybe one day we will find out that we are not alone in this vast universe, and that life can thrive in places we never imagined.