Scientists believe that some potentially habitable planets are tidally locked, with one side permanently pointed toward their star. This means that one hemisphere experiences eternal daylight, while the other hemisphere is in eternal darkness.
Research on organisms on Earth that live in sun-poor environments shows that the absence of sunlight does not hinder life. Such creatures offer a glimpse into what alien life without circadian rhythms might look like, according to Marie Cohen of Britain’s Open University.
Researchers estimate that our Milky Way Galaxy, which contains between 100 and 400 billion stars, is home to billions of potentially habitable planets.
About 70% of the stars in the Milky Way are red dwarfs, and about 40% of them have at least one planet orbiting within their habitable zone. This means that these planets could potentially have liquid water on their surfaces, increasing the chances of life.
However, rocky planets orbiting red dwarfs are different from Earth. Red dwarfs are cooler than the Sun, so their planets orbit more closely around each other and experience stronger gravitational forces.
As a result, many of these planets are tidally locked, with one side always facing their star, similar to how the moon always shows the same face to Earth.
Unlike Earth, these planets would not have a traditional day-night cycle or seasonal changes. On Earth, life is strongly influenced by circadian rhythms, which are attuned to the alternation of day and night. These rhythms regulate biochemistry, body temperature, cell regeneration, behavior and more.
Cohen suggests it is still unclear how crucial rest periods, such as sleep, are for life. It is possible that organisms that develop without cyclic time do not require rest and function continuously.
On Earth, many organisms in sunless environments synchronize their biorhythms with non-light signals, such as temperature, humidity, or chemical changes. Some species even have internal circadian clocks that are adapted to daily and seasonal shifts.
What might extraterrestrial life look like?
Cohen notes that recent studies suggest that rocky planets orbiting red dwarfs may experience cycles that simulate days and seasons.
Simulations indicate that the contrast between the perpetually lit and dark sides of these planets likely creates fast-moving winds and atmospheric waves. If these planets host water, the dayside could generate dense cloud formations with frequent thunderstorms.
The interaction between wind, clouds and atmospheric waves can cause climate shifts, resulting in temperature and humidity cycles.
These cycles could last tens to hundreds of Earth days, although they would not be consistent with the planet’s rotation period. Even if the star remains stationary in the planet’s sky, the environmental conditions would change.
Life on these planets could evolve biorhythms that synchronize with these environmental cycles, or evolution could come up with more unusual adaptations. For example, aliens living on the dayside may migrate to the nightside to sleep and recover.
This glimpse into alien evolution shows how life might adapt to the unique environments of tidally closed planets, where traditional day-night cycles are lacking.
