Scientists believe that some potentially habitable planets are tidally locked, with one side constantly facing a star. This means that one hemisphere experiences perpetual daylight while the other is in perpetual darkness.
Studies of organisms on Earth living in sunless environments show that the lack of sunlight does not make life arduous. Such creatures give us a glimpse of what alien life might be like without a circadian rhythm, according to Marie Cohen of the Open University in the UK.
Scientists estimate that there are billions of potentially habitable planets in our galaxy, the Milky Way, which contains 100 to 400 billion stars.
About 70% of the stars in the Milky Way are red dwarfs, and about 40% of them have at least one planet orbiting in their habitable zone. This means that these planets could potentially have liquid water on their surfaces, increasing the chances of life.
However, the rocky planets orbiting red dwarfs are different from Earth. Red dwarfs are cooler than the Sun, so their planets orbit closer, experiencing stronger gravitational forces.
As a result, many of these planets are tidally locked, meaning that one side of them always faces the star, just as the Moon always keeps the same side facing Earth.
Unlike Earth, these planets would not have time-honored day-night cycles or seasonal changes. On Earth, life is deeply influenced by circadian rhythms that are tuned to the alternation of day and night. These rhythms regulate biochemistry, body temperature, cell regeneration, behavior, and much more.
Cohen I suggest it is still not clear how crucial rest periods such as sleep are to life. It is possible that organisms that evolved without cyclical time might not need rest, functioning continuously.
On Earth, many organisms in sunless environments synchronize their biorhythms with nonlight signals such as temperature, humidity, or chemical changes. Some species even have internal circadian clocks that are tuned to diurnal and seasonal changes.
What could extraterrestrial life look like?
Cohen notes that recent research suggests that rocky planets orbiting red dwarfs may experience cycles that simulate days and seasons.
Simulations suggest that the contrast between the perpetually lit and obscure sides of these planets likely creates fast-moving winds and atmospheric waves. If these planets have water, the day side could generate dense cloud formations with repeated lightning storms.
The interaction between wind, clouds and atmospheric waves can cause climate change, resulting in cycles of temperature and humidity.
These cycles can last from tens to hundreds of Earth days, although they do not coincide with the planet's spin period. Even if a star remains stationary in the planet's sky, environmental conditions change.
Life on these planets may develop biorhythms in sync with these environmental cycles, or evolution may produce more unusual adaptations. For example, aliens living on the day side may migrate to the night side to sleep and regenerate.
This insight into alien evolution shows how life can adapt to the unique environment of synchronous planets, which lack time-honored day and night cycles.
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