Life on Alien Planets: How a Day-Night Cycle Might Change Evolution

Introduction

When we think of alien life, we often imagine creatures that are similar to us, with day and night cycles that we’re familiar with. However, research suggests that many planets that could support life may not have a day-night cycle. This raises intriguing questions about how life might evolve on these planets and how it might differ from life on Earth.

Billions of Potentially Habitable Planets

The Milky Way galaxy is estimated to have between 100 billion and 400 billion stars. Of these, 70% are tiny, cool red dwarfs, also known as M-dwarfs. A recent study estimated that 41% of M-dwarf stars have a planet orbiting in their “Goldilocks” zone, where the planet has the right temperature to support liquid water.

M-Earths: Rocky Planets in the Goldilocks Zone

These planets, known as M-Earths, are different from our Earth in fundamental ways. Because M-dwarf stars are much cooler than the Sun, the planets are close-in, which makes the gravitational pull of the star on the planet incredibly strong. This creates friction that resists and slows the planet’s spin over time, resulting in most M-Earths being tidally locked, where one hemisphere always faces the star while the other always faces away.

Tidally Locked Planets and the Circadian Rhythm

A tidally locked planet’s year is the same length as its day. The Moon is tidally locked to the Earth, which is why we always see only one face of the Moon and never its dark side. A tidally locked planet may seem exotic, but most potentially habitable planets are probably like this. Our closest planetary neighbor, Proxima Centauri b, is likely a tidally locked M-Earth.

Unlike our Earth, M-Earths have no days, no nights, and no seasons. But life on Earth, from bacteria to humans, has circadian rhythms tuned to the day-night cycle. Sleep is only the most obvious of these rhythms, but the circadian cycle affects biochemistry, body temperature, cell regeneration, behavior, and much more.

What Can We Learn from Organisms on Earth?

We can look to organisms on Earth that thrive far from daylight, such as cave-dwellers, deep-sea life, and microorganisms in dark environments like the Earth’s crust and the human body. Many of these life-forms do have biorhythms, synchronized to stimuli other than light. Naked mole rats spend their entire lives underground, never seeing the sun, but they have circadian clocks attuned to daily and seasonal cycles of temperature and rainfall.

Recent Research and Climate Models

Recent research shows that M-Earths could have cycles that replace days and seasons. Scientists have adapted climate models to simulate what the environment on an M-Earth would look like, including our neighbor Proxima Centauri b. In these simulations, the contrast between dayside and nightside seems to generate rapid jets of wind and atmospheric waves, which could cause regular cycles in temperature, humidity, and rainfall.

Conclusion

While we can’t know for sure how life would evolve on M-Earths, it’s clear that it would be vastly different from life on Earth. The lack of a day-night cycle would likely lead to the development of biorhythms synchronized to environmental cycles, such as temperature and humidity fluctuations. The possibilities are endless, and it’s exciting to consider the potential for life to thrive in ways we can’t yet imagine.

FAQs

Q: How many potentially habitable planets are there in the Milky Way galaxy?
A: The Milky Way galaxy is estimated to have between 100 billion and 400 billion stars. Of these, 70% are tiny, cool red dwarfs, also known as M-dwarfs. A recent study estimated that 41% of M-dwarf stars have a planet orbiting in their “Goldilocks” zone, where the planet has the right temperature to support liquid water.

Q: What is an M-Earth?
A: M-Earths are rocky planets that orbit in the habitable zone of M-dwarf stars. They are different from our Earth in fundamental ways, with strong gravitational forces and tidal locking, where one hemisphere always faces the star while the other always faces away.

Q: How would life on M-Earths differ from life on Earth?
A: Life on M-Earths would likely be vastly different from life on Earth. The lack of a day-night cycle would lead to the development of biorhythms synchronized to environmental cycles, such as temperature and humidity fluctuations. This could result in unique adaptations and behaviors that we can’t yet imagine.

Q: Is there evidence of life on Proxima Centauri b?
A: There is currently no evidence of life on Proxima Centauri b. However, it is a potentially habitable planet that is likely tidally locked, which could lead to unique environmental conditions and potential adaptations.