But now it seems that if these worlds also have oceans, then tides could drive a biological clock just like a rising and setting sun would, according to a simulation by Avi Loeb and Manasvi Lingam at Harvard University.

We used to think that not having a day-night cycle would make it harder for life to emerge, because of the lack of a circadian clock. On Earth, these built-in timekeepers play an important role in several biological contexts, including reproduction.

Of course, it’s possible to imagine aliens without biological clocks, but how life could evolve without one is difficult to imagine.

“There is abundant evidence that biological clocks are essential to modern life on Earth, and that biological clocks may have evolved very early in the history of life on Earth, either in cyanobacteria or in other single-celled organisms,” says Jennifer Macalady at Pennsylvania State University.

Tides make life tick

Loeb and Lingam’s study shows tides could play the role of timekeeper. If a planet with one side always facing its star has an elliptical orbit, the change in distance as it circles the star would create the back and forth pull needed for tides.

“Because of the small size of the habitable zone around stars like TRAPPIST-1, the tides are orders of magnitude larger than on Earth,” Loeb says.

Tides would form ponds as they go out, and can also help dissipate heat caused by volcanic activity or proximity to the planet’s star, mimicking periodic evaporation caused by a rising and setting sun. Each cycle would concentrate chemicals essential for the emergence of life. Eventually, life could evolve with its days and nights governed by the flow of the ocean.

This might sound alien, but even on Earth some animals’ biological clocks are set by the tides rather than the sun. So-called circatidal rhythms govern the lives of certain fish and crabs.

What kinds of worlds are likely to have a permanent day and night side? Some planets orbiting dwarf stars, like the nearby Proxima Centauri or TRAPPIST-1, are much closer to their host stars than we are to our sun. Because of this proximity, gravity from the star tugs much more at one side of a planet than the other, causing it to show the same face to its star throughout an orbit.

On such a “tidally locked” planet, the side facing away from the star would never receive light and could be too cold for life, while the side facing it could be too hot. The areas where life might develop would be on the border – the zone experiencing a constant “sunrise” or “sunset”.

“Work like this is important to inform our understanding of how life might evolve on the planets around other stars,” says Matt Burleigh at the University of Leicester, UK. “Oceanic tides may well be another factor that needs to be taken into account when speculating on life on planets around red dwarf stars like Proxima Centauri.”

Algal signals

Loeb and Lingam say these planets might be revealed to us through algal blooms that would change how the planet reflects its star’s light, in sync with the tides. But even if there is a signature there, seeing it might be tricky, says Jaymie Matthews at the University of British Columbia in Canada.

“Even if tidal blooms in an alien ocean are more extensive than in Earth’s, they are unlikely to be global,” he says.

“And they are not long-lasting, so the signature in the spectrum of exoplanet light will be diluted and intermittent.”

Source: New Scientist