What would alien life be like on a planet with no day and night cycle?

When we search for life we are often looking for planets with conditions that mimic our own. But it might not be like this. Many exoplanets in the Universe have very different orbits to our own, and some are tidally locked, meaning the same surface of the planet is always facing their star.

This gives rise to a day and night side. Previously researchers had thought this would be bad for the chances of life developing, but now research suggests otherwise. In fact, these planets might be the best place to look for alien life.

While we know stars like our Sun can give rise to life, they are not the most common star out there. Stars called M dwarfs are ten times more common than Sun-like stars, so trying to understand how life might develop around these stars is an important quest in modern astronomy.

M dwarf stars are smaller, lighter and less bright than our Sun, and as a result the planets around them are completely different to our solar system. They are also easy to spot, and survive for a much longer time than stars like our own Sun.

“We now know that most M dwarfs are hosts to closely-packed planetary systems characterised by a paucity of Jupiter-mass planets and the presence of multiple rocky planets,” says Aomawa Shields, astronomer at the Harvard-Smithsonian Center for Astrophysics. “Roughly a third of these rocky M-dwarf planets orbit within the habitable zone, where they have the potential to support liquid water on their surfaces.”

Planets around M dwarf stars, known as M dwarf planets, are often in an eccentric orbit, meaning instead of always being the same distance from their star, they go from extremely close to extremely far away. This can cause them to be ‘tidally-locked’, meaning the same half is always facing towards their star, and the same half facing away. Some of the planet is in constant night, some in constant day and other parts are experiencing constant sunsets and sunrises.

A new paper, published on the preprint server arXIv, has shown exactly what an eccentric orbit like this could mean for habitability of the planet. A group of researchers from Peking University, China and McGill University, Canada, modelled the orbits of M dwarf planets.

“The orbital configurations of M-dwarf planets are always tidally-locked at resonance states, which are quite different from those around Sun-like stars,” the authors said. “Thus, the conclusions drawn from previous studies focusing on Sun-like-star planets may not be applicable to potentially habitable exoplanets around M dwarfs. M-dwarf planets need to be investigated separately.”

The group found planets in the habitable zone, where liquid water could exist on the surface, are unlikely to move out of that zone even during their eccentric orbits. However, the actual habitable zone is smaller around an M dwarf star than a Sun-like star. Regardless of these differences, M dwarf stars are where we should be searching for life, the authors say. “Exoplanets around M dwarfs are the most promising targets in searching for alien life,” the paper explains.

Biological clocks

In a paper published earlier this year, Avi Loeb and Manasvi Lingam at Harvard University simulated what life would be like on a planet with no day and night, particularly looking into the idea of biological clocks. Our biological clock on Earth, which is known to have played an important part in the evolution of life as we know it, comes from the day and night cycle.

“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, from Penn State University. “Cyanobacteria have been an important component of the biosphere for most of Earth’s history, 3-4 billion years.”

But, on a planet with no day and night, the pair found a biological clock could come from other biological processes, such as the tides. “We suggest that periodic tides could replace this cycle in tidally-locked planets around M-dwarfs,” Loeb says. “The period there is set by the orbital period which is also of order days.”

“Of course, one can imagine ‘alien’ animals that do not have these clocks,” he says. “But the main point of our paper is that tides serves many other important purposes for the development of life, such as allowing ponds to be generated again and again. As the ponds partially evaporate in each cycle, they increase the concentration of chemicals that are essential for the emergence of life.”

Searching for M dwarf planets

The search for life with no day and night looks promising, and because of this many new missions are setting out to look. NASA’s Transiting Exoplanet Survey Satellite (TESS), due to launch in March next year, will be one of the first to study more than 200,000 stars for signs of planets. As we continue to search, we will start to understand more about these strange worlds and whether or not they might have the conditions for life, albeit very different to that we know.

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