In most science fiction films and TV series, aliens are depicted as relatively similar to humans. As a rule, they are bipedal, upright creatures that need water, food, and sleep. But what is the likelihood that representatives of extraterrestrial civilizations will have so much in common with us? Since we have not yet found signs of life beyond Earth, such thoughts may seem frivolous, but scientists are unlikely to agree with this. This is because in recent years, many exoplanets have been discovered – distant worlds orbiting other stars. The results of numerous studies have even shown that some of them are suitable for life. True, on many exoplanets where life could originate, there is no change of day and night. But how, in this case, would the creatures inhabiting such worlds look and live?
Conditions on other planets are unlikely to be identical to those on Earth. This is especially true for the change of day and night. Image: newscientist.com
Distant Worlds
We live in an amazing time, because we know the answer to a question that has troubled humanity for centuries: beyond Earth, there are many planets that revolve around other stars. In the Milky Way alone, according to researchers, the number of stars varies from 100 to 400 billion, which means that in our Galaxy there are billions of potentially habitable planets. Just think about these numbers.
The first exoplanets were discovered in the 1990s, and today astronomers have confirmed the existence of 5,000 distant worlds. Thousands of other space objects have also been found – candidates for exoplanets. All of them require further observations to finally confirm their status. Not to mention other galaxies, many of which are significantly larger than the Milky Way.
The habitable region or Goldilocks zone is the distance of a planet from its host star that provides conditions suitable for the formation of life. Image: techinsider.ru
But let's get back to the stars that we can observe. Seventy percent of them are tiny, cool red dwarfs or M-dwarfs. A detailed review of exoplanets published in 2013 found that 41% of stars of this type have planets orbiting around the “Goldilocks” zone – the optimal distance at which the planet's temperature is sufficient to support liquid water.
Read also: The Milky Way May Have Millions of Habitable Planets – Scientists Tell Where to Look
Of course, scientists do not yet know whether the exoplanets discovered contain water, let alone life. However, there are 28.7 billion planets in the Goldilocks zones of M-dwarfs alone. And that's not counting other types of stars, such as our Sun.
Earth-like planets
Recall that our planet, like Mercury, Venus and Mars, is a rocky celestial object. For this reason, worlds orbiting in the “Goldilocks” zone around red dwarfs are called Earth-like. These exoplanets, however, differ from Earth: firstly, red dwarfs are much colder than the Sun, and secondly, they are located close to each other, which makes the gravitational attraction between the star and the planet extremely strong.
This gravitational force slows down the rotation of exoplanets around red dwarfs, meaning that most Earth-like planets always have one hemisphere facing the star, while the other hemisphere faces away from it. Recall that the closest such exoplanet to Earth is Proxima Centauri b, located in the Alpha Centauri system at a distance of four light years.
Exoplanet Proxima Centauri b. Image: nasa.com
This relatively close world, unlike our planet, is not subject to the change of seasons, as well as day and night. However, all living organisms on Earth – from bacteria to humans – have circadian rhythms corresponding to the 24-hour cycle of day and night. But if one side of Proxima Centauri b always faces the star, and the other – into space, what creatures can inhabit it?
More on the topic: What are circadian rhythms? Setting up your internal clock
Day and night on other planets
Circadian rhythms are a kind of biological clock that regulate body temperature, cell regeneration, sleep, behavior, and more. Researchers believe that organisms that evolved without day and night simply would not need rest, and therefore no sleep.
To support this idea, look no further than organisms that thrive away from daylight on our planet. Naked mole rats, for example, spend their entire lives underground, yet even they have circadian rhythms, albeit attuned to daily and seasonal cycles of temperature and precipitation. Deep-sea mussels and shrimp, for their part, swim in time with the ocean tides, suggesting that biorhythms have inherent advantages.
If there is life on exoplanets orbiting red dwarfs, these creatures probably don't know what sleep is. Image: natgeofe.com
Moreover, the results of recent studies have shown that exoplanets orbiting red dwarfs may have cycles that replace days and seasons. Thus, scientists believe that the contrast between the day and night sides apparently creates rapid gusts of wind and atmospheric waves. And if there is water on the planet, then thick clouds full of lightning are likely to form on its day side.
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The interaction between winds, atmospheric waves and clouds can change temperature, humidity and rainfall. The length of these cycles will vary depending on the planet, but they will not be related to its rotation period. As long as the star remains stationary, the environment will change.
Deep underground and in the depths of the ocean, there are organisms that don't depend on sunlight. Image: futurecdn.net
Perhaps life on such extreme planets would develop biorhythms that are synchronized with these cycles. Or perhaps evolution would find a stranger solution – we could imagine species that live on the day side of the planet and migrate to the dark side for rest and recuperation, astronomers suggest.
All this data really makes you think about the fact that we are not alone in the vastness of the infinite Universe. And if life (especially intelligent life) exists somewhere beyond Earth, it will be very different from us and will definitely be amazing.