Binary systems
Typical estimates suggest more than 50% of star systems are binary systems. This may be due in part to sample bias, as bright, massive stars tend to belong to binary systems and are the easiest to observe and catalogue; Another more precise analysis has suggested that the most common stars, which are less bright, do not usually have a companion and that therefore up to two-thirds of all star systems are solitary.[21].
The separation between stars in a binary system ranges from less than one astronomical unit (AU, the distance between the Earth and the Sun) to several hundred. In the latter case, the gravitational effects will be negligible on a planet orbiting one of the stars, and its planetary habitability will not be disrupted unless the orbit is very eccentric (see Nemesis, for example). However, when the separation is significantly smaller, a stable orbit may be impossible. If a planet's distance from its primary star is greater than one-fifth of the minimum distance the other star approaches, orbital stability is not guaranteed.[22] The mere fact that planets can form in binary systems has long been unclear, as gravitational forces could interfere with planet formation. Theoretical work by Alan Boss") at the Carnegie Institute has shown that gas giants can form around binary systems in the same way they do with solitary stars.[23].
A study of Alpha Centauri, the closest star system to the Sun, suggests that binary systems should not be ruled out in the search for habitable planets. Centauri A and B are separated by 11 AU at closest approach (23 AU on average), and both may have stable habitable zones. A study of the long-term orbital stability of simulated planets in this system shows that planets located approximately three AU from either star can remain stable (i.e., the semimajor axis deviates by less than 5%). A conservative estimate of the ZH of Centauri A places it at 1.2 or 1.3 AU and that of Centauri B at 0.73 or 0.74 AU, well into the stable region in both cases.[24].
Red dwarf systems
Determining the habitability of a red dwarf can help determine how common life is in the universe, since red dwarfs make up 70 to 90 percent of all stars in the galaxy. Brown dwarfs are probably more numerous than red dwarfs. However, they are not usually classified as stars, and could never support life as we know it, since the little heat they emit disappears quickly.
For many years, astronomers have dismissed red dwarfs as a potential abode for life. Their small size (from 0.1 to 0.6 solar masses) means that their nuclear reactions occur at an exceptionally slow rate, and they emit very little light (from 3% to 0.01% of that produced by the Sun). Any planet orbiting a red dwarf would have to be very close to its star to reach a surface temperature similar to that of Earth; from 0.3 AU (just inside Mercury's orbit "Mercury (planet)") for a star like Lacaille 8760 to 0.032 AU for a star like Proxima Centauri (such a world would have a 6.3-day year).[25] At these distances, the star's gravity would cause tidal coupling. The day side of the planet would forever point towards the star, while the night side would always point in the opposite direction. The only way potential life could avoid hell or freeze would be if the planet had an atmosphere thick enough to transfer the star's heat from the day side to the night side. It was long assumed that such a thick atmosphere would prevent sunlight from reaching the surface, preventing photosynthesis.
This pessimism has been softened by research. Studies by Robert Harbele and Manoj Joshi of NASA's Ames Research Center in California have shown that a planet's atmosphere (assuming it was composed of the greenhouse gases CO and H O) would need to be only 100 mb, 10% of Earth's atmosphere, for heat to be effectively transferred to the night side.[26] This is well within the levels required for photosynthesis, although water would still be frozen on the night side. night for some of its models. Martin Heath of Greenwich Community College) has shown that sea water could also circulate without freezing if ocean basins were deep enough to allow free flow beneath the nightside ice sheet. Further research—including a study of the amount of photosynthetically active radiation—suggests that orbitally coupled planets in red dwarf systems would be habitable at least for higher plants.[27]
The drawback of tidal coupling can disappear if we consider the possibility that the planet has a satellite or consider the satellite itself as a candidate for habitability.
• - If the habitability of the planet is studied, the satellite could have produced the coupling of the planet's rotation with its own movement around it, preventing the planet from always showing the same face to the star. In the solar system, an example is found in Pluto "Pluto (dwarf planet)"), which rotates on itself in the same period (6.4 days) that it takes its satellite Charon "Charon (satellite)") to complete one revolution.
• - If the habitability of the satellite is studied, it is found that most of the satellites in the solar system (including the Moon) rotate always showing the same face to the planet and some of them do so in periods that are suitable for habitability. However, no satellite in the solar system is large enough to be considered habitable.
However, size is not the only factor that can make a red dwarf incompatible with life. On a planet orbiting a red dwarf, photosynthesis would be impossible on the night side, since it would never see the sun. On the daytime side, since the sun would never rise or set, the areas under the shadow of a mountain would remain that way forever. Known photosynthesis would be complicated by the fact that a red dwarf produces most of its radiation in the infrared, and on Earth this process depends on visible light. There are several positive aspects to this scenario. For example, many terrestrial ecosystems rely on chemosynthesis rather than photosynthesis, something that would be possible in a red dwarf system. A static position of the sun eliminates the need for plants to turn their leaves towards it, deal with changes in the sun/shade pattern, or have to switch during the night from photosynthesis to stored energy. In the absence of a day-night cycle, including weak morning and evening light, there will be much more energy available at a certain radiation level.
Red dwarfs are much more variable and violent than their older, more stable cousins. They are often covered in sunspots that can dim their light by up to 40% for months at a time, while other times they emit giant flares that can double their brightness in a matter of minutes.[28] This variation would be very harmful to life, although it could also stimulate evolution by increasing mutation rates and rapidly changing climatic conditions.
However, red dwarfs have a big advantage over other stars in terms of habitability for life: they live a long time. It took humanity 4.5 billion years to appear on Earth, and life as we know it will have suitable conditions for about 500 million more years.[29] Red dwarfs, on the other hand, can live for billions of years, because their nuclear reactions are much slower than those of larger stars, which means that life could have more time to evolve and survive. Furthermore, although the probability of finding a planet in the habitable zone of a particular red dwarf is small, the total amount of habitable zone around all red dwarfs together is equal to the total amount around Sun-like stars, given their ubiquity.[30].