Imagine that you are walking on a planet illuminated by a red sun. There are no sunrises or sunsets here.
A large incandescent fireball hangs constantly in the sky. The shadows from large stones, hills and mountains have not changed for millennia. But fast clouds are rushing across the sky, bringing cold, moist air from the hemisphere, where eternal night reigns. Sometimes gusts of wind are so strong that they can lift into the air not only a gaping astronaut, but also heavy equipment. Is there a place in this world for living organisms? Or are the planets near the red stars lifeless cosmic bodies with hellish heat on the day side and fierce cold on the night side? This is not the first time this question has arisen in the scientific community, and there are several reasons for this.
Find what you can't see
Searching for exoplanets is a rather difficult scientific task, since most of them cannot be observed directly with a telescope. There are quite a few ways to detect them, but most often in news reports, the radial velocity method (Doppler method) and the transit method are mentioned. The essence of the first is that scientists are studying the spectrum of a star, trying to use the Doppler effect to notice in it signs of the presence of one or more planets. The fact is that in the process of its orbital motion, the planet also attracts a star to itself, forcing it, as it were, to "wiggle" in time with the period of revolution. The amplitude of such wobbles depends on the mass of the planet, the distance between the planet and the star, as well as the angle at which the observer from the Earth looks into the planet's orbit. If an exoplanet is massive enough and orbits close to its star, and its orbit is edge-on from the solar system, then the chances of finding it will be high. However, with an increase in the radius of the orbit or a decrease in the mass of an alien planet, it becomes more difficult to find it. So this method will be much more effective in finding heavy planets in orbits close to the star. Moreover, the method of radial velocities determines only the lowest possible value of the planet's mass, since by studying the displacement of spectral lines, researchers cannot find out the angle at which an alien star system is visible. It was in this way that the planets around Proxima Centauri and the star Gliese 581 were discovered.
In order to conduct searches with the second method, scientists very accurately measure the brightness of the star, trying to find the moment when the exoplanet will pass between it and the Earth. At this moment, the brightness of the star will drop slightly, and the researchers will be able to draw some conclusions about the parameters of the alien star system. The method is also interesting because in some cases it allows you to get an idea of the exoplanet's atmosphere. The fact is that during transit, the light of a star passes through the upper layers of the atmosphere, therefore, by analyzing the spectra, one can try to at least roughly estimate its chemical composition. For example, in this way astronomers have discovered in the atmosphere of the planet HD 209458b, better known as Osiris, traces of oxygen and carbon. True, it is somewhat easier to explore Osiris, because it is a huge planet, slightly less than Jupiter in mass, but located extremely close to its star. The disadvantages of the transit method include the low probability that the plane of the planet's orbit lies directly on the line of sight between the solar system and another star. The probability is estimated as the ratio of the radius of the extrasolar planet to the radius of the star. Moreover, this probability will decrease with increasing orbital radius and decreasing exoplanet size. For example, the probability of detecting our Earth from neighboring stars by the transit method is only 0.47%. And even if the orbits of the Earth and the Sun turn out to be with some alien observer on the same line of sight, this does not at all guarantee an accurate detection of our planet.For reliable confirmation, the passage of the Earth across the disk of the Sun would have to be noticed several times in order to accurately determine the orbital period. The situation is partially saved by the fact that a large number of stars can be viewed at once by the transit method. For example, the famous Kepler telescope continuously observes about 100,000 stars. The transit method, like the radial velocity method, will be more sensitive to large planets in close orbits.
Of course, in addition to radial velocities and transits, there are several other methods that make it possible to detect extrasolar planets. For example, there is a method of gravitational microlensing, astrometry, or direct optical observations. These methods are just more effective for planets located at relatively large distances from their stars. However, so far all these search methods are far from being so effective, and the number of planets discovered with their help does not exceed several dozen.
Of course, many would like to find a planet suitable for life, "the second Earth", as some journalists have called it. However, we have only one known example of the origin of life on the planet - our own Earth. To simplify the formulation of the problem, scientists have introduced the concept of the so-called "habitable zone" or "Goldilocks zone." This is the region of space around the star where the amount of energy received is sufficient for the existence of liquid water on the surface. Of course, such a concept does not take into account, for example, the reflectivity of an exoplanet, the composition of the atmosphere, the tilt of the axis, and so on, but it allows us to roughly estimate the prevalence of space bodies of interest to us. The name "Goldilocks zone" is associated with the tale of the three bears (originally - "Goldilocks and the three bears"), in which a girl, finding herself in the house of three bears, tries to get comfortable there: she tastes porridge from different bowls and lies on different beds. And the first star to find a planet in the habitable zone was Gliese 581. Two planets at once, Gliese 581 c and d, on the warm and cold border of the habitable zone, were discovered by the radial velocity method on the HARPS spectrograph of the La Silla Observatory in Chile. Moreover, judging by the lower limit of their possible masses (5.5 and 7 Earth masses, respectively), these may well be rocky bodies.
Later, in 2010, scientists from the University of California at Santa Cruz and the Carnegie Institution in Washington announced the discovery of the planet Gliese 581 g, which is located right in the middle of the habitable zone. The planet was even given an unofficial name - Zarmina - in honor of the wife of the head of the exoplanet search group Stephen Vogt. The discovery rocked the public. The star system was now constantly appearing in the news bulletins of the "yellow" newspapers and on the pages of science fiction. It was from the planet Gliese 581 g that evil aliens arrived who attacked the Earth in the 2012 film "Sea Battle". However, other scientific groups did not confirm the discovery of Gliese 581 g, explaining the results rather by an error in the processing of observations and the activity of the star itself. The quarrels between the Vogt group and other "exoplanets" continued for several years and ended not in his favor. Zarmina existed, most likely, only in the imagination of researchers.
But new discoveries were not long in coming. With the advent of the Kepler telescope, the planets in the habitable zone rained down one after another. Kepler-186f, Kepler-438 b, Kepler-296 e, Kepler-442 b and many other exoplanets have been discovered during the operation of this space telescope. But it turned out that the vast majority of them have one thing in common - they all revolve around red dwarfs. Red dwarfs are low-mass and cool stars with surface temperatures around 3500K. This is not much higher than the filament coil temperature. Such stars shine dimly, but they live for a long time, as they consume hydrogen reserves very slowly.A red dwarf with a mass 10 times less than the Sun, in theory, will shine for trillions of years, which is many orders of magnitude greater than the age of the Universe. By the way, the recently discovered Proxima b and TRAPPIST-1 planets also revolve around similar faint stars. Proxima b is the closest exoplanet to us, and it is located in the habitable zone. Most likely, this is a rocky body, which means that the existence of seas and oceans there is not excluded in the presence of an atmosphere. True, the planet was discovered by the method of radial velocities, so we do not yet know the exact value of its mass and density. Well, the TRAPPIST-1 star has several planets at once, theoretically, can have conditions for the existence of liquid water on the surface. In fact, such an abundance of planets in the zone of life of red dwarfs does not mean at all that they appear there more often than, for example, in yellow stars. Since stars of late spectral types (cool and red) sometimes emit 10,000 times less energy than the Sun, the habitable zone is located much closer to them. And here a selection of methods for searching for extrasolar planets is already starting to work. If the "Goldilocks zone" is closer to the star, then it is easier to find exoplanets in it. Moreover, it is believed that red dwarfs are the most common type of stellar population, and there are approximately 70% of them in our Galaxy. It turns out that we will open them much more often.
Worlds under the red sun
After the first publications about the discovery of planets near Gliese 581, a dispute about their possible habitability flared up in the scientific community. If life could arise and develop around red stars, this would seriously increase its prevalence in the Universe. Moreover, the biosphere on planets under the red sun could exist much longer than the terrestrial one, which means that there would be more chances to develop before the emergence of an intelligent species. Indeed, even our star, seemingly such a stable star, in 1 billion years can become so bright that the Earth's surface will turn into a desert. Life will certainly survive below the surface, but it will survive rather than develop. But the red centenarian could support its biosphere for tens, if not hundreds of billions of years. It's a tempting idea, but research shows that red dwarfs are far from straightforward. And in order for life to arise and develop in such a star system, it will have to overcome many very serious problems.
When we look at the moon, we always see the same pattern of the seas - dark spots on the surface of our satellite. This is because the Earth and its satellite rotate synchronously and the Moon makes one revolution around its axis in the same time it takes to go around the Earth. And this is not a coincidence. Its rotation around the axis was suspended by tidal forces from our planet. And this picture is very common in the solar system. Satellites of Mars and giant planets, the Pluto-Charon system - it can take a long time to enumerate cosmic bodies with synchronous rotation. Even Mercury, which at first glance does not obey this principle, is also in orbital resonance. Sidereal days there last 58.65 Earth days, and the planet makes a revolution around the Sun in 88 days. That is, the day of Mercury lasts 2/3 of its year. By the way, because of this effect, as well as the rather elongated orbit of the planet, there are moments in the firmament of Mercury when the movement of the Sun across the sky suddenly stops and then goes in the opposite direction.
Calculations show that, most likely, all planets in the habitable zone of red dwarfs will always face the star with one hemisphere. At best, a resonance like the rotation of Mercury is possible. For a long time it was believed that under such conditions one hemisphere would be red-hot under the constant direct rays of the luminary, and the other would be the kingdom of eternal cold. Moreover, on the night side, it will even be possible for some atmospheric gases to freeze.But a model of the atmosphere of Earth-like planets captured by tidal forces, created by scientists at the California Institute of Technology in 2010, shows that even with a slow rotation of the air envelope, heat will be quite efficiently transferred to the night side. As a result, the temperature of the night side should not drop below 240K (-33Co). And also quite strong winds should walk on such a planet. According to the models of atmospheres developed by Ludmila Karone and her colleagues at the Catholic University of Leuven, a superrotation effect should occur in the upper atmosphere. A very fast wind constantly circulates along the equator of such a planet, the speed of which reaches 300 km / h and even higher. Air travel in such a world would be a very risky business.
Another 3D simulation, carried out by a team of scientists led by Manoja Joshi, showed that only 10% of the pressure of the earth's atmosphere is enough to effectively transfer heat to the night side of the planet. It also follows from this model that in the sunflower point of the planet (the region closest to the star) there will be not a scorched desert, but a giant atmospheric cyclone - an eternal hurricane that does not move, but stands in one place. This data was used by the National Geographic Channel in the creation of the documentary mini-series Aurelia and the Blue Moon, where Joshi himself acted as a consultant. True, for the development of life, just one comfortable temperature is not enough. Further research showed that if the exoplanet does not have a very large supply of water, then there is a risk that most of it will move to the night side with the winds and freeze there. Gradually, ice masses will move back from the night side, but nevertheless there is a risk that the planet will become a dry desert. How quickly moisture will be transported to and from the night side depends on many factors, including the configuration of the continents, the chemical composition and density of the atmosphere, and so on. At the same time, a sufficiently deep ocean will remain liquid under the ice, which will also prevent its complete freezing. By the way, modeling the very process of the formation of earth-like planets in red dwarfs just shows a much higher water content in comparison with the Earth. The work of Yann Alibert and Willie Benza, published in the journal Astronomy and Astrophysics, shows that in some cases the proportion of H2O can be up to 10% by weight. Interestingly, if the planets, on the contrary, have a dense atmosphere, then there is a possibility of overcoming tidal capture. The moment of rotation of the dense atmosphere will be transmitted to the planet, due to which day and night can again begin to change on it. True, these days and nights can last quite a long time.
Another, even more serious problem is that red dwarfs are often very turbulent objects. Most of them are variable stars, that is, stars that change their luminosity as a result of some physical processes taking place inside or near them. For example, quite often these stars show variability of the BY Dragon type. Variations in brightness with this type of activity are associated with the rotation of the star around its axis, since its surface is covered with a large number of spots similar to the sun. Sunspots are areas where strong (up to several thousand gauss) magnetic fields enter the photosphere, which impede the transfer of heat from deeper layers. Thus, the temperature at the spots is lower than that of the surrounding photosphere, which makes them appear darker in a telescope with a light filter.
Sun-like spots are also present on red dwarfs, but they occupy a much larger area. As a result, in a short time, the brightness of the star can change by 40%, which is likely to negatively affect the hypothetical life.
But a much more dangerous property of red stars is their flare activity.A significant proportion of red dwarfs are variable stars of the UV Ceti type. These are flare stars, which, at the moment of an outbreak, increase their luminosity several times, and in the range from radio to X-ray. The flares themselves can last from minutes to several hours, and the interval between them - from an hour to several days. Scientists believe that the nature of these flares is the same as that of flares on the Sun, but the power is much higher. In addition to an increase in luminosity in all ranges, at the moment of a flash, charged particles are emitted, which contribute to the loss of the atmosphere, especially light elements such as hydrogen. The famous Proxima Centauri also belongs to the variable stars of the UV Ceti type. But what does scientific research say about the ability to withstand such a hostile environment?
According to some astrophysicists - for example, according to the popularizer of science and astronomer from the University of Southern Illinois Pamela Gay - most red dwarfs are active for about the first 1.2 billion years of life, after which they have a decrease in both the frequency and intensity of flares. Theoretically, in the case of partial preservation or reappearance of the atmosphere, the biosphere could begin to develop after the star has passed the active stage of evolution. But not all scientists are of the opinion about the short stage of the active phase. Nikolai Samus, a leading researcher at the Department of Nonstationary Stars and Stellar Spectroscopy at the Institute of Astronomy of the Russian Academy of Sciences, told Naked Science about this: “Flare activity is very common in red dwarfs. It should fade with age, but red dwarfs of very late classes and really low luminosities “age” for so long that all of them actually observed can be considered young. On the whole, at least a quarter of M dwarfs are Me (active dwarfs with powerful spectral emission lines. - Ed.), And almost all of them have either sunspot or flare variability, or both. In the later subclasses of M, up to 100% of the stars are variable”. By the way, the age of that very Proxima Centauri is almost 5 billion years, but the star remains very active and regularly demonstrates powerful flares.
The situation is partially saved by the planet's magnetic field. Calculations show that even the slow rotation of tidally captured planets will be enough to generate a magnetic field as long as the inner part of the planet remains molten. But modeling the rate of loss of atmospheres, carried out by astrophysicist Jorge Zuluaga and his colleagues, showed that even if the planet has a powerful magnetic field, it will rather intensively lose its atmosphere due to interaction with matter ejected during the flare. According to this study, the situation is slightly better in super-earths with a mass of 3 or more times the mass of the Earth, but even there the losses are significant. According to this model, the exoplanet Gliese 667Cc should have completely lost its atmosphere, but Gliese 581d and HD 85512b should have retained it. Interestingly, earlier models, for example, the study by Maxim Krodachenko and his colleagues, published in the journal Astrobiology, predicted, on the contrary, very weak magnetic fields of the planet, unable to protect the atmosphere from powerful emissions of stellar matter.
Currently, research on red dwarfs is complicated by the fact that they are rather faint stars that are difficult to study at large distances. The question still remains to be answered as to what fraction of these stars remain active for billions of years and on what it depends. Both Proxima Centauri, and Gliese 581, and even the recent hero of the news reports TRAPPIST-1 demonstrate flare activity, which means that the atmospheres of the planets will be irradiated with both ultraviolet light and a stream of charged particles. The models basically show the possibility of preserving the atmosphere even in such harsh conditions, but the question of the possibility of the existence of the biosphere is still open.By the way, already at the beginning of 2017, Jorge Zuluaga published an article in which he showed the possibility of Proxima Centauri b to have a powerful magnetic field.
But, let's say, on the planet, despite all the difficulties, primitive forms of life have appeared. On Earth, photosynthesis is the energy basis of all living things, except for bacteria that feed on inorganic substances, such as sulfur bacteria. Most of atmospheric oxygen is a byproduct of photosynthesis. However, can photosynthesis use the light of the red sun? There are several forms of chlorophyll that use light from different parts of the spectrum. These are mainly chlorophylls a and b, which differ slightly in absorbed frequencies. Most of the chlorophyll of higher plants absorbs the blue and red portion of the solar spectrum, making the leaves appear green. Depending on the lighting conditions, the ratio between the two types of chlorophyll and its concentration can vary. For example, in shade-loving plants, the chlorophyll content can be 5-10 times higher than in plants that love bright light. An interesting adaptation exists in red algae, which, thanks to additional pigments, can absorb light from almost the entire visible part of the spectrum.
In 2014, a shade-tolerant strain of cyanobacteria Leptolyngbya JSC-1, living in hot springs, was discovered. These bacteria are capable of using near-infrared light (700 to 800 nm). Interestingly, when it enters a more illuminated area, this cyanobacterium is able to rebuild the photosynthetic mechanism. There is also encouraging information coming from the ocean floor. Another international team of biologists discovered the sulfur bacterium GSB1, which contains chlorophyll, in the vicinity of a deep-sea thermal spring off the coast of Costa Rica. Since sunlight does not penetrate 2.4 km, the researchers hypothesized that the sulfur bacteria uses an infrared light source emitted by hot hydrothermal vents (~ 750 nm). The study was published in the journal Proceedings of the National Academy of Sciences. Thus, hypothetical red dwarf life forms should not starve to death.
Currently, computer simulations are perhaps the only way to assess conditions on the surface of an exoplanet near a red dwarf. Observational technology is not yet able to clarify the chemical composition, much less distinguish any details on the surface. But the simulation results depend on many factors, and sometimes the calculations of different scientific groups give almost opposite results. New telescopes will help to finally understand the question of the viability of red dwarfs. In 2020, the launch of the James Webb Space Telescope is due. It is assumed that he will be able to conduct spectroscopic studies of the atmospheres of some exoplanets. Also in the Atacama Desert in Chile, the construction of the E-ELT (European Extremely Large Telescope) is already underway, the diameter of the main mirror of which will be almost 40 meters. More distant projects involve the launch of several space telescopes capable of operating in the interferometer mode, while obtaining ultra-clear resolution. Also recently, an even more extravagant project has been gaining popularity in the scientific community - observing an exoplanet using a gravitational lens from the Sun. The essence of the method is that a small telescope is sent at a distance of 547 astronomical units from the Sun to its so-called gravitational focus. Gravitational lensing is the process of bending electromagnetic radiation by the gravitational field of a heavy object, just like a conventional lens bends a light beam. In fact, mankind will receive a giant telescope with the Sun as an objective, with the help of which it will be possible to see the relief, the outlines of the continents and the cloud cover of distant exoplanets, for example, the planets of the TRAPPIST-1 system or Proxima b.Such a "gravitational" telescope will have a magnification of 10 ^ 11 times, which is similar to a ground-based instrument with a diameter of 80 km.