How many potentially habitable planets are there in our Galaxy?

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How many potentially habitable planets are there in our Galaxy?
How many potentially habitable planets are there in our Galaxy?

In June 2020, Canadian astronomers calculated that there could be five billion Earth-like planets in the Milky Way orbiting Sun-like stars. However, this is only the visible part of the iceberg of habitable planets. The point is not only that there are more of them around stars of other types: the very conditions on exoplanets in other systems can be much more favorable for life than ours. Let's try to understand why.


Stars are of different types. Those that are more massive - such as the yellow dwarf of the Sun (spectral type G2) or the white star Sirius A (spectral type A1) - are visible for many light years. Moreover, as their mass increases, their luminosity grows nonlinearly: Sirius is only twice as heavy as our star, but shines 25 times brighter.

The opposite is also true: the heaviest and largest red dwarf (spectral type M0) is only a couple lighter than the Sun, but shines about 15 times fainter. The lightest of them (M9) are a dozen times lighter than the Sun, but inferior in luminosity by many thousands of times.

This gives rise to the effect of the "invisible part of the iceberg": there are a lot of red dwarfs around the Earth (and in the Universe in general), but it is really difficult to see them. The closest star to Earth - Proxima Centauri - is just such a dwarf: despite the extremely small distance of 4, 3 light years, it cannot be seen in the sky with the naked eye. It is difficult to establish the exact number of such objects in the Universe. Estimates vary from 70% to 90% of all luminaries in existence - but most of these estimates are close to 75-76%.


Canadian astronomers, who tried to count the number of terrestrial planets around yellow dwarfs, used data from space telescopes, in which images were recorded 200 thousand stars. They found that about 7% of all 400 billion luminaries in our Galaxy are yellow dwarfs. There are 28 billion of these stars in total. Based on the available data on the occurrence of planets in class G stars, scientists have calculated that in 18% of cases in each of these systems there may be a planet the size of the Earth, and in the habitable zone. There may be five billion such planets in total.

Now let's look at this situation from the point of view of not yellow dwarfs like our star, but orange (12% of all stars) and red (76%). It turns out that there are about 300 billion reds in the Milky Way alone - 11 times more than yellow ones. At the same time, according to the calculations of other astronomical groups, there are a lot of planets close in mass to the Earth and lying in the habitable zone there. 40% of all red dwarfs can have such exoplanets - and then potentially inhabited planets around such stars up to 120 billion.


Of course, the mere presence of a celestial body in a conventional habitable zone is far from a guarantee of the presence of life there. The most important question of modern exoplanetary astronomy: are such worlds really suitable for life?

What's wrong with the light of red dwarfs

Contrary to the name, a person who ends up on a planet near a red dwarf will not see red luminaries in the sky. It's all about the peculiarities of our vision: it catches photons of different wavelengths from the light source and "folds" them, getting not the true color of the object, but the "synthetic" one. Our Sun is a classic example. It emits most of its energy in the green part of the visible spectrum.To understand this, it is enough to look at the foliage around: it is precisely such that it effectively reflects this part of the solar radiation and avoids overheating during a sharp transition from shadow to sunlight.

The same is the case with an incandescent light bulb and a red dwarf. Their true color is red, but we perceive it as an ocher yellow in the end. However, if there really is life on worlds with an ocher-yellow Sun, then highly developed terrestrial vegetation there will have exactly a red color, otherwise it will be difficult for it to adapt to a sharp change in lighting when a cloud passes over it.

For a long time, some researchers believed that mainly red and infrared radiation from red stars would become a serious problem for plants on planets around them. Indeed: the energy of such photons is lower than that of green light, which dominates the radiation of the Sun. Will red light and infrared light turn out to be enough for photosynthesis? After all, it is known that ordinary chlorophyll practically cannot use light waves in the far red part of the spectrum (with waves from 700 nanometers and more)?


Of course, oxygen can be formed not only with the help of chlorophyll: for example, bacteria have a protein called bacteriorhodopsin, similar to ordinary rhodopsin, with which we, for example, perceive light. However, "bacteriorhodopsin" photosynthesis does not normally form oxygen: this means that a complex biosphere - with oxygen-breathing multicellular organisms - cannot be built on its basis.

So is the light of red stars unsuitable for "feeding" complex life? To find out the answer to this question, you do not have to fly to distant stars. In 2010, chlorophyll f (approximate descriptive formula C55H70O6N4Mg) was discovered on the west coast of Australia.


Unlike other types of chlorophyll, it provides "classic" photosynthesis with the release of oxygen - but from photons with a wavelength of up to 720 nanometers. The reasons for its use by photosynthetic organisms in water are clear: the shorter the wavelength of electromagnetic radiation, the more it is absorbed by water. Therefore, in some cases it is more profitable to use the far part of the red range.

Will outbreaks kill all living things?

Another feature of red dwarfs that astronomers often dwell on is their tendency to violent flares, which our Sun showed only in the first millions of years of its life. During such an event, the level of X-ray and ultraviolet radiation from the luminary rises sharply - so much so that, as it is sometimes argued, this can "sterilize" life on the surface of such a planet.

Based on this, astronomers tried to calculate what the consequences of such strong UV and X-ray flares of low-mass stars could be. They found that some planets in the habitable zone of the red dwarf TRAPPIST-1 (39, 6 light years from us) could thus lose water, in mass equal to 15 Earth's oceans. The loss mechanism is simple: ultraviolet light splits water vapor molecules into hydrogen and oxygen. The molecules of the former are too light, so they quickly scatter into outer space.

More recently, it was found that the actual level of ultraviolet radiation during TRAPPIST-1 flares is 50 times higher than it was assumed when calculating the possible loss of water. An apocalyptic picture is emerging: earthlike planets in red dwarfs should be waterless compared to Earth, and UV radiation on their surface during outbreaks, it seems, can sterilize any terrestrial life.


But let's turn to the real parameters of the seven planets of the TRAPPIST-1 system. Are they really so waterless? In the habitable zone there are three planets at once - TRAPPIST-1e, f and g. The density of the first is 1.024 terrestrial, the second - 0.816 terrestrial, and the third - 0.759 terrestrial. It is clearly seen that two of the three planets should contain significantly more light elements than our Earth.For planets very close to it in mass, water is the main source of light components, because the gravity of such bodies is not capable of holding a large hydrogen or helium atmosphere.

Maybe we are talking about the exception and there is a lot of water on planets in the habitable zone only on worlds in the TRAPPIST-1 system? No, and in other systems of red dwarfs, the planets in the habitable zone almost always have the density of the Earth or even lower. Therefore, they cannot be truly waterless deserts - even with strong UV and X-ray flares.


How They Preserved Water and the Opportunity for Life to Grow

When asked why we see such a picture today, astronomers have not yet found the answer. Indeed, ultraviolet light splits water, and it is believed that the loss of water on Mars took place precisely due to solar UV radiation.

There are two possible answers to the question of how the planets of red dwarfs did not lose water.

The first scenario can be called "while the fat dries up, the thin one evaporates." The fact is that most red dwarf systems with open planets look unnatural. Their planets are located extremely crowded, at a very small distance from their star. The nearest of the seven planets of the same TRAPPIST-1 lies 1.73 million kilometers from its star, and the most distant one is only 9.27 million kilometers from it. Think about it: seven planets at 7.54 million kilometers!

In the solar system, the seven planets closest to the star are scattered in orbits from 58 million kilometers (Mercury) to 2.88 billion kilometers. Their separation in space is 370 times stronger than that of the TRAPPIST-1 exoplanets. Between Mercury and the Sun in our system, seven seven-planetary systems TRAPPIST-1 would fit at once - and there would still be room.


Of course, red dwarfs are smaller than the sun. And their protoplanetary disks should also be smaller, but hardly 370 times. All this forces scientists to assume the possibility of the formation of red dwarf planets in orbits farther from the star, followed by migration closer to the star. A migraine like the one we described in the January issue of our solar system magazine - but more radical.

In this case, the proportion of light elements on planets in the habitable zone of low-mass stars will initially be very high - much more than on Earth. Then powerful stellar flares, even if they deprive it of large masses of water, will still not reduce the water "reserve" on such planets below the earth's level.

The second possible scenario for survival under ultraviolet light is, of course, the ozone layer. According to astronomers' calculations, in the presence of a noticeable oxygen atmosphere and a constant ozone layer, even during moments of serious flares, the average level of UV radiation reaching the surface of a terrestrial planet in the red dwarf system will not be much higher than on Earth. This will be especially true where the density of the atmosphere is noticeably higher than that of the Earth.

This scenario has one weak point: a noticeable amount of free oxygen must come from somewhere. Judging by the experience of terrestrial life, it appears only billions of years after the formation of the planet - due to the activities of those photosyntheticians that produce oxygen (as we said above, not all photosynthetic organisms do this). But where could a serious ozone layer come from on young planets, if ultraviolet light should kill all life on the surface?

Here the answer may lie in the serious protection that even a thin layer of water gives any living creature from ultraviolet radiation. If a photosynthetic organism grows in an aquatic environment, UV does not interfere with it. And when it accumulates free oxygen, then it will become much safer on land (in terms of reducing the level of ultraviolet radiation).

Tidal capture: is life possible under a stationary sun?

Skeptics will recall another often-cited problem of planets in red dwarfs - tidal capture.As we have shown above, the distance from a typical habitable planet in such a system to its star is a matter of millions of kilometers. Neighboring planets in such tightness will look like moons in the sky, and their own moons will look much brighter than earthly ones.

The small distance to the star means that sooner or later its gravity will "fix" its planets. As the Earth forces the Moon to always look at itself with only one side, red dwarfs will most often shine forever on one side of their inhabited planets, while the other half will remain in eternal shadow. This is what is called tidal capture in relation to a star and a planet. Will not life "burn out" on the sunflower side and freeze on the shady side?

The answer to this question came a few years ago, when astronomers first used detailed models of the behavior of exoplanet atmospheres to understand what would happen to them in the event of tidal capture.

It turned out that constant heating inevitably creates extremely powerful ascending tropospheric currents and dense clouds at the “sunflower” point. So dense that we are simply not talking about excessive overheating of the "sunflower" side of the exoplanet. Moreover: the ascending currents turned out to be strong enough for the heated air masses to then quickly move to the "shadow", eternally dark side of the planet. In fact, the distribution of temperatures on such a planet will only slightly differ from that on Earth - although half of the surface here will never see the local sun.

Of course, this does not mean that the biosphere of the worlds in red dwarfs will develop in the same way as in our country. Yes, the overall bioproductivity will be comparable: on the sunflower side, photosynthesis will go on all hours of the local day, and not half the time, as we do. But on the eternally shady side, the photosynthetics we are used to will never become the dominant form of life. There will be dominated by chemoautotrophs - organisms that decompose certain compounds and thus live. This will be a rather strange half of the world - it is unlikely that the developed animals of the day side will ever enter. Life will have to huddle near hydrothermal vents under water and near volcanoes on land.

By the way, there are examples of photosynthetic organisms on Earth that live without sunlight. We are talking about green sulfur bacteria that live at depths of up to 2.4 kilometers, where sunlight does not get. Therefore, they only use the dim glow from the nearby hydrothermal vents. The source of energy of this glow is the heating of compounds emerging from under the surface of the planet, therefore the light used by sulfur bacteria is red, plus part of the infrared range.

Obviously, such "thermal" photosynthetics can be found on the shadow sides of planets in red dwarfs. But it is also clear that complex life cannot unfold on such a basis: the shadow side will forever remain a reserve for primitive forms of life.

Superinhabited worlds

But red dwarfs have features that scientists unequivocally interpret as favorable for life. Even the largest of the red stars (with a mass of a quarter of the sun) live for at least a trillion years. The least massive ones are just over 10 trillion years old. The word "live" should not be misleading: while the Universe is not even 14 billion years old, therefore, in fact, not a single red dwarf has yet managed to reach the end of its life path and become first a blue and then a black dwarf.

The reason for the super-long life cycle is the low consumption of hydrogen and the impossibility, due to its small size, to enter the phase of the red supergiant, which in five billion years will become the Sun. Therefore, red dwarfs with biospheres should give them maximum time for evolution. On our planet, complex terrestrial life with higher plants and animals has existed for only half a billion years.If people do not come up with something extraordinary, in another billion years this life will disappear: the luminosity of the Sun is gradually increasing.

In theory, a "carbon air conditioner" works on Earth - a mechanism by which it does not overheat. When there is too much solar radiation, CO2 in the Earth's atmosphere is more quickly bound by rocks, after which the strength of the greenhouse effect decreases, and the temperature drops again to an acceptable level.

But the problem is that such drops in carbon dioxide content are dangerous in themselves. In the last ice age, CO2 in the air was 180 parts per million, and already at 150 parts per million, all trees will die. Some grasses will be able to photosynthesize even with less carbon dioxide in the air, but below 50 ppm, almost all complex plants will begin to die.

We humans are able to solve the problem of a gradual increase in the radiation of our yellow dwarf: for example, by building large mirrors in orbit that reflect part of the solar radiation. But on a red dwarf, such a problem, in principle, does not arise: complex life there has not 1.5 billion years for natural development, but at least hundreds of billions. On low-mass red dwarfs of the TRAPPIST-1 type, we are talking about trillions of years.

In theory, this is a huge advantage for the development of almost any complex biosphere. Existing hundreds, if not thousands of times longer than on Earth, it is able to rise to more complex life forms with a high probability. Who knows: maybe even to reasonable ones?

Preserve of stable temperatures

Another positive feature of red and to some extent orange dwarfs is the absence of ice ages.

In general, such events were, until recently, a rarity on Earth. More or less regularly, they began only two million years ago, and before that the planet was much warmer. Five million years ago, beech trees grew on the coast of Antarctica, and Novaya Zemlya, three million years ago, remained covered with deciduous forests. Since then, after a decrease in the concentration of CO2 in the atmosphere, the planet entered a period of chronically unstable climate, which it did not know before.

Every several tens of thousands of years, ice begins to advance in low latitudes, along the way, sharply reducing the productivity of the biosphere. It's not just a drop in temperatures: because of them, the volume of precipitation also falls, making the planet deserted. Just 20 thousand years ago, more than half of the Earth's land mass was either arctic or sandy deserts.

But for red dwarfs, this scenario cannot work: their climate is much more stable. The Sun has a little less than half of the radiation energy in the infrared part of the spectrum. This radiation is not reflected from the water ice, but absorbed by it, leading to melting. The remaining half of the energy of the sun's rays is in the visible and UV ranges, and these waves are effectively reflected by ice into space, which leads to an increase in the cooling of the Earth. Therefore, the start of any glaciation generates a positive feedback: more ice - even colder planet - even more ice. And so in a circle.

In red dwarfs, up to 95% of radiation falls on the infrared part of the spectrum, so ice there cannot "cool" the planet by itself. So any temporary onset of ice (for example, after a volcanic or asteroid winter or a short-term decline in stellar activity) turns out to be extremely short-term. No ice ages can be long-term there.

This means not only a greater productivity of the biosphere as a whole than on Earth, but also a "speedy" recovery after each major extinction. Judging by our planet, they - that in the case of dinosaurs, that in the case of the Great Extinction of the end of the Permian - occur precisely during the period of cold snaps and the active onset of ice. But in low-mass stars, such an offensive will be short, which is why the depth of mass extinctions of species may turn out to be less than on our planet.

Let's summarize.The threats to life on planets in red dwarfs in the light of the latest scientific evidence look noticeably exaggerated. Judging by the high content of light components in them, they are rich in water. On a large number of them, there is no destructive cold of the eternally shadow hemisphere, and the overheating of the always day hemisphere.

Moreover, there are a dozen times more of them than the planets around yellow dwarfs, and in theory, there should be about the same number of "red-star" biospheres in the Universe. Of the approximately 50 billion potentially inhabited planets of the Galaxy, more than 40 orbit around red and orange dwarfs, and only five around yellow ones like our star.

But the point is not only that there are numerically more "red-star" exoplanets of the terrestrial type. Complex life on each of the inhabited worlds of red stars will exist hundreds of times longer than ours. In other words, the overwhelming majority of all living beings can be inhabitants of just such planets - and not at all "Earth's twins".

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