How many potentially habitable planets are there in our galaxy?

(ORDO NEWS) — 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 conditions themselves 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. The more massive ones, such as the yellow dwarf of the Sun (spectral class G2) or the white star Sirius A (spectral class A1), are visible for many light years.

Moreover, as their mass increases, their luminosity grows non-linearly: Sirius is only twice as heavy as our star, but it shines 25 times brighter.

The opposite is also true: the heaviest and largest red dwarf (spectral class M0) is only a couple lighter than the Sun, but shines about 15 times weaker.

The lightest of them (M9) are a dozen times lighter than the Sun, but inferior to it in luminosity 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 the 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 accurately determine the number of such objects in the universe. Estimates vary from 70% to 90% of all existing luminaries – but most of these estimates are close to 75-76%.

Canadian astronomers trying to count the number of terrestrial planets around yellow dwarfs used data from space telescopes that captured 200,000 stars.

They found out that approximately 7% of all 400 billion luminaries in our Galaxy are yellow dwarfs. There are 28 billion such stars in total.

Based on the available data on the occurrence of planets in class G stars, scientists calculated that in 18% of cases in each of these systems there can 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 yellows.

At the same time, according to the calculations of other astronomical groups, there are a lot of planets that are close to Earth in mass and lie in the habitable zone there.

40% of all red dwarfs can have such exoplanets – and then there are up to 120 billion potentially habitable planets around such stars.

Of course, the mere presence of a celestial body in a conditional 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 is wrong with the light of red dwarfs

Contrary to the name, a person who finds himself on a planet near a red dwarf will not see a red luminary in the sky.

It’s all about the peculiarities of our vision: it catches photons of different wavelengths from a light source and “adds” them, getting not the true color of the object, but “synthetic”. A classic example is our Sun.

It radiates most of its energy in the green part of the visible spectrum.

To understand this, just look at the foliage around: it is just such that it effectively reflects this part of the solar radiation and avoid overheating during a sharp transition from shade to sunlight.

The same is true of the incandescent light bulb and the red dwarf. Their true color is red, but we perceive it as a result of ocher yellow.

However, if there really is life on worlds with an ocher-yellow Sun, then the highly developed terrestrial vegetation there will have exactly 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, part of the researchers believed that the mainly red and infrared radiation of red stars would become a serious problem for plants on the 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 radiation be sufficient for photosynthesis?

After all, it is known that ordinary chlorophyll practically cannot use light waves of 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 bacteriorhodopsin, similar to ordinary rhodopsin, with which we, for example, perceive light.

However, “bacteriorhodopsin” photosynthesis normally does not form oxygen: it 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, it is not necessary 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 “classical” 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.

Flashes will destroy all living things?

Another feature of red dwarfs that astronomers often dwell on is their propensity for the strongest flares that our Sun showed only in the first million years of its life.

During such an event, the level of X-ray and ultraviolet radiation from the star rises sharply – so much so that, as is sometimes claimed, 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 lose water in this way, equal in mass to 15 Earth’s oceans.

The loss mechanism is simple: ultraviolet light splits water vapor molecules into hydrogen and oxygen. The molecules of the first are too light, so they quickly dissipate into outer space.

Relatively recently, it was found that the actual level of ultraviolet radiation during the TRAPPIST-1 flares is 50 times higher than was considered when calculating the possible water losses.

An apocalyptic picture emerges: Earth-like planets near red dwarfs should be waterless compared to Earth, and UV radiation on their surface during flares, as 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 that waterless? There are three planets in the habitable zone at once – TRAPPIST-1e, f and g.

The density of the first is 1.024 Earth, the second is 0.816 Earth, and the third is 0.759 Earth. It is clearly seen that two of the three planets should contain noticeably 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 able to hold a large hydrogen or helium atmosphere.

Maybe we are talking about an exception and a lot of water on planets in the habitable zone only on worlds in the TRAPPIST-1 system?

No, and in other red dwarf systems, planets in the habitable zone almost always have a density of Earth or even lower. Therefore, they cannot be truly waterless deserts – even despite strong UV and X-ray flares.

How they saved water and the opportunity for life to develop

To the question of why we observe such a picture today, astronomers have not yet found an answer.

Indeed, ultraviolet light splits water, and it is believed that the loss of water on Mars took place precisely because of solar UV radiation.

There are two possible answers to the question of how planets near red dwarfs have not lost water.

The first scenario can be called “while the fat one dries up, the thin one will evaporate.” The fact is that most red dwarf systems with open planets look unnatural.

Their planets are extremely crowded, at a very small distance from their star.

The closest of the seven planets of the same TRAPPIST-1 lies 1.73 million kilometers from its star, and the most distant is only 9.27 million kilometers from it. Think about it: seven planets for 7.54 million kilometers!

In the solar system, the seven closest planets to a 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-planet TRAPPIST-1 systems 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 by a factor of 370.

All this makes scientists assume the possibility of the formation of red dwarf planets in orbits more distant from the sun, with subsequent migration closer to the star.

A migration similar to the one we described in the January issue of our Solar System Journal, 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 large – 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 permanent ozone layer, even during moments of serious flares, the average level of UV radiation reaching the surface of a terrestrial planet in a 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.

Such a 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 activity of those photosynthetics 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 should kill all life that is 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 builds up free oxygen, it will become much safer on land (in terms of reducing the level of ultraviolet radiation).

Tidal capture: is life possible under a fixed sun?

Skeptics will recall another oft-cited red dwarf planet problem: 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 close quarters will look like moons in the sky, and their own moons will be much brighter than the earth’s.

The small distance to the star means that its gravity will “fix” its planets sooner or later. Just as the Earth causes the Moon to always look at itself with only one side, red dwarfs will most often shine forever on one side of their habitable planets, while the other half will remain lying in eternal shadow.

This is what is called tidal trapping of a star and a planet. Won’t 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 a tidal capture.

It turned out that constant heating inevitably creates extremely powerful ascending tropospheric currents and dense clouds in the “subsolar” point.

So dense that there is simply no question of excessive overheating of the “sunflower” side of the exoplanet.

Moreover, the ascending currents turned out to be strong enough so that the heated air masses after that quickly moved to the “shadow”, eternally dark side of the planet.

In fact, the distribution of temperatures on such a planet would be only slightly different from the Earth’s – although half of the surface here will never see the local sun.

Of course, this does not mean that the biosphere of the worlds of red dwarfs will develop in the same way as ours.

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, like ours.

But on the eternally shadowy side, the photosynthetics we are accustomed to will never become the dominant form of life.

Chemoautotrophs will dominate there – organisms that decompose certain compounds and thus live.

It will be a rather strange half of the world – it is unlikely that advanced animals of the day side will ever enter there. Life will have to huddle near hydrothermal vents underwater and near volcanoes on land.

By the way, on Earth there are examples of photosynthetic organisms 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 fall.

Therefore, they use only the dim glow from nearby hydrothermal vents.

The energy source 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.

It is obvious that such “thermal” photosynthetics can also be found on the shadow sides of planets near 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 life forms.

Superhabitable worlds

But red dwarfs have features that scientists unambiguously 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 most low-mass 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 a red supergiant, which the Sun will become in five billion years.

Therefore, red dwarfs with biospheres should give them maximum time to evolve. 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.

Theoretically, a “carbon 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 quickly binds to rocks, after which the strength of the greenhouse effect decreases, and the temperature drops back to acceptable levels.

But the problem is that such falls in carbon dioxide are dangerous in and of 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 with less carbon dioxide in the air, but below 50 parts per million, almost all complex plants will 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 does not arise in principle: complex life there has not 1.5 billion years for natural development, but at least hundreds of billions.

For low-mass red dwarfs like TRAPPIST-1, 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 reasonable?

Reserve 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 rare on Earth until recently. 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 remained covered with broad-leaved forests three million years ago.

Since then, after the decrease in the concentration of CO2 in the atmosphere, the planet has entered a period of chronically unstable climate, which was not known before.

Every several tens of thousands of years, ice begins to advance into low latitudes, sharply reducing the productivity of the biosphere along the way.

It’s not just a matter of lowering temperatures: because of them, the amount of precipitation also falls, which is why the planet becomes deserted.

Just 20,000 years ago, more than half of the Earth’s land mass was either arctic or sandy desert.

But for red dwarfs, this scenario cannot work: their climate is much more stable. The Sun has slightly less than half of its radiation energy in the infrared part of the spectrum.

Such radiation is not reflected from the water ice, but is 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 increased cooling of the Earth.

Therefore, the start of any glaciation generates positive feedback: more ice – even colder planet – even more ice. And so in a circle.

In red dwarfs, up to 95% of the radiation falls on the infrared part of the spectrum, so the ice there cannot “cool down” the planet by itself.

So any temporary advance of ice (for example, after a volcanic or asteroidal winter or a short-term decline in stellar activity) turns out to be extremely short-lived. No ice ages there can be long-term.

This means not only greater than on Earth, the productivity of the biosphere as a whole, but also a “fast” recovery after each major extinction.

Judging by our planet, they – both in the case of dinosaurs and in the case of the Great Extinction of the end of the Permian – occur precisely during the period of cooling and active advance of ice.

But for low-mass stars, such an onset 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. Threats to life on planets near red dwarfs in the light of recent scientific data look markedly 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 fatal cold of the eternally shady hemisphere, and no overheating of the always daytime hemisphere.

At the same time, there are a dozen times more of them than there are 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 habitable planets in the Galaxy, more than 40 orbit red and orange dwarfs, and only five orbit yellow ones like our star.

But the point is not only that there are more “red-star” exoplanets of the Earth type. Complex life on each of the habitable red star worlds will last hundreds of times longer than ours.

In other words, the vast majority of all living beings can be inhabitants of just such planets – and not at all “twins of the Earth.”

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