The possibility of life on aquatic planets

2024 Author: Seth Attwood | [email protected]. Last modified: 2023-12-16 15:55

Most of the planets we know of are larger in mass than Earth, but less than Saturn. Most often, among them there are "mini-neptunes" and "super-earths" - objects a couple of times more massive than our planet. The discoveries of recent years give more and more grounds to believe that super-Earths are planets whose composition is very different from ours. Moreover, it turned out that the terrestrial planets in other systems are likely to differ from the Earth in much richer light elements and compounds, including water. And that's a good reason to wonder how fit they are for life.

The aforementioned differences between "ex-earth" and the Earth are explained by the fact that three quarters of all stars in the Universe are red dwarfs, luminaries much less massive than the Sun. Observations show that the planets around them are often in the habitable zone - that is, where they receive about the same energy from their star as the Earth from the Sun. Moreover, there are often extremely many planets in the habitable zone of red dwarfs: in the "Goldilocks belt" of the TRAPPIST-1 star, for example, there are three planets at once.

And this is very strange. The habitable zone of red dwarfs lies in millions of kilometers from the star, and not 150-225 million, as in the solar system. Meanwhile, several planets at once cannot form in millions of kilometers from their star - the size of its protoplanetary disk will not allow. Yes, a red dwarf has it less than a yellow one, like our Sun, but not a hundred or even fifty times.

The situation is further complicated by the fact that astronomers have learned to more or less accurately "weigh" planets in distant stars. And then it turned out that if we relate their mass and size, it turns out that the density of such planets is two or even three times less than the Earth's. And this is, in principle, impossible if these planets were formed in millions of kilometers from their star. Because with such a close arrangement, the radiation of the luminary should literally push the bulk of the light elements outward.

This is exactly what happened in the solar system, for example. Let's take a look at the Earth: it was formed in the habitable zone, but water in its mass is not more than one thousandth. If the density of a number of worlds in red dwarfs is two to three times lower, then the water there is no less than 10 percent, or even more. That is, a hundred times more than on Earth. Consequently, they formed outside the habitable zone and only then migrated there. It is easy for stellar radiation to deprive light elements of the zones of the protoplanetary disk close to the luminary. But it is much more difficult to deprive a ready-made planet that has migrated from the distant part of the protoplanetary disk of light elements - the lower layers there are protected by the upper ones. And water loss is inevitably rather slow. A typical super-earth in the habitable zone will not be able to lose even half of its water, and during the entire existence of, for example, the solar system.

So, the most massive stars in the Universe often have planets in which there is a lot of water. This, most likely, means that there are much more such planets than such as Earth. Therefore, it would be good to figure out whether in such places there is a possibility of the emergence and development of complex life.

Need more minerals

And this is where the big problems begin. There are no close analogues of super-earths with a large amount of water in the solar system, and in the absence of examples available for observation, planetary scientists literally have nothing to start from. We have to look at the phase diagram of the water and figure out what parameters will be for different layers of the oceanid planets.

Phase diagram of the state of water. Ice modifications are indicated by Roman numerals. Almost all ice on Earth belongs to group I_h, and a very small fraction (in the upper atmosphere) - to I_c… Image: AdmiralHood / wikimedia commons / CC BY-SA 3.0

It turns out that if there is 540 times more water on a planet the size of the Earth than here, then it will be completely covered by an ocean more than a hundred kilometers deep. At the bottom of such oceans, the pressure will be so great that ice of such a phase will begin to form there, which remains solid even at very high temperatures, since the water is held solid by a tremendous pressure.

If the bottom of the planetary ocean is covered with a thick layer of ice, liquid water will be deprived of contact with solid silicate rocks. Without such contact, the minerals in it will, in fact, have nowhere to come from. Worse, the carbon cycle will be disrupted.

Let's start with minerals. Without phosphorus, life - in the forms known to us - cannot be, because without it there are no nucleotides and, accordingly, no DNA. It will be difficult without calcium - for example, our bones are composed of hydroxylapatite, which cannot do without phosphorus and calcium. Problems with the availability of certain elements sometimes arise on Earth. For example, in Australia and North America in a number of localities there was an abnormally long absence of volcanic activity and in soils in some places there is a severe lack of selenium (it is part of one of the amino acids, necessary for life). From this, cows, sheep and goats are deficient in selenium, and sometimes this leads to the death of livestock (the addition of selenite to livestock feed in the United States and Canada is even regulated by law).

Some researchers suggest that the mere factor of the availability of minerals should make the oceans-planets real biological deserts, where life, if there is, is extremely rare. And we are simply not talking about really complex forms.

Broken air conditioner

In addition to mineral deficiencies, theorists have discovered a second potential problem of planets-oceans - perhaps even more important than the first. We are talking about malfunctions in the carbon cycle. On our planet, he is the main reason for the existence of a relatively stable climate. The principle of the carbon cycle is simple: when the planet becomes too cold, the absorption of carbon dioxide by the rocks slows down sharply (the process of such absorption proceeds quickly only in a warm environment). At the same time, "supplies" of carbon dioxide with volcanic eruptions are going at the same pace. When the gas binding decreases and the supply does not decrease, the CO₂ concentration naturally rises. The planets, as you know, are in the vacuum of interplanetary space, and the only significant way of heat loss for them is its radiation in the form of infrared waves. Carbon dioxide absorbs such radiation from the planet's surface, which is why the atmosphere is slightly warmed up. This evaporates water vapor from the water surface of the oceans, which also absorbs infrared radiation (another greenhouse gas). As a result, it is CO₂ that acts as the main initiator in the process of heating the planet.

It is this mechanism that leads to the fact that glaciers on Earth end sooner or later. He also does not allow it to overheat: at excessively high temperatures, carbon dioxide is more quickly bound by rocks, after which, due to the tectonics of the earth's crust plates, they gradually sink into the mantle. CO level₂falls and the climate becomes cooler.

The importance of this mechanism for our planet can hardly be overestimated. Imagine for a second a breakdown of a carbon air conditioner: say, volcanoes have stopped erupting and no longer deliver carbon dioxide from the bowels of the Earth, which once descended there with old continental plates. The very first glaciation will literally become eternal, because the more ice on the planet, the more solar radiation it reflects into space. And a new portion of CO₂ will not be able to unfreeze the planet: it will have nowhere to come from.

This is exactly how, in theory, it should be on the planets-oceans. Even if volcanic activity at times can break through the shell of exotic ice at the bottom of the planetary ocean, there is little good about it. Indeed, on the surface of the sea world, there are simply no rocks that could bind excess carbon dioxide. That is, its uncontrolled accumulation can begin and, accordingly, overheating of the planet.

Something similar - true, without any planetary ocean - happened on Venus. There is no plate tectonics on this planet either, although why this happened is not really known. Therefore, volcanic eruptions there, breaking through at times through the crust, put a lot of carbon dioxide into the atmosphere, but the surface cannot bind it: continental plates do not sink down and new ones do not rise up. Therefore, the surface of the existing slabs has already bound all the CO₂, which could, and cannot absorb more, and it is so hot on Venus that lead will always remain a liquid there. And this despite the fact that, according to modeling, with the Earth's atmosphere and carbon cycle, this planet would be a habitable twin of the Earth.

Is there life without air conditioning?

Critics of "terrestrial chauvinism" (the position that life is possible only on "copies of the Earth", planets with strictly terrestrial conditions) immediately asked the question: why, in fact, everyone decided that minerals would not be able to break through a layer of exotic ice? The stronger and more impenetrable the lid is over something hot, the more energy accumulates under it, which tends to break out. Here is the same Venus - plate tectonics does not seem to exist, and carbon dioxide escaped from the depths in such quantities that there is no life from it in the literal sense of the word. Consequently, the same is possible with the removal of minerals upward - solid rocks during volcanic eruptions completely fall upward.

Even so, another problem remains - the “broken air conditioner” of the carbon cycle. Could an ocean planet be habitable without it?

There are many bodies in the solar system on which carbon dioxide does not at all play the role of the main regulator of the climate. Here is, say, Titan, a large moon of Saturn.

Titanium. Photo: NASA / JPL-Caltech / Stéphane Le Mouélic, University of Nantes, Virginia Pasek, University of Arizona

The body is negligible in comparison with the Earth's mass. However, it was formed far from the Sun, and the radiation of the luminary did not "evaporate" from it the light elements, including nitrogen. This gives Titan an atmosphere of almost pure nitrogen, the same gas that dominates our planet. But the density of its nitrogen atmosphere is four times that of ours - with gravity it is seven times weaker.

At the first glance at Titan's climate, there is a steady feeling that it is extremely stable, although there is no "carbon" air conditioner in its direct form. Suffice it to say that the temperature difference between the pole and the equator of Titan is only three degrees. If the situation was the same on Earth, the planet would be much more evenly populated and generally more suitable for life.

Moreover, calculations by a number of scientific groups have shown: with an atmosphere density five times higher than that of the Earth, that is, a quarter higher than on Titan, even the greenhouse effect of nitrogen alone is quite enough for temperature fluctuations to drop to almost zero. On such a planet, day and night, both at the equator and at the pole, the temperature would always be the same. Earthly life can only dream of such a thing.

Planets-oceans in terms of their density are just at the level of Titan (1, 88 g / cm ³), and not the Earth (5, 51 g / cm ³). Let's say, three planets in the TRAPPIST-1 habitable zone 40 light years from us have a density from 1.71 to 2.18 g / cm³. In other words, most likely, such planets have more than sufficient density of nitrogen atmosphere to have a stable climate due to nitrogen alone. Carbon dioxide cannot turn them into red-hot Venus, because a really large mass of water can bind a lot of carbon dioxide even without any plate tectonics (carbon dioxide is absorbed by water, and the higher the pressure, the more it can contain it).

Deep sea deserts

With hypothetical extraterrestrial bacteria and archaea, everything seems to be simple: they can live in very difficult conditions and for this they do not need an abundance of many chemical elements at all. It is more difficult with plants and a highly organized life living at their expense.

So, ocean planets can have a stable climate - very likely more stable than the Earth has. It is also possible that there is a noticeable amount of minerals dissolved in water. And yet, life there is not at all Shrovetide.

Let's take a look at the Earth. Except for the last millions of years, its land is extremely green, almost devoid of brown or yellow spots of deserts. But the ocean does not look green at all, except for some narrow coastal zones. Why is that?

The thing is that on our planet the ocean is a biological desert. Life requires carbon dioxide: it "builds" plant biomass and only from it can animal biomass be fed. If there is CO in the air around us₂ more than 400 ppm as it is now, the vegetation is blooming. If it were less than 150 parts per million, all trees would die (and this could happen in a billion years). With less than 10 parts of CO₂ per million all plants would die in general, and with them all really complex forms of life.

At first glance, this should mean that the sea is a real expanse for life. Indeed, the earth's oceans contain a hundred times more carbon dioxide than the atmosphere. Therefore, there should be a lot of building material for plants.

In fact, nothing is further from the truth. The water in the Earth's oceans is 1.35 quintillion (billion billion) tons, and the atmosphere is just over five quadrillion (million billion) tons. That is, there is noticeably less CO in a ton of water.₂than a ton of air. Aquatic plants in Earth's oceans almost always have much less CO₂ at their disposal than terrestrial ones.

To make matters worse, aquatic plants only have a good metabolic rate in warm water. Namely, in it, CO₂ least of all, because its solubility in water decreases with increasing temperatures. Therefore, algae - in comparison with terrestrial plants - exist under conditions of constant colossal CO deficiency.₂.

That is why scientists' attempts to calculate the biomass of terrestrial organisms show that the sea, which occupies two-thirds of the planet, makes an insignificant contribution to the total biomass. If we take the total mass of carbon - the key material in the dry mass of any living creature - the inhabitants of the land, then it is equal to 544 billion tons. And in the bodies of the inhabitants of the seas and oceans - only six billion tons, crumbs from the master's table, a little more than a percent.

All this may lead to the opinion that although life on the planets-oceans is possible, it will be very, very unsightly. The biomass of the Earth, if it were covered by one ocean, all other things being equal, would be, in terms of dry carbon, only 10 billion tons - fifty times less than it is now.

However, even here it is too early to put an end to the water worlds. The fact is that already at a pressure of two atmospheres, the amount of CO₂, which can dissolve in seawater, more than doubles (for a temperature of 25 degrees). With atmospheres four to five times denser than Earth's - and this is exactly what you would expect on planets like TRAPPIST-1e, g and f - there can be so much carbon dioxide in the water that the water of the local oceans will begin to approach the Earth's air. In other words, aquatic plants on planets and oceans find themselves in much better conditions than on our planet. And where there is more green biomass, and animals have a better food base. That is, unlike the Earth, the seas of planets-oceans may not be deserts, but oases of life.

Sargasso planets

But what to do if the ocean planet, due to a misunderstanding, still has the Earth's atmosphere density? And everything is not so bad here. On Earth, algae tend to attach to the bottom, but where there are no conditions for this, it turns out that aquatic plants can swim.

Some of the sargassum algae use air-filled sacs (they resemble grapes, hence the Portuguese word "sargasso" in the name of the Sargasso Sea) to provide buoyancy, and in theory this allows you to take CO₂ from air, and not from water, where it is scarce. Due to their buoyancy, it is easier for them to do photosynthesis. True, such algae reproduce well only at rather high water temperatures, and therefore on Earth they are relatively good only in some places, such as the Sargasso Sea, where the water is very warm. If the ocean planet is warm enough, then even the earth's atmospheric density is not an insurmountable obstacle for marine plants. They may well take CO₂ from the atmosphere, avoiding the problems of low carbon dioxide in warm water.

Sargasso algae. Photo: Allen McDavid Stoddard / Photodom / Shutterstock

Interestingly, floating algae in the same Sargasso Sea give rise to a whole floating ecosystem, something like a "floating land". Crabs live there, for which the buoyancy of algae is enough to move on their surface as if it were land. Theoretically, in calm areas of the ocean planet, floating groups of sea plants can develop quite "land" life, although you will not find the land itself there.

Check your privilege, earthling

The problem of identifying the most promising places for the search for life is that so far we have little data that would allow us to single out the most likely carriers of life among the candidate planets. By itself, the concept of "habitable zone" is not the best assistant here. In it, those planets are considered suitable for life that receive from their star a sufficient amount of energy to support liquid reservoirs at least on a part of their surface. In the solar system, both Mars and the Earth are in the habitable zone, but at the first complex life on the surface is somehow imperceptible.

Mainly because this is not the same world as Earth, with a fundamentally different atmosphere and hydrosphere. The linear representation in the style of "the planet-ocean is the Earth, but only covered with water" can lead us into the same delusion that at the beginning of the 20th century there was about the suitability of Mars for life. Real oceanids can differ sharply from our planet - they have a completely different atmosphere, different mechanisms for stabilizing the climate, and even different mechanisms for supplying marine plants with carbon dioxide.

A detailed understanding of how the water worlds actually work allows us to understand in advance what the habitable zone will be for them, and thereby quickly approach detailed observations of such planets in James Webb and other promising large telescopes.

Summing up, one cannot but admit that until very recently our ideas about which worlds are really inhabited and which are not, suffered too much from anthropocentrism and geocentrism. And, as it turns out now, from "sushcentrism" - the opinion that if we ourselves arose on land, then it is the most important place in the development of life, not only on our planet, but also in other suns. Perhaps the observations of the coming years will not leave a stone unturned from this point of view.