Scientists have identified a second condition for the goldilocks zone. Search for planets in the habitable zone

Look at the scattering of stars in the black night sky - they all contain amazing worlds like our solar system. According to the most conservative estimates, the Milky Way galaxy contains more than a hundred billion planets, some of which may be similar to Earth.

New information about “alien” planets - exoplanets- opened the Kepler space telescope, exploring the constellations in anticipation of the moment when a distant planet appears in front of its star.

The orbital observatory was launched in May 2009 specifically to search for exoplanets, but four years later it failed. After many attempts to return the telescope to operation, NASA was forced to decommission the observatory from its “space fleet” in August 2013. However, over the years of observations, Kepler has received so much unique data that it will take several more years to study them. NASA is already preparing to launch Kepler's successor, the TESS telescope, in 2017.

Super-Earths in the Goldilocks Belt

Today, astronomers have identified nearly 600 new worlds out of 3,500 candidates for the “exoplanet” title. It is believed that among these celestial bodies, at least 90% may turn out to be “true planets”, and the rest - double stars, “brown dwarfs” that have not grown to stellar size, and clusters of large asteroids.

Most of the new planet candidates are gas giants like Jupiter or Saturn, as well as super-Earths—rocky planets several times larger than ours.

Naturally, not all planets fall within the field of view of Kepler and other telescopes. Their number is estimated at only 1-10%.

To be sure to identify an exoplanet, it must be repeatedly recorded on the disk of its star. It is clear that most often it turns out to be located close to its sun, because then its year will last only a few Earth days or weeks, so astronomers will be able to repeat observations many times.

Such planets, in the form of hot balls of gas, often turn out to be “hot Jupiters,” and every sixth one is like a flaming super-Earth covered in seas of lava.

Of course, under such conditions, protein life of our type cannot exist, but among hundreds of inhospitable bodies there are pleasant exceptions. So far, more than a hundred terrestrial planets have been identified, located in the so-called habitable zone, or goldilocks belt.

This fairy-tale character was guided by the principle of “no more, no less.” Likewise, rare planets included in the “life zone” should have a temperature within the limits of the existence of liquid water. Moreover, 24 planets from this number have a radius less than two radii of the Earth.

However, so far only one of these planets has the main features of the Earth’s twin: it is in the Goldilocks zone, close to Earth’s size, and is part of a yellow dwarf system similar to the Sun.

In the world of red dwarfs

However, astrobiologists, persistently searching for extraterrestrial life, do not lose heart. Most of the stars in our galaxy are small, cool, dim red dwarfs. According to modern data, red dwarfs, being about half the size and cooler than the Sun, make up at least three-quarters of the “stellar population” of the Milky Way.

Orbiting these solar cousins ​​are miniature systems the size of Mercury's orbit, and they too have their own Goldilocks belts.

Astrophysicists at the University of California at Berkeley even compiled a special computer program, TERRA, with the help of which they identified a dozen terrestrial twins. All of them are close to their zones of life around the small red luminaries. All this greatly increases the chances of the presence of extraterrestrial centers of life in our galaxy.

Previously, it was believed that red dwarfs, in the vicinity of which Earth-like planets were found, are very quiet stars, and flares accompanied by plasma emissions rarely occur on their surface.

As it turned out in fact, such luminaries are even more active than the Sun.

Powerful cataclysms constantly occur on their surface, generating hurricane gusts of “stellar wind” that can overcome even the powerful magnetic shield of the Earth.

However, many Earth twins can pay a very high price for being close to their star. Radiation flows from frequent flares on the surface of red dwarfs can literally “lick” part of the atmosphere of planets, making these worlds uninhabitable. At the same time, the danger of coronal ejections is enhanced by the fact that the weakened atmosphere will poorly protect the surface from charged particles of hard ultraviolet radiation and X-rays from the “stellar wind”.

In addition, there is a danger of suppression of the magnetospheres of potentially habitable planets by the strong magnetic field of red dwarfs.

Broken magnetic shield

Astronomers have long suspected that many red dwarfs have powerful magnetic fields that could easily penetrate the magnetic shield surrounding potentially habitable planets. To prove this, a virtual world was built in which our planet rotates around a similar star in a very close orbit in the “life zone.”

It turned out that very often the magnetic field of a dwarf not only greatly deforms the Earth’s magnetosphere, but even drives it under the surface of the planet. In this scenario, in just a few million years we would have no air or water left, and the entire surface would be scorched by cosmic radiation.

Two interesting conclusions follow from this. The search for life in red dwarf systems may be completely futile, and this is another explanation for the “great silence of the cosmos.”

However, perhaps we cannot detect extraterrestrial intelligence because our planet was born too early...

Who can live on distant exoplanets? Could there be such creatures?

The sad fate of the firstborn

Analyzing data obtained using the Kepler and Hubble telescopes, astronomers discovered that the process of star formation in the Milky Way has slowed down significantly. This is due to the growing shortage of building materials in the form of dust and gas clouds.

Nevertheless, there is still a lot of material left in our galaxy for the birth of stars and planetary systems. Moreover, in a few billion years our stellar island will collide with the giant galaxy the Andromeda Nebula, which will cause a colossal surge in star formation.

Against this backdrop of future galactic evolution, the sensational news was recently announced that four billion years ago, during the emergence of the Solar System, only a tenth of the potentially habitable planets existed.

Considering that it took several hundred million years for the birth of the simplest microorganisms on our planet, and several more billions of years for developed life forms to form, there is a high probability that intelligent aliens will appear only after the extinction of the Sun.

Perhaps here lies the answer to the intriguing Fermi paradox, which was once formulated by an outstanding physicist: where are these aliens? Or does it make sense to look for answers on our planet?

Extremophiles on Earth and in space

The more we become convinced of the uniqueness of our place in the Universe, the more often the question arises: can life exist and develop in worlds completely different from ours?

The answer to this question is given by the existence of amazing organisms on our planet - extremophiles. They got their name for their ability to survive in extreme temperatures, toxic environments and even airless space. Marine biologists have found similar creatures in underground geysers - “sea smokers.”

There they thrive under enormous pressure and in the absence of oxygen at the very edge of hot volcanic vents. Their “colleagues” are found in salty mountain lakes, hot deserts and subglacial reservoirs of Antarctica. There are even “tardigrade” microorganisms that survive the vacuum of space. It turns out that even in the radiation environment near red dwarfs, some “extreme microbes” can arise.

Acid lake located in Yellowstone. Red plaque - acidophilus bacteria

Tardigrades are capable of existing in the vacuum of space

Academic evolutionary biology believes that life on Earth arose from chemical reactions in a “warm, shallow body of water” permeated by ultraviolet radiation and ozone from raging “lightning storms.” On the other hand, astrobiologists know that the chemical “building blocks” of life are found on other worlds. For example, they were noticed in gas and dust nebulae and satellite systems of our gas giants. This, of course, is still far from a “full life,” but the first step towards it.

The “standard” theory of the origin of life on Earth recently received a major blow from... geologists. It turns out that the first organisms are much older than previously thought, and were formed in a completely unfavorable environment of a methane atmosphere and boiling magma pouring out from thousands of volcanoes.

This makes many biologists think about the old hypothesis of panspermia. According to it, the first microorganisms originated somewhere else, say, on Mars, and came to Earth in the core of meteorites. Perhaps ancient bacteria had to travel a longer distance in cometary nuclei that arrived from other star systems.

But if this is so, then the paths of “cosmic evolution” can lead us to “brothers by origin” who drew the “seeds of life” from the same source as us...

The weather forecast for most exoplanets is disappointing. The scorching sun, annual floods and deep snow make life much more difficult for local inhabitants.

Scientists are interested in the habitability of other planets for a number of reasons, political, financial, humanitarian and scientific. They want to understand how our own climate is changing.

How we will live in the climate of the future and what we can do to stop the rising tide of the greenhouse effect. After all, in just a little while the heavenly Earth will be hopelessly lost.

It is unlikely that we will seriously concern ourselves with the search for clean energy sources or persuade politicians to take up climate issues at the expense of financial gain. A much more interesting question is: when will we see aliens?

The habitable zone, also known as the “Goldilocks zone,” is the region around a star where the average temperature of the planet allows the liquid water we are so accustomed to to exist. We are hunting for liquid water not only for future use, but also to find a landmark: perhaps there may be other life out there somewhere.

The problems outside this zone are quite obvious. If it is too hot, the environment will become an unbearable steam bath, or will begin to break the water into oxygen and hydrogen.

Oxygen will then combine with carbon to form carbon dioxide, and hydrogen will escape into space. This happens with Venus.

If the planet is too cold, the water will form solid pieces. There may be pockets of liquid water under the ice crust, but overall it's not a pleasant place to live.

We found this on Mars and the moons of Jupiter and Saturn. And if a potentially habitable zone can be roughly defined, it is a place where liquid water could exist.

Unfortunately, this equation involves more than just the distance to the star and the amount of energy produced. The atmosphere of the planet plays a serious role.

You will be surprised, but Venus and Mars are in the potentially habitable zone of the solar system. The atmosphere of Venus is so thick that it traps the sun's energy and creates a life-inhospitable furnace that will melt any hint of life faster than you can say "two cups of tea to this gentleman." On Mars, everything is completely opposite.

The thin atmosphere cannot retain heat at all, so the planet is very cold. Improve the atmospheres of both planets and you will get worlds that can easily harbor life.

Perhaps we could push them together and mix up the atmospheres? Need to think. When we look at other worlds in the Milky Way and try to understand whether there is life there, it is not enough to simply estimate their location in the Goldilocks zone.

We need to know the shape of the atmosphere. Astronomers have found planets located in habitable zones around other stars, but these worlds do not appear to be particularly well-positioned for life.

They orbit red dwarf stars. In principle, living in conditions of reddish reflections is not so bad, but there is one problem.

Red dwarfs tend to behave very poorly when they are young. They generate powerful flares and coronal mass ejections.

This clears the surface of any planet that gets too close. True, there is some hope.

After a few million years of high activity, these red dwarf stars settle down and begin sucking up their hydrogen reserves with a potential of trillions of years. If life can survive long enough in the early stages of a star's existence, it can expect a long, happy life. When you're thinking about a new home among the stars or trying to find new life in the universe, look for planets in the potentially habitable zone.

Habitable zone (Goldilocks zone)

Once upon a time there was a solar system, and then one day - a long time ago, about four billion years ago - it realized that it was almost formed. Venus appeared near the very Sun - and she was so close to the Sun that the energy of the sun's rays evaporated her entire supply of water. But Mars was far from the Sun - and all its water froze. And only one planet - the Earth - turned out to be just at such a distance from the Sun - “just right” - that the water on it remained liquid, and therefore life could arise on the surface of the Earth. This belt around the Sun became known as the habitable zone. The tale of the three bears is told to children in many countries, and in England its heroine is called Goldilocks. She also liked everything to be “just right.” In the three bears' house, one bowl of porridge was too hot. The other one is too cold. And only the third was “just right” for Goldilocks. And in the house of the three bears there were three beds, and one was too hard, the other was too soft, and the third was “just right,” and Goldilocks fell asleep in it. When the three bears returned home, they discovered not only that the porridge from the third bowl was missing, but also Goldilocks, who was sleeping sweetly in the little bear's bed. I don’t remember how it all ended, but if I were the three bears - omnivorous predators at the very top of the food chain - I would have eaten Goldilocks.

Goldilocks might be interested in the relative habitability of Venus, Earth, and Mars, but in reality the plot of these planets is much more complex than three bowls of porridge. Four billion years ago, planetary surfaces were still being bombarded by water-rich comets and mineral-rich asteroids, albeit much less frequently than before. During this game of cosmic billiards, some planets migrated from their native places closer to the Sun, and some were thrown into orbits of larger diameter. And many of the dozens of formed planets ended up in unstable orbits and fell into the Sun or Jupiter. Several more planets were simply thrown out of the solar system. The remaining units in the end rotated precisely in those orbits that turned out to be “just right” to survive billions of years on them. The Earth settled in an orbit with an average distance to the Sun of approximately 150 million kilometers. At this distance, the Earth intercepts a very modest fraction of the total energy emitted by the Sun - only two billionths. If we assume that the Earth absorbs all this energy, then the average temperature of our planet is about 280 K, that is, 7 ° C - halfway between winter and summer temperatures.

At normal atmospheric pressure, water freezes at 273 K and boils at 373 K, so, to our great joy, almost all the water on Earth is in a liquid state. However, there is no need to rush. Sometimes in science you get the right answers based on the wrong premises. In fact, the Earth absorbs only two-thirds of the solar energy that reaches it. The rest is reflected back into space by the earth's surface (especially the oceans) and cloud cover. If we add the reflection coefficient to the formula, then the average temperature of the Earth drops to 255 K, which is much lower than the freezing point of water. These days, there must be some other mechanism at work that keeps the average temperature at a more comfortable level. Again, take your time. All theories of stellar evolution tell us that four billion years ago, when life was forming from the proverbial primordial soup on Earth, the Sun was a third dimmer than it is today, which means that the average temperature of the Earth was below freezing. Maybe the Earth in the distant past was simply closer to the Sun? However, after a period of intense bombardment that has long since ended, we do not know of any mechanisms that would shift stable orbits within the solar system. Maybe the greenhouse effect was stronger in the past? We don't know for sure. But we know that habitable zones in the original sense of these words have only a distant relation to whether life can exist on planets located within the boundaries of these zones.

The famous Drake equation, which is always referred to in the search for extraterrestrial intelligence, allows us to give a rough estimate of how many civilizations can, in principle, be found in the Milky Way galaxy. The equation was derived in the 60s of the 20th century by the American astronomer Frank Drake, and at that time the concept of a habitable zone was limited to the idea that planets should be at a distance from their star that is “just right” for the existence of life. The meaning of one version of the Drake equation is something like this: let's start with the number of stars in the galaxy (hundreds of billions). Let's multiply this huge number by the fraction of stars that have planets. Multiply the resulting number by the fraction of planets located in the habitable zone. Now let's multiply the result by the fraction of planets on which life has developed. Let us multiply the result by the proportion of planets on which intelligent life has developed. Let us multiply the result by the proportion of planets where technological progress has reached such a stage that interstellar communication can be established.

If we now take into account the rate of star formation and the life expectancy of a technologically advanced civilization, we get the number of advanced civilizations that are probably waiting for our phone call this very minute. Small, cool, low-luminosity stars live hundreds of billions, maybe even trillions, of years, which means their planets have enough time to grow two or three species of living organisms, but their habitable zones are too close to the star. A planet that formed in this zone quickly falls into the so-called tidal capture of the star and always rotates with one side facing it, which causes a strong imbalance in the heating of the planet - all the water on the “front” side of the planet will evaporate, and all the water on the “back” will freeze . If Goldilocks lived on such a planet, we would find that she eats her porridge, spinning on her axis like a grilled chicken - on the very border between eternal sunshine and eternal darkness. Habitable zones around long-lived stars have another drawback - they are very narrow, so the planet has very little chance of accidentally ending up in an orbit with a radius that is “just right.”

But around hot, large, bright stars there are huge habitable zones. However, these stars, unfortunately, are rare and live only a few million years before exploding, so their planets can hardly be considered candidates in the search for life as we know it, unless they undergo some very rapid evolution. And it is unlikely that the animals capable of inventing differential calculus will be the first to emerge from the primeval mucus. The Drake Equation can be considered Goldilocks mathematics, a method for estimating what the odds are that somewhere in the galaxy everything has worked out “just right.” However, the Drake equation in its original form does not include, for example, Mars, which is located far beyond the habitable zone of the Sun. Meanwhile, Mars is full of winding, dry rivers with deltas and floodplains, and this irrefutably proves that at one time in the past there was plenty of liquid water on Mars.

But what about Venus, the “sister” of the Earth? It falls exactly within the habitable zone of the Sun. This planet, completely covered by a thick layer of clouds, has the highest reflectivity in the entire solar system. There are no obvious reasons why Venus might be bad and uncomfortable. However, it exhibits a monstrous greenhouse effect. Venus's thick atmosphere is mostly carbon dioxide and absorbs almost 100% of the little radiation that reaches its surface. The temperature on Venus is 750 K, and this is a record in the entire solar system, although the distance from the Sun to Venus is almost twice that of Mercury.

Since the Earth has supported life throughout its entire evolution - billions of years of turbulent vicissitudes - this means that life itself probably provides some kind of feedback mechanism that maintains liquid water on the planet. This idea was developed by biologists James Lovelock and Lynn Margulis in the 70s, and it is called the Gaia hypothesis. This fairly popular but controversial hypothesis suggests that the collection of species on Earth at any given time acts as if it were a collective organism that continually, albeit unintentionally, adjusts the composition of the Earth's atmosphere and climate so that it is conducive to the presence and development of life - that is, the presence of liquid water on the surface. I think this is very interesting and worthy of study. The Gaia hypothesis is a favorite hypothesis of New Age philosophies. But I'm willing to bet that some long-dead Martians and Venusians probably also advocated this idea a billion years ago...

If we expand the concept of a habitable zone, it turns out that it just needs any source of energy to melt the ice. One of Jupiter's moons, icy Europa, is heated by the tidal forces of Jupiter's gravitational field. Just like a racquetball that heats up from frequent impacts, Europa heats up from the dynamic loads that Jupiter pulls on one side more than the other. What is the result? Current observational data and theoretical calculations show that beneath a kilometer-thick crust of ice on Europa lies an ocean of liquid water or, possibly, slushy snow. Given the abundance of life in the ocean depths on Earth, Europa is the most tempting candidate for life in the solar system beyond Earth. Another recent breakthrough in our understanding of what the habitable zone is is living organisms recently dubbed "extremophiles": organisms that not only survive, but even thrive in conditions of extreme cold or extreme heat. If there were biologists among the extremophiles, they would probably think that they are normal, and extremophiles are all those who live well at room temperature. Among the extremophiles are heat-loving thermophiles, which usually live near underwater mountain ridges in the middle of the oceans, where water, heated under enormous pressure to a temperature much higher than the normal boiling point, splashes out from under the earth's crust into the cold thickness of the ocean. The conditions there are similar to those in a kitchen pressure cooker: a particularly durable saucepan with an airtight lid allows you to heat water under pressure to a temperature above boiling, while avoiding boiling as such.

On the cold ocean floor, minerals rise from hot springs, creating giant porous tubes ten stories high - hot in the middle, a little cooler at the edges, where they directly touch the ocean water. At all these temperatures, the chimneys are inhabited by countless species of living beings who have never seen the Sun and who do not care whether it exists or not. These tough nuts are fueled by geothermal energy, which is a combination of what's left over from the Earth's formation, and the heat that constantly seeps into the Earth's crust due to the radioactive decay of natural, but unstable isotopes of long-familiar chemical elements - including, for example, aluminum-26, which lasts for millions of years, and potassium-40, which lasts for billions. The ocean floor is probably one of the most stable ecosystems on Earth. What would happen if a giant asteroid collided with the Earth and all life on its surface would die out? Ocean thermophiles will live and live as if nothing had happened. Perhaps after each wave of extinction they even evolve and repopulate the earth's land. What will happen if the Sun, for mysterious reasons, disappears from the center of the solar system, and the Earth falls out of orbit and drifts in outer space? This event will not even make it into the Thermophile newspapers. However, five billion years will pass, and the Sun will turn into a red giant, expand and absorb the entire inner solar system. The Earth's oceans will boil away, and the Earth itself will evaporate. Now this will be a sensation.

If thermophiles are everywhere on Earth, a serious question arises: what if life began deep in the depths of stray planets that were kicked out of the solar system during its formation? Their “geo” thermal reservoirs would last for billions of years. And what can be said about the countless planets that were forcibly expelled from all other solar systems that managed to form in our Universe? Could it be that interstellar space is teeming with life that arose and evolved in the depths of homeless planets? The habitable zone is not a neatly delineated area around a star that receives the ideal, “just right” amount of sunlight - in fact, it is everywhere. So the house of the three bears may also not occupy any special place in the world of fairy tales. A bowl of porridge, the temperature of which was “just right,” could be found in any home, even in the houses of the three little pigs. We found that the corresponding factor of the Drake equation - the same one that is responsible for the existence of planets within the habitable zone - may well increase to almost 100%.

So our fairy tale has a very promising ending. Life is not necessarily a rare and unique phenomenon; it is perhaps as common as the planets themselves. And thermophilic bacteria lived happily ever after - about five billion years.

Water, water, water all around

Judging by the appearance of some of the driest and most inhospitable places in our solar system, you might think that water, which is abundant on Earth, is a rare luxury in the rest of the galaxy. However, of all triatomic molecules, water is the most abundant, and by a wide margin. And in the list of the most common elements in space, the components of water - hydrogen and oxygen - occupy first and third places. So there is no need to ask where water came from in this or that place - it is better to ask why it is not available everywhere. Let's start with the solar system. If you are looking for a place without water and without air, you don’t have to go far: you have the Moon at your disposal. With low atmospheric pressure on the Moon - it is almost zero - and two-week days when the temperature is close to 100 °C, water quickly evaporates. During the two-week night the temperature drops to -155°C: under these conditions almost anything will freeze.

The Apollo astronauts took with them to the Moon all the air, all the water, and all the air conditioning systems they needed for the trip there and back. However, in the distant future, expeditions will probably no longer need to carry water and various products made from it. Data from the Clementine space probe puts to rest once and for all a long-standing debate about whether there are frozen lakes at the bottom of deep craters at the Moon's North and South Poles. If we take into account the average number of collisions of the Moon with interplanetary debris per year, we have to assume that among the debris falling to the surface there must be quite large icy comets. What does "big enough" mean? There are enough comets in the solar system that, if they melted, would leave a puddle the size of Lake Erie.

Of course, you can't expect a brand new lake to survive many hot lunar days with temperatures close to 100°C, but any comet that hits the lunar surface and evaporates dumps some of its water molecules at the bottom of deep craters near the poles. These molecules are absorbed into the lunar soil, where they remain forever, since such places are the only places on the Moon where literally “the Sun does not shine.” (If you were convinced that one side of the Moon was always dark, then you were misled by a variety of reputable sources, which undoubtedly included Pink Floyd's 1973 album The Dark Side of the Moon. ) As sun-starved inhabitants of the Arctic and Antarctic know, in these places the Sun never rises high above the horizon - neither during the day nor throughout the year. Now imagine that you live at the bottom of a crater, the edge of which is higher than the point in the sky as far as the Sun rises. In such a crater, and even on the Moon, where there is no air and nothing to scatter the light so that it gets into the shady corners, you will have to live in eternal darkness.

Your refrigerator is also cold and dark, but the ice there still evaporates over time (if you don’t believe me, look at what the ice cubes look like when you return from a long absence), however, at the bottom of these craters it is so cold that evaporation, in essentially stops (at least within the framework of our conversation, we can well assume that it does not exist). There is no doubt that if we ever build a colony on the Moon, it will need to be located near such craters. In addition to the obvious advantages - the colonists will have plenty of ice, something to melt, clean and drink - hydrogen can also be extracted from water molecules by separating it from oxygen. Hydrogen and part of the oxygen will go into rocket fuel, and the colonists will breathe the rest of the oxygen. And in your free time from space expeditions, you can go ice skating on a frozen lake made from extracted water.

So, ancient crater data tells us that comets fell on the Moon, which means the same thing happened to the Earth. If you consider that the Earth is larger and its gravity is stronger, you can even conclude that comets fell to the Earth much more often. This is true - from the very birth of the Earth to the present day. Moreover, the Earth did not emerge from the vacuum of space in the form of a ready-made spherical coma. It grew from condensed protosolar gas, from which the Sun itself and all the other planets were formed. The earth continued to grow as small solid particles stuck to it, and then through constant bombardment by asteroids, which were rich in minerals, and comets, which were rich in water. In what sense is it constant? It is suspected that the frequency of comets falling on Earth in the early stages of its existence was enough to provide water to all its oceans. However, there are still some questions (and room for debate). The water from the comets we are studying now, compared to the water from the oceans, has a lot of deuterium - a type of hydrogen that has an extra neutron in its nucleus. If the oceans were filled by comets, then the comets that fell to Earth at the beginning of the solar system had a slightly different chemical composition.

Did you think you could safely go outside? Well, no: recent studies of the water content in the upper layers of the Earth's atmosphere have shown that chunks of ice the size of houses regularly fall to Earth. These interplanetary snowballs quickly evaporate upon contact with air, but manage to contribute to the Earth's water budget. If the frequency of falls has been constant throughout Earth's 4.6 billion year history, then these snowballs may also have contributed to Earth's oceans. Add to this water vapor, which, as we know, enters the atmosphere during volcanic eruptions, and it turns out that the Earth received its supply of water on the surface in a variety of ways. Our majestic oceans now cover two-thirds of the earth's surface, but make up only one five-thousandth of the earth's mass. It would seem a very small share, but it is still as much as one and a half quintillion tons, 2% of which at any given time is in the form of ice. If Earth ever experiences a strong greenhouse effect like Venus, our atmosphere will absorb excess amounts of solar energy, air temperatures will rise, and the oceans will boil and quickly evaporate into the atmosphere. This will be bad. Not only will the flora and fauna of the Earth die out - this is obvious - one of the compelling (literally) reasons for universal destruction will be that the atmosphere, saturated with water vapor, will become three hundred times more massive. We'll all be flattened.

Venus is different from the other planets in the solar system in many ways, including its thick, dense, heavy atmosphere of carbon dioxide, the pressure of which is one hundred times that of Earth's atmosphere. We would have been flattened there too. However, in my ranking of the most amazing features of Venus, the first place is occupied by the presence of craters, which all formed relatively recently and are distributed evenly over the entire surface. This seemingly innocuous feature suggests a single catastrophe on a planetary scale that reset the cratering clock and erased all evidence of past impacts. This is possible, for example, with an erosive climatic phenomenon like a global flood. And also - large-scale geological (not venereological) activity, say, lava flows that turned the entire surface of Venus into the dream of an American motorist - a completely paved planet. Whatever reset the clock happened abruptly and instantly. However, not everything is clear here. If there really was a global flood on Venus, where did all the water go now? Gone below the surface? Evaporated into the atmosphere? Or was Venus flooded not by water at all, but by some other substance?

Our curiosity and ignorance are not limited to Venus alone - they extend to other planets. Mars was once a real swamp - with winding rivers, floodplains, deltas, a network of small streams and huge canyons carved by running water. We already have enough evidence that if anywhere in the solar system there were abundant sources of water, it was on Mars. However, today the surface of Mars is completely dry, and it is not clear why. Looking at Mars and Venus - our planet's brother and sister - I also look at Earth in a new way and think about how unreliable our water sources on the Earth's surface may be. As we already know, Percival Lowell's imagination led Percival Lowell to suggest that colonies of inventive Martians had built an ingenious network of canals on Mars to deliver water from the polar glaciers to the more populated mid-latitudes. To explain what he saw (or thought he saw), Lowell invented a dying civilization that had somehow lost its water. In his detailed but astonishingly flawed treatise, Mars as the Abode of Life (1909), Lowell bemoans the inevitable decline of the Martian civilization of his imagination:

The drying out of the planet will continue, undoubtedly, until its surface loses the ability to support all life. Time will surely blow it away like dust. However, when its last spark goes out, the dead planet will rush through space like a ghost, and its evolutionary career will end forever.

(Lowell, 1908, p. 216)

Lowell got one thing right. If there was once a civilization (or any living organisms) on the Martian surface that required water, then at some unknown stage in Martian history and for some unknown reason, all the water on the surface actually dried up, which led exactly to such an ending as Lowell describes. Perhaps the missing Martian water simply went underground and became captured by permafrost. How can this be proven? In large craters on the surface of Mars, streaks of dried mud overflowing are more common than in small ones. Assuming that the permafrost lies quite deep, getting to it would require a violent impact. The release of energy from such a collision would have melted the ice below the surface upon contact, causing dirt to splash out. Craters with these features are more common in cold subpolar latitudes, precisely where you would expect a layer of permafrost to lie closer to the surface. According to some estimates, if all the water, which we suspect is hidden in the permafrost on Mars and, as we know for sure, is enclosed in glaciers at the poles, melted and was evenly distributed over its surface, Mars would turn into a continuous ocean in tens of meters deep. The search for life on Mars, both modern and fossil, should include looking at a variety of places, especially below the surface of Mars.

When astrophysicists began to think about where liquid water, and by association, life, might be found, they were initially inclined to consider planets that orbit at a certain distance from their star - at such a distance that water would remain on their surface fluid, not too far and not too close. This zone is commonly referred to as the habitable zone, or Goldilocks zone (see previous chapter), and it was a reasonable estimate to begin with. However, she did not take into account the possibility of life arising in places where there were other sources of energy, thanks to which water, where it should have turned into ice, remained in a liquid state. This could provide a slight greenhouse effect. As well as an internal source of energy, such as residual heat from the formation of a planet or the radioactive decay of unstable heavy elements, each of which contributes to the internal heating of the Earth and, therefore, to its geological activity. In addition, planetary tides also serve as a source of energy - this is a more general concept than just the dancing of the heaving ocean with the Moon. As we have already seen, Io, a moon of Jupiter, is subject to constant stress due to changing tidal forces, since its orbit is not completely circular and Io moves closer and further away from Jupiter. Io is located at such a distance from the Sun that under other conditions it should have frozen forever, but due to constant tidal changes it has earned the title of the celestial body with the most violent geological activity in the entire solar system - it has everything: volcanoes spewing lava , and fiery chasms, and tectonic shifts. Sometimes modern Io is likened to the young Earth, when our planet had not yet cooled down after birth.

No less interesting is Europa, another satellite of Jupiter, which also draws heat from tidal forces. Scientists have long suspected, and have recently confirmed (based on images from the Galileo space probe), that Europa is covered in thick, migrating sheets of ice, beneath which lies an ocean of slush or liquid water. A whole ocean of water! Just imagine what ice fishing is like there. And in fact, engineers and scientists from the Jet Propulsion Laboratory are already thinking about sending a space probe to Europa, which will land on the ice, find a hole in it (or cut or sink it itself), lower a deep-sea video camera into it, and we Let's see what's there and how. Since life on Earth most likely originated in the ocean, the existence of life in the oceans of Europe is by no means an empty fantasy; it may well happen. In my opinion, the most amazing quality of water is not the well-deserved label of "universal solvent" that we all learned about in chemistry lessons at school, nor the unusually wide range of temperatures over which water remains liquid. The most amazing thing about water is that while almost all substances, including water itself, become denser when cooled, water, when cooled below 4°C, becomes less and less dense. When it freezes at zero degrees, it becomes less dense than in a liquid state at any temperature, and this is annoying for water pipes, but very good for fish. In winter, when the air temperature drops below zero, water with a temperature of 4 degrees sinks to the bottom and remains there, and a floating layer of ice very slowly grows on the surface and isolates the warmer water from the cold air.

If this density inversion did not occur with water at temperatures below 4 degrees, then at an air temperature below the freezing point, the outer surface of the reservoir would cool and sink to the bottom, and warmer water would rise to the top. Such forced convection would quickly cool the entire mass of water to zero, after which the surface would begin to freeze. Denser ice would sink - and the entire thickness of water would freeze from the bottom to the surface. In such a world there would be no ice fishing because all the fish would be frozen—frozen alive. And lovers of ice fishing would sit either under a layer of not yet frozen water, or on a block of a completely frozen reservoir. Icebreakers would not be needed to travel across a frozen Arctic: the Arctic Ocean would either freeze to the bottom or remain open to normal shipping because the ice layer would lie below. And you could walk on the ice as much as you want and not be afraid of falling through. In such a parallel world, ice floes and icebergs would sink, and in 1912 the Titanic would calmly sail to its destination - New York.

The existence of water in the galaxy is not limited to the planets and their satellites. Molecules of water, as well as several other familiar household chemicals such as ammonia, methane and ethyl alcohol, are detected every now and then in interstellar gas clouds. Under certain conditions - low temperature and high density - a group of water molecules can re-radiate the energy of a nearby star into space in the form of amplified high-intensity directed microwave radiation. The physics of this phenomenon are very similar to everything that happens with visible light in a laser. But in this case it is better to talk not about a laser, but about a maser - this is how the phrase “Microwave amplification by the stimulated emission of radiation” is abbreviated. So water is not only everywhere in the galaxy - sometimes it also radiates at you from the depths of space.

We know that water is necessary for life on Earth, but we can only assume that it is a necessary condition for the emergence of life in any corner of the galaxy. However, chemically illiterate people often believe that water is a deadly substance that is better not to come into contact with. In 1997, Nathan Zoner, a fourteen-year-old high school student in Eagle Rock, Idaho, conducted an objective study of anti-technology prejudice and related “chemophobia” that gained well-deserved fame. Nathan invited passers-by on the street to sign a petition demanding strict control or a ban on the use of dihydrogen monoxide. The young experimenter gave a list of the nightmarish properties of this substance, devoid of taste and smell:

Dihydrogen monoxide is the main component of acid rain;

Sooner or later this substance dissolves everything it comes into contact with;

If accidentally inhaled, it can be fatal;

In its gaseous state it leaves severe burns;

It has been found in tumors from terminal cancer patients.

Forty-three of the fifty people approached by Zohner signed the petition, six were undecided, and one was an ardent supporter of dihydrogen monoxide and refused to sign.

Living space

If you ask a person where he is from, the answer will usually be the name of the city where he was born, or some place on the earth's surface where he spent his childhood. And this is absolutely correct. However

An astrochemically accurate answer would be: “I come from the debris from the explosions of many massive stars that died more than five billion years ago.” Outer space is the main chemical factory. It was launched by the Big Bang, which supplied the Universe with hydrogen, helium and a drop of lithium - the three lightest elements. The remaining ninety-two naturally occurring elements created the stars, including each and every carbon, calcium, and phosphorus in every single living organism on Earth, humans and otherwise. Who would need all this rich assortment of raw materials if it remained locked in the stars? But when stars die, they return the lion's share of their mass to the cosmos and pepper nearby gas clouds with a full complement of atoms that then enrich the next generation of stars.

If the right conditions arise—the right temperature and the right pressure—many atoms come together and simple molecules arise. After which many molecules become larger and more complex, and the mechanisms for this are both intricate and inventive. Ultimately, complex molecules self-organize into living organisms of one kind or another, and this probably happens in billions of corners of the Universe. In at least one of them, molecules became so complex that they developed intelligence, and then the ability to formulate and communicate to each other the ideas outlined in the icons on this page.

Yes, yes, not only people, but also all other living organisms in space, as well as the planets and moons on which they live, would not exist if not for the remains of spent stars. In general, you are made up of garbage. You'll have to come to terms with this. It's better to be happy. After all, what could be more noble than the idea that the Universe lives within us all? You don't need rare ingredients to cook up life. Let's remember which elements occupy the first five places in terms of abundance in space: hydrogen, helium, oxygen, carbon and nitrogen. With the exception of chemically inert helium, which does not like to create molecules with anyone, we get the four main components of life on Earth. They bide their time in the massive clouds that envelop the stars in the galaxy, and begin creating molecules as soon as the temperature drops below a couple thousand degrees Kelvin. Molecules of two atoms are formed at once: this is carbon monoxide and a hydrogen molecule (two hydrogen atoms bonded to each other). Lower the temperature a little more and you get stable three- or four-atom molecules like water (H2O), carbon dioxide (CO2) and ammonia (NH3) - simple but high-quality products of biological cuisine. If the temperature drops a little more, a whole host of molecules of five and six atoms will appear. And since carbon is not only widespread, but also very active from a chemical point of view, it is included in most molecules - in fact, three-quarters of all “types” of molecules observed in the interstellar medium contain at least one carbon atom. Promising. However, space is a rather dangerous place for molecules. If they are not destroyed by the energy of supernova explosions, then ultraviolet radiation from nearby ultra-bright stars completes the matter.

The larger the molecule, the less resistant it is to attack. If the molecules are lucky and live in relatively quiet or sheltered areas, they can survive to become part of grains of cosmic dust, and eventually into asteroids, comets, planets and people. But even if the stellar onslaught leaves none of the original molecules alive, there will still be plenty of atoms and time to create complex molecules - not only during the formation of a given planet, but also on the yielding surface of the planet and below it. Some of the most common complex molecules include adenine (a nucleotide or “base” that is part of DNA), glycine (a protein precursor), and glycoaldehyde (a hydrocarbon). All these and similar ingredients are necessary for the emergence of life in the form we are familiar with and, undoubtedly, are not found only on Earth.

However, all this bacchanalia of organic molecules is not yet life, just as flour, water, yeast and salt are not yet bread. Although the transition from raw material to living being itself remains a mystery, it is obvious that several conditions are required for this to happen. The environment should encourage molecules to experiment with each other and at the same time protect against unnecessary injury. Liquids are especially good for this, as they provide both close contact and greater mobility. The more opportunities for chemical reactions the environment provides, the more inventive the experiments of its inhabitants. It is important to take into account another factor, which is indicated by the laws of physics: chemical reactions require an uninterrupted source of energy.

When we consider the wide range of temperatures, pressures, acidities and radiations at which life on Earth can thrive, and remember that what is one microbe's cozy corner is another's torture chamber, it becomes clear why scientists no longer have the right to propose additional living conditions in other places. An excellent illustration of the limitations of such conclusions is given in the charming book “Cosmotheoros” by the 17th century Dutch astronomer Christiaan Huygens: the author is convinced that hemp should be cultivated on other planets - otherwise, what would ship ropes be made of to steer ships and navigate the seas? Three hundred years have passed, and we are content with just a handful of molecules. If you mix them well and put them in a warm place, you can expect that in just a few hundred million years we will have thriving colonies of microorganisms. Life on earth is unusually prolific, there is no doubt about that. What about the rest of the Universe? If there is a celestial body anywhere else that is at least somewhat similar to our planet, perhaps it performed similar experiments with similar chemical reagents and these experiments were orchestrated by the same physical laws that are the same throughout the Universe.

Let's take carbon for example. He is able to create a variety of connections both with himself and with other elements and therefore is included in an incredible number of chemical compounds - in this he has no equal in the entire periodic table. Carbon creates more molecules than all other elements combined (10 million - how about that?). Typically, to create a molecule, atoms share one or more outer electrons, grabbing each other like cam joints between freight cars. Each carbon atom is capable of creating such bonds with one, two, three or four other atoms - but a hydrogen atom, say, with only one, oxygen - with one or two, nitrogen - with three.

When carbon combines with itself, it creates many molecules from all sorts of combinations of long chains, closed rings, or branched structures. These complex organic molecules are capable of feats that small molecules can only dream of. For example, they are capable of performing one task at one end and another at the other, twisting, folding, intertwining with other molecules, creating substances with more and more new properties and qualities - there are no barriers to them. Perhaps the most striking carbon-based molecule is DNA, a double helix in which the individual appearance of every living organism is encrypted. What about water? When it comes to ensuring life, water has a very useful quality - it remains liquid over a very wide, according to most biologists, temperature range. Unfortunately, most biologists only consider Earth, where water remains liquid within 100 degrees Celsius. Meanwhile, in some places on Mars the atmospheric pressure is so low that water is not liquid at all - as soon as you pour yourself a glass of H2O, all the water will boil and freeze at the same time! However, no matter how unfortunate the current state of the Martian atmosphere is, in the past it allowed the existence of huge reserves of liquid water. If life once existed on the surface of the red planet, it was only at that time.

As for the Earth, water on its surface is very good, sometimes even too good and even deadly. Where did she come from? As we have already seen, it is logical to assume that it was partly brought here by comets: they can be said to be saturated with water (frozen, of course), there are billions of them in the Solar System, among them there are quite large ones, and when the Solar System was just forming, they constantly bombarded young Earth. Volcanoes erupt not only because the magma is very hot, but also because the surging hot magma turns underground water into steam, and the steam rapidly expands, causing an explosion. Steam no longer fits into the underground voids, and the lid is ripped off the volcano, causing H2O to come to the surface. Given all this, it should not be surprising that the surface of our planet is full of water. With all the diversity of living organisms on Earth, they all have common sections of DNA. A biologist who has never seen anything other than the Earth in his life only rejoices at the versatility of life, but an astrobiologist dreams of diversity on a larger scale: of life based on completely alien DNA or something else altogether.

Unfortunately, so far our planet is the only biological example. However, an astrobiologist can afford to collect hypotheses about living organisms that live somewhere in the depths of space by studying organisms that live in extreme environments here on Earth. Once you start looking for these extremophiles, it turns out that they live almost everywhere: in nuclear waste dumps, in acid geysers, in acidic rivers saturated with iron, in deep-sea springs spewing chemical suspensions, and near underwater volcanoes, in permafrost. , in piles of dross, in industrial salt ponds and in all sorts of places where you probably wouldn't go on your honeymoon, but which are probably quite typical for most other planets and moons. Biologists once believed that life began in some “warm pool,” as Darwin wrote (Darwin 1959, p. 202); However, the evidence that has accumulated recently makes us lean towards the idea that the first living organisms on Earth were extremophiles.

As we'll see in the next part, for the first half-billion years of its existence, the solar system resembled nothing more than a shooting range. Large and small boulders constantly fell onto the surface of the Earth, leaving behind craters and crushing rocks into dust. Any attempt to launch the “Life” project would be immediately stopped. However, about four billion years ago, the bombardment eased and the temperature of the earth's surface began to drop, allowing the results of complex chemical experiments to survive and thrive. Old textbooks count down time from the birth of the solar system, and their authors usually claim that the Earth took 700-800 million years to form. But this is not so: experiments in the planet’s chemical laboratory could begin no earlier than the celestial bombardment subsided. Feel free to subtract 600 million years of “warfare” - and it turns out that single-celled mechanisms got out of the primordial liquid in just 200 million years. Although scientists still cannot understand how exactly life began, nature does not seem to have any difficulties with it.

Astrochemists have come a long way in just a few decades: until recently they knew nothing at all about molecules in space, but today they have already discovered many different compounds almost everywhere. Moreover, in the last ten years, astrophysicists have confirmed that planets also revolve around other stars and that every star system, not just the Solar one, is full of the same four main ingredients of life as our own cosmic home. Of course, no one expects to find life on a star, even on a “cold” one, where it is only a thousand degrees, but life on Earth is often found in places where the temperature reaches several hundred degrees. All these discoveries taken together lead us to conclude that in fact the Universe is not at all alien and unknown to us - in fact, we are already familiar with it at a fundamental level. But how closely do we know each other? What is the probability that any living organisms are like those on Earth - carbon-based and preferring water to all other liquids? Consider, for example, silicon, one of the most abundant elements in the Universe. In the periodic table, silicon is directly below carbon, which means they have the same configuration of electrons in their outer shell. Silicon, like carbon, can form bonds with one, two, three or four other atoms. Under the right conditions, it can also form chain molecules. Since silicon has about the same potential for creating chemical compounds as carbon, it is reasonable to assume that life could arise from it.

However, there is one difficulty with silicon: in addition to being ten times less common than carbon, it also creates very strong bonds. In particular, if you combine silicon and hydrogen, you will get not the rudiments of organic chemistry, but stones. On Earth, these chemical compounds have a long shelf life. And for a chemical compound to be beneficial for a living organism, it needs bonds that are strong enough to withstand not too strong attacks from the environment, but not so indestructible as to cut off the possibility of further experiments. How necessary is liquid water? Is this really the only medium suitable for chemical experiments, the only medium capable of delivering nutrients from one part of a living organism to another? Maybe living organisms just need any liquid. Ammonia, for example, is quite common in nature. And ethyl alcohol. Both are made from the most abundant elements in the universe. Ammonia mixed with water freezes at a much lower temperature than just water (-73°C rather than 0°C), which expands the temperature range at which there is a chance of finding living organisms that love the liquid. There is another option: on a planet where there are few sources of internal heat, for example, it rotates far from its star and is frozen to the bones, methane, which is usually in a gaseous state, can also play the role of the necessary liquid. Such compounds have a long shelf life. And for a chemical compound to be beneficial for a living organism, it needs bonds that are strong enough to withstand not too strong attacks from the environment, but not so indestructible as to cut off the possibility of further experiments.

How necessary is liquid water? Is this really the only medium suitable for chemical experiments, the only medium capable of delivering nutrients from one part of a living organism to another? Maybe living organisms just need any liquid. Ammonia, for example, is quite common in nature. And ethyl alcohol. Both are made from the most abundant elements in the universe. Ammonia mixed with water freezes at a much lower temperature than just water (-73°C rather than 0°C), which expands the temperature range at which there is a chance of finding living organisms that love the liquid. There is another option: on a planet where there are few sources of internal heat, for example, it rotates far from its star and is frozen to the bones, methane, which is usually in a gaseous state, can also play the role of the necessary liquid.

In 2005, the Huygens space probe (named after you-know-who) landed on Titan, Saturn's largest moon, which is rich in organic compounds and has an atmosphere ten times thicker than Earth's. Apart from the planets Jupiter, Saturn, Uranus and Neptune, each of which consists entirely of gas and has no solid surface, only four celestial bodies in our solar system have a noteworthy atmosphere: Venus, Earth, Mars and Titan. Titan is by no means a random object of study. The list of molecules that can be found there inspires respect: this includes water, ammonia, methane, and ethane, as well as the so-called polycyclic aromatic hydrocarbons - molecules from many rings. The water ice on Titan is so cold that it is as hard as cement. However, a combination of temperature and pressure liquefies the methane, and the first Huygens images show streams, rivers and lakes of liquid methane. The chemical environment on Titan's surface is in some ways reminiscent of the environment on the young Earth, which is why so many astrobiologists consider Titan a "living" laboratory for studying the Earth's distant past. Indeed, experiments carried out two decades ago showed that if we add water and a little acid to the organic suspension that is obtained by irradiating the gases that make up Titan's turbid atmosphere, this will give us sixteen amino acids.

Recently, biologists learned that the total biomass below the surface of planet Earth is probably greater than on the surface. Current research into especially hardy living organisms shows time after time that life knows no barriers or boundaries. Researchers studying the conditions for the origin of life are no longer “nutty professors” looking for little green men on nearby planets, but rather generalist scientists who own a wide range of tools: they must be specialists not only in astrophysics, chemistry and biology, but also in geology and planetary science, since they have to look for life anywhere.

An example of a system for finding the habitable zone depending on the type of stars.

In astronomy, habitable zone, habitable zone, life zone (habitable zone, HZ) is a conditional region in space, determined from the calculation that the conditions on the surface of those in it will be close to the conditions on and will ensure the existence of water in the liquid phase. Accordingly, such planets (or theirs) will be favorable for the emergence of life similar to that on Earth. The probability of life arising is greatest in the habitable zone in the vicinity ( circumstellar habitable zone, CHZ ), located in the habitable zone ( galactic habitable zone, GHZ), although research on the latter is still in its infancy.

It should be noted that the location of a planet in the habitable zone and its favorableness for life are not necessarily related: the first characteristic describes the conditions in the planetary system as a whole, and the second - directly on the surface of the celestial body.

In English-language literature, the habitable zone is also called Goldilocks zone (Goldilocks Zone). This title is a reference to an English fairy tale Goldilocks and the Three Bears, known in Russian as “Three Bears.” In the fairy tale, Goldilocks tries to use several sets of three similar objects, in each of which one of the objects turns out to be too large (hard, hot, etc.), the other is too small (soft, cold...), and the third, intermediate between them , the item turns out to be “just right.” Likewise, to be in the habitable zone, a planet must be neither too far from the star nor too close to it, but at the “right” distance.

Habitable zone of a star

The boundaries of the habitable zone are established based on the requirement for the presence of liquid water on the planets located in it, since it is a necessary solvent in many biochemical reactions.

Beyond the outer edge of the habitable zone, the planet does not receive enough solar radiation to compensate for radiative losses, and its temperature will drop below the freezing point of water. A planet located closer to the star than the inner boundary of the habitable zone will be excessively heated by its radiation, causing water to evaporate.

The distance from the star where this phenomenon is possible is calculated from the size and luminosity of the star. The center of the habitable zone for a particular star is described by the equation:

Habitable zone in the solar system

There are different estimates of where the habitable zone extends:

Galactic habitable zone

Considerations that the location of a planetary system within a galaxy should influence the possibility of the development of life led to the concept of the so-called. "galactic habitable zone" ( GHZ, galactic habitable zone ). The concept was developed in 1995 Guillermo Gonzalez, despite its challenge.

The galactic habitable zone is, according to currently available ideas, a ring-shaped region located in the plane of the galactic disk. The habitable zone is estimated to be located in a region 7 to 9 kpc from the galactic center, expanding with time and containing stars 4 to 8 billion years old. Of these stars, 75% are older than the Sun.

In 2008, a group of scientists published extensive computer simulations suggesting that, at least in galaxies like the Milky Way, stars like the Sun could migrate over long distances. This contradicts the concept that some areas of the galaxy are more suitable for the formation of life than others.

Search for planets in the habitable zone

Planets in habitable zones are extremely interesting to scientists who are searching for both extraterrestrial life and future homes for humanity.

The Drake equation, which attempts to determine the likelihood of extraterrestrial intelligent life, includes a variable ( n e) as the number of habitable planets in star systems with planets. Finding Goldilocks helps clarify the values ​​for this variable. Extremely low values ​​may support the unique Earth hypothesis, which states that a series of extremely unlikely events led to the origin of life on . High values ​​can reinforce the Copernican principle of mediocrity in position: a large number of Goldilocks planets means that the Earth is not unique.

The search for Earth-sized planets in the habitable zones of stars is a key part of the mission, which uses (launched March 7, 2009, UTC) to survey and collect characteristics of planets in the habitable zones. As of April 2011, 1,235 possible planets had been discovered, of which 54 were located in habitable zones.

The first confirmed exoplanet in the habitable zone, Kepler-22 b, was discovered in 2011. As of February 3, 2012, four reliably confirmed planets are known to be in the habitable zones of their stars.