What is the "Goldilocks Zone"? Scientists have identified a second condition for the goldilocks zone.

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, it won’t be long before 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. Where more interesting question: 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 satellites 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 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 life, it could have a long life ahead. 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.

According to a Yale University (USA) researcher, in the search for habitable worlds, we need to make room for the second Goldilocks condition.

For many decades, it was thought that the key factor in determining whether a planet could support life was its distance from its sun. In our solar system, for example, Venus is too close to the Sun, Mars is too far away, and Earth is just right. Scientists call this distance the “habitable zone” or “Goldilocks zone.”

It was also believed that planets were able to self-regulate their internal temperatures through mantle convection and underground rock displacement caused by internal heating and cooling. The planet may initially be too cold or too hot, but will eventually reach the right temperature.

New study published in the journal Science Advances August 19, 2016, indicates that simply being in the habitable zone is not enough to support life. The planet must initially have the required internal temperature.

A new study has shown that for life to originate and sustain, the planet must have a certain temperature. Credit: Michael S. Helfenbein/Yale University

“If you collect all kinds of scientific data about how the Earth has evolved in the last few billion years and try to make sense of it, you end up realizing that convection in the mantle is pretty indifferent to internal temperature,” said Jun Korenaga, author of the study and Professor of Geology and Geophysics at Yale University. Korenaga presented the general theoretical basis, which explains the degree of self-regulation expected for convection in the mantle. The scientist suggested that self-regulation is unlikely to be a characteristic of terrestrial planets.

“The lack of a self-regulatory mechanism has enormous implications for planetary habitability. Research into planet formation suggests that Earth-like planets form through powerful impacts, and the outcome of this highly random process is known to be highly variable,” Korenaga writes.

The diversity of sizes and internal temperatures would not hinder planetary evolution if self-regulation of the mantle occurred. The things we take for granted on our planet, including the oceans and continents, would not exist if the Earth's internal temperature was not within a certain range, meaning that the beginning of Earth's history was not too hot or too cold.

The NASA Astrobiology Institute supported the research. Korenaga is a co-investigator on NASA's Alternative Earths project team. The team is focused on how Earth maintained a persistent biosphere throughout most of its history, how the biosphere manifests itself in planetary-scale “biosignatures,” and the search for life within and beyond the solar system.

The habitable zone, which in English is called habitable zone, is an area in space with the most favorable conditions for earth-type life. Term habitat means that almost all conditions for life are met, we just don’t see it. Suitability for life is determined by the following factors: the presence of water in liquid form, a sufficiently dense atmosphere, chemical diversity (simple and complex molecules based on H, C, N, O, S and P) and the presence of a star that brings the necessary amount of energy.

History of study: terrestrial planets

From an astrophysical point of view, there were several incentives for the emergence of the concept of a habitable zone. Consider our solar system and the four terrestrial planets: Mercury, Venus, Earth and Mars. Mercury has no atmosphere and is too close to , so it is not very interesting to us. This is a planet with a sad fate, because even if it had an atmosphere, it would be carried away by the solar wind, that is, a stream of plasma continuously flowing from the corona of the star.

Let's consider the remaining terrestrial planets in the solar system - these are Venus, Earth and Mars. They arose in almost the same place and under the same conditions ~ 4.5 billion years ago. And therefore, from the point of view of astrophysics, their evolution should be quite similar. Now, at the beginning space age As we progressed in studying these planets using spacecraft, the results obtained showed extremely different conditions on these planets. We now know that Venus has very high pressure and is very hot on the surface, 460–480 °C are temperatures at which many substances even melt. And from the first panoramic photographs of the surface, we saw that it was completely lifeless and practically not adapted to life. The entire surface is one continent.

// Image: Planets terrestrial group– Mercury, Venus, Earth, Mars. (commons.wikimedia.org)

On the other hand, Mars. It's a cold world. Mars has lost its atmosphere. This is again a desert surface, although there are mountains and volcanoes. Atmosphere from carbon dioxide very sparse; if there was water there, it was all frozen. Mars has a polar cap, and the latest results from a mission to Mars suggest that ice exists beneath the sandy regolith.

And the Earth. Very favorable temperature, the water does not freeze (at least not everywhere). And it was on Earth that life arose - both primitive and multicellular, intelligent life. It would seem that we are seeing a small part of the solar system in which three planets, called terrestrial planets, formed, but their evolution is completely different. And at these first ideas about possible ways evolution of the planets themselves and the idea of ​​a habitable zone arose.

Boundaries of the habitable zone

Astrophysicists observe and explore the world around us, the outer space that surrounds us, that is, our Solar system and the planetary systems of other stars. And in order to somehow systematize where to look, what objects to be interested in, you need to understand how to determine the habitable zone. We always believed that other stars must have planets, but instrumental capabilities allowed us to discover the first ones - planets located outside the solar system - only 20 years ago.

How are the internal and external boundaries of the habitable zone determined? In our solar system, the habitable zone is believed to be between 0.95 and 1.37 astronomical units from the Sun. We know that the Earth is 1 astronomical unit (AU) from the Sun, Venus is 0.7 AU. e., Mars - 1.5 a. e. If we know the luminosity of a star, then it is very easy to calculate the center of the habitable zone - you just need to take the square root of the ratio of the luminosity of this star and relate it to the luminosity of the Sun, that is:

Rae=(Lstar/Lsun)½.

Here Rae is the average radius of the habitable zone in astronomical units, and Lstar and Lsun are bolometric luminosity indicators of the desired star and the Sun, respectively. 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 biomechanical 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.

More strictly, the internal boundary is determined both by the distance of the planet from the star, and the composition of its atmosphere, and especially the presence of so-called greenhouse gases: water vapor, carbon dioxide, methane, ammonia and others. As is known, greenhouse gases cause heating of the atmosphere, which in the case of a catastrophically growing greenhouse effect (for example, early Venus) leads to the evaporation of water from the surface of the planet and loss from the atmosphere.

The external border is another side of the issue. It can be much further when there is little energy coming from the Sun and the presence of greenhouse gases in the atmosphere of Mars is not enough for the greenhouse effect to create a mild climate. As soon as the amount of energy becomes insufficient, greenhouse gases (water vapor, methane, etc.) from the atmosphere condense, fall as rain or snow, and so on. And greenhouse gases themselves have accumulated under the polar cap on Mars.

It is very important to say one word about the habitable zone for stars outside our Solar System: potential - a zone of potential habitability, that is, the conditions necessary, but not sufficient for the formation of life, are met in it. Here we need to talk about the habitability of the planet, when a whole series of geophysical and biochemical phenomena and processes come into play, such as the presence of magnetic field, plate tectonics, the length of a planetary day, and so on. The listed phenomena and processes are now being actively studied in a new direction of astronomical research - astrobiology.

Search for planets in the habitable zone

Astrophysicists simply look for planets and then determine whether they are in the habitable zone. From astronomical observations you can see where this planet is located, where its orbit is located. If in the habitable zone, then interest in this planet immediately increases. Next, we need to study this planet in other aspects: atmosphere, chemical diversity, presence of water and heat source. This takes us a little beyond the concept of “potential”. But the main problem is that all these stars are located very far away.

It's one thing to see a planet around a star like the Sun. There are a number of exoplanets similar to our Earth - the so-called sub- and super-Earths, that is, planets with radii close to or slightly larger than the radius of the Earth. Astrophysicists study them by examining the atmosphere; we do not see the surface - only in isolated cases, the so-called direct imaging, when we see only a very distant point. Therefore, we must study whether this planet has an atmosphere, and if so, what is its composition, what gases are there, and so on.

// Image: Exoplanet (red dot on the left) and brown dwarf 2M1207b (middle). The first image taken using direct imaging technology in 2004. Credit: ESO/VLT

In a broad sense, the search for life outside the Solar System, and even in the Solar System, is a search for so-called biomarkers. Biomarkers are believed to be chemical compounds of biological origin. We know that the main biomarker on Earth, for example, is the presence of oxygen in the atmosphere. We know that on early earth there was very little oxygen. The simplest, primitive life arose early, multicellular life arose quite late, not to mention intelligent. But then oxygen began to form due to photosynthesis, and the atmosphere changed. And this is one of the possible biomarkers. Now, from other theories, we know that there are a number of planets with oxygen atmospheres, but the formation of molecular oxygen there is caused not by biological, but by ordinary physical processes, say, the decomposition of water vapor under the influence of stellar ultraviolet radiation. Therefore, all the enthusiasm about the fact that as soon as we see molecular oxygen, it will already be a biomarker is not entirely justified.

Kepler mission

The Kepler Space Telescope (CT) is one of the most productive astronomical missions (after the space telescope, of course). It is aimed at finding planets. Thanks to Kepler CT, we have made a quantum leap in the study of exoplanets.

The Kepler CT was focused on one type of discovery - so-called transits, when a photometer - the only instrument on board the satellite - tracked the change in the brightness of a star as a planet passed between it and the telescope. This provided information about the planet’s orbit, its mass, and temperature conditions. And this made it possible to identify about 4,500 potential planet candidates during the first part of this mission.

// Kepler Space Telescope (NASA)

In astrophysics, astronomy and, probably, in all natural sciences, it is customary to confirm discoveries. The photometer detects that the star's brightness is changing, but what could this mean? Maybe some internal processes in the star lead to changes; planets pass - it darkens. Therefore, it is necessary to look at the frequency of changes. But in order to say for sure that there are planets there, you need to confirm this in some other way - for example, by changing the radial velocity of the star. That is, now there are about 3600 planets - these are planets confirmed by several methods of observation. And there are almost 5,000 potential candidates.

Proxima Centauri

In August 2016, confirmation of the presence of a planet, named Proxima b, was received near the star Proxima Centauri. Why is everyone so interested in this? For a very simple reason: it is the closest star to our Sun at a distance of 4.2 light years (that is, light covers this distance in 4.2 years). It is the closest exoplanet to us and possibly the closest to the solar system heavenly body, on which life can exist. The first measurements were taken in 2012, but since this star is a cool red dwarf, a very long series of measurements had to be carried out. And a number of scientific teams at the European Southern Observatory (ESO) have been observing the star for several years. They made a website, it's called Pale Red Dot(palereddot.org - editor's note), that is, a 'pale red dot', and observations were posted there. Astronomers used different observers, and the results of the observations could be tracked in the public domain. Thus, it was possible to follow the process of discovery of this planet almost online. And the name of the observing program and website goes back to the term Pale Red Dot, proposed by the famous American scientist Carl Sagan for images of the planet Earth transmitted by spacecraft from the depths of the solar system. When we try to find a planet like Earth in other star systems, we can try to imagine what our planet looks like from the depths of space. This project was called Pale Blue Dot(‘pale blue dot’), because from space, due to the luminosity of the atmosphere, our planet is visible as a blue dot.

Planet Proxima b found itself in the habitable zone of its star and relatively close to Earth. If we, planet Earth, are 1 astronomical unit from our star, then this new planet is 0.05, that is, 200 times closer. But the star shines weaker, it is colder, and already at such distances it falls into the so-called tidal capture zone. Just as the Earth captured the Moon and they rotate together, the same situation is here. But at the same time, one side of the planet is warm, and the other is cold.

// Image: Artist's impression of the proposed landscape of Proxima Centauri b (ESO/M. Kornmesser)

There are such climatic conditions, a system of winds that exchanges heat between the warm part and the dark part, and on the borders of these hemispheres there can be quite favorable conditions for life. But the problem with the planet Proxima Centauri b is that the parent star is a red dwarf. Red dwarfs live quite a long time, but they have one specific property: they are very active. Stellar flares, coronal mass ejections, and so on occur there. Quite a lot has already been published scientific articles according to this system, where, for example, they say that, unlike the Earth, there is 20–30 times higher levels of ultraviolet radiation. That is, in order for there to be favorable conditions on the surface, the atmosphere must be dense enough to protect against radiation. But this is the only exoplanet closest to us that can be studied in detail using the next generation of astronomical instruments. Observe its atmosphere, see what is happening there, whether there are greenhouse gases, what the climate is like there, whether there are biomarkers there. Astrophysicists will study the planet Proxima b, which is a hot object for research.

Prospects

We are waiting for several new ground and space telescopes, new instruments to be launched. In Russia it will be the Spektr-UV space telescope. The Institute of Astronomy of the Russian Academy of Sciences is actively working on this project. In 2018, the American Space Telescope will be launched. James Webb is the next generation compared to CT named after. Hubble. Its resolution will be much higher, and we will be able to observe the composition of the atmosphere of those exoplanets that we know about, and somehow resolve their structure and climate system. But you need to understand that this is a general astronomical instrument - naturally, there will be a lot of competition there, just like at the CT named after. Hubble: someone wants to look at galaxies, someone wants to look at stars, someone wants to look at something else. Several dedicated exoplanet exploration missions are planned, such as NASA's TESS ( Transiting Exoplanet Survey Satellite). In fact, in the next 10 years we can expect significant advances in our knowledge about exoplanets in general and about potentially habitable exoplanets like Earth in particular.

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 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:

(\displaystyle d_(AU)=(\sqrt (L_(star)/L_(sun)))), where: is the average radius of the habitable zone in , is the bolometric index (luminosity) of the star, is the bolometric index (luminosity) .

Habitable zone in the solar system

Exist various estimates where the habitable zone extends to:

Internal border, a.e.	External border, a. e.	Source	Notes
0,725	1,24	Dole 1964	Estimation assuming optically transparent and fixed albedo.
0,95	1,01	Hart et al. 1978, 1979	K0 stars can no longer have a habitable zone
0,95	3,0	Fogg 1992	Assessment using carbon cycles
0,95	1,37	Kasting et al. 1993
-	1-2% further...	Budyko 1969, Sellers 1969, North 1975	...leads to global glaciation.
4-7% closer...	-	Rasool & DeBurgh 1970	...and the oceans won't condense.
-	-	Schneider and Thompson 1980	Criticism of Hart.
-	-	Casting 1991
-	-	Casting 1988	Water clouds can narrow the habitable zone because they increase albedo, thereby counteracting the greenhouse effect.
-	-	Ramanathan and Collins 1991	The greenhouse effect for infrared radiation has a stronger effect than the increased albedo due to clouds, and Venus should have been dry.
-	-	Lovelock 1991
-	-	Whitemire et al. 1991

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 available data. this moment 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 an extensive study computer modelling, according to which, at least in galaxies like Milky Way, stars like the Sun can 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 ​​can confirm the hypothesis unique Earth, which argues that a series of extremely unlikely cases and 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.