The habitable zone, which in English is called the habitable zone — a region in space with the most favorable conditions for life terrestrial. The term habitat means that met almost all the conditions for life, we just don’t see it. Fitness for life is determined by the following factors: the presence of water in liquid form, a dense atmosphere, chemical variety (simple and complex molecules based on H, C, N, O, S, and P) and the presence of stars, which brings the required amount of energy.
History of study: earth-like planets
From the point of view of astrophysics, there were several incentives to the notion of the habitable zone. Consider our Solar system four terrestrial planets: mercury, Venus, Earth and Mars. Mercury has no atmosphere, and it is too close to the Sun, so not very interesting to us. It is a planet with a sad fate because, even if she had an atmosphere she would have been carried away by the solar wind that is continuously flowing from the crown of the star’s plasma flow.
Consider other earth-like planets in the Solar system are Venus, Earth and Mars. They appeared almost in the same place 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 of the space age, when we have advanced in the study of these planets with spacecraft, the results showed extremely different conditions on these planets. We now know that Venus has very high pressure and very hot on the surface, 460-480 °C is the temperature at which a substance will melt even. And with the first panoramic pictures of the surface we saw that it is completely lifeless and almost not adapted to life. The entire surface is a single continent.
On the other hand, Mars. It’s a cold world. Mars lost atmosphere. Again, this is the desert surface, although there are mountains and volcanoes. Carbon dioxide atmosphere is very thin, if the water was there, the whole thing froze. Mars has a polar cap, and the latest results of missions to Mars suggests that beneath the sandy surface — regolith — is there ice.
And Earth. Very favorable temperature, water does not freeze (at least not everywhere). And what on Earth did life — as a primitive, multicellular, intelligent life. It would seem that we see a small part of the Solar system, which formed three planets, called terrestrial planets, but their evolution is quite different. And these first ideas about possible ways of evolution of the planets and the idea of the habitable zone.
The boundaries of the habitable zone
Astrophysicists observe and investigate the world around us, surrounding us space that is our Solar system and planetary systems around other stars. And to organize, where to look, what objects interested in, you need to understand how to determine the habitable zone. We have always believed that other stars must have planets, but the instrumental ability has allowed us to open the first exoplanets — planets located outside the Solar system — only 20 years ago.
How to define inner and outer boundaries of the habitable zone? It is believed that our Solar system the habitable zone is at a distance of from 0.95 to 1.37 astronomical units from the Sun. We know that the Earth is 1 astronomical unit (a. E.) from the Sun, Venus is 0.7 a. E., Mars 1.5 a. E. If we know the luminosity of the star, to calculate the center of the zone of habitability is very easy — you just need to take the square root of the ratio of the luminosity of this star and attributed to the luminosity of the Sun, that is:
Here Rае is the average radius of the habitable zone in astronomical units, and Lзвезда and Lсолнце — bolometric luminosity indicators searched the stars and the Sun, respectively. The boundaries of the habitable zones are established on the basis of the requirement for in it the planets water in the liquid state, because it is a necessary solvent in many biomechanical reactions. Beyond the outer boundary of the habitable zone, the planet receives enough solar radiation to compensate for losses by radiation, and its temperature falls below the freezing point of water. A planet closer to the sun than the inner boundary of the habitable zone is excessively heated its radiation causing the water to evaporate.
More strictly, the internal boundary is defined as the distance of the planet from the star and the composition of its atmosphere and in particular the presence of so-called greenhouse gases: water vapor, carbon dioxide, methane, ammonia, and others. As you know, greenhouse gases cause warming of the atmosphere, which in the case of dramatically increasing the greenhouse effect (e.g., early Venus) leads to the evaporation of water from the surface of the planet and loss from the atmosphere.
The outside border is another side of the issue. It can be much further when energy from the Sun comes to few and the presence of greenhouse gases in the atmosphere of Mars enough to the greenhouse effect created a mild climate. As soon as it becomes insufficient amounts of energy, greenhouse gases (water vapor, methane and so on) from the atmosphere is condensed, falls as rain or as snow, and so on. And in fact greenhouse gases have accumulated under the polar cap on Mars.
About zone of habitability for stars outside of our Solar system it is very important to speak one word: potential zone of potential habitability, that is, it complied with the conditions necessary but not sufficient for the formation of life. Here we can talk about of viable planets, when the game enters a number of geophysical and biochemical phenomena and processes, such as the presence of the planet’s magnetic field, plate tectonics, length of planetary day and so on. These phenomena and processes are being actively studied in a new direction astronomy — astrobiology.
The search for planets in the habitable zone
Astrophysics just looking for planets, and then determine whether they are in the habitable zone. From astronomical observations we can see where this planet is, where it located its orbit. If in the habitable zone, immediately the interest in this planet increases. Next you need to study this planet in other aspects: the atmosphere, the chemical diversity, the presence of water and a heat source. This is a little bit takes us over the brackets of the concept of “potential”. But the main problem is that these stars are very far away.
It’s one thing to see the planet from a star, like the Sun. There are a number of exoplanets similar to our Earth, the so — called sub – and super-earths, i.e. planets with radii similar to or slightly greater than the radius of the Earth. Astrophysicists studying them, exploring the atmosphere, the surface we do not see only in a few cases, the so-called direct imaging, when we see only a very distant point. We must therefore examine whether this planet has an atmosphere, and if so, what its composition is, what sort of gases, and so on.
Image: Exoplanet (the red dot on the left) and the brown dwarf, 2M1207b (in the middle). First picture taken using the technology of direct imaging in 2004. (ESO/VLT)
In the broad sense of finding life outside the Solar system, and in the Solar system is the search for so-called biomarkers. I believe that the biomarkers are 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 early Earth had very little oxygen. Simple, primitive life arose before multicellular life arose quite late, not to mention reasonable. But later due to photosynthesis began to form the oxygen that changed the atmosphere. And this is one of the possible biomarkers. Now from the other theories we know that there are a number of planets with oxygen atmospheres, but the formation of molecular oxygen there caused not biological, but ordinary physical processes, for example by decomposition of water vapor under the influence of the stellar UV radiation. Therefore, all the enthusiasm about the fact that, once we see how molecular oxygen, it will be a biomarker — it is not entirely justified.
The Mission Of “Kepler”
Space telescope (CT) “Kepler” is one of the most successful astronomical missions (of course, after the space telescope. The Hubble). It aims to search for planets. Thanks CT “Kepler” we have made a quantum leap in the study of exoplanets.
CT “Kepler” has been focused on one way open — the so-called transits, when the photometer is the only instrument on Board the satellite is tracked the change in the brightness of the stars at the time of passage of the planet between it and the telescope. It gave information about the orbit of a planet, its mass, temperature. And it is possible to determine at the first part of this mission, about 4500 candidates in the world.
In astrophysics, astronomy and everything science made confirmed discoveries. The photometer detects that the star of changing brightness, but what does that mean? Maybe stars of some internal processes lead to the changes that are on the planet — she is obscured. It is therefore necessary to look at the frequency changes. But to accurately say that there is a planet, we still have some way to confirm this — for example, a change in the star’s radial velocity. That is now about 3,600 planets is confirmed in several ways observations of the planet. But potential candidates almost 5000.
In August 2016 confirmation of the presence of a planet named Proxima b, the star Proxima Centauri. Why is everyone so interested? For a very simple reason: this is the closest to our Sun the star at a distance of 4.2 light-years (that is, the light covers this distance in 4.2 years). It is the closest exoplanet to us and, perhaps, the nearest to the Solar system celestial body on which life could exist. The first measurements were obtained in 2012, but since this star is a red dwarf, it was necessary to spend a very long series of measurements. A number of research groups at the European southern Observatory (ESO) observed the star for several years. They made a web site, it’s called Pale Red Dot (palereddot.org — approx. ed.), ‘pale-red’ point, and there laid out observation. Astronomers were attracted by different observers, and you can track the results of observations in the public domain. So, it was possible to follow by the process of discovery of this planet is almost online. And the name of the program observations and site goes back to the term Pale Red Dot, proposed by the famous American scientist Carl Sagan for images of the Earth transmitted by the spacecraft from the depths of the Solar system. When we try to find a planet like Earth, in other star systems, then we can try to imagine how our planet looks from outer space. This project called the Pale Blue Dot (‘pale blue dot’) because of the space because of the luminosity of the atmosphere of our planet is visible as a blue dot.
Planet Proxima b been found in the habitable zone of its star and relatively close to the Ground. If we, Earth are on 1 astronomical unit from its star, this new planet is 0.05, that is 200 times closer. But the star shines weaker, she is more cold, and at such distances it falls into the so-called zone of tidal capture. How the Earth captured the moon, and they rotate together, the same situation here. But one side of the planet warmed, and the second cold.
There are climatic conditions, the wind system exchanges heat between the heated part and the dark part, and on the borders of these hemispheres 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 a long time, but they have one peculiar property: they are very active. There are stellar flares, coronal mass ejections, and so on. Published quite a few scientific articles on this system, where, for example, say that, unlike Earth, there is 20-30 times higher levels of ultraviolet radiation. That is, on the surface had favorable conditions, the atmosphere must be dense enough to protect from radiation. But it’s the only nearest exoplanet, which it will be possible to study in detail with next-generation astronomical instruments. To observe its atmosphere, to see what is happening there, whether there are greenhouse gases, what’s the climate if there’s a biomarker. Astrophysics will study the planet Proxima b is a hot target for research.
We are waiting for some new ground and space telescopes, new tools that are launched. In Russia it will be a space telescope “Spectrum-UV”. Institute of astronomy RAS is actively working on this project. In 2018 will be launched by the American space telescope. James Webb is the next generation compared to CT them. Hubble. His resolution will be much higher, and we can have those exoplanets that we know of, to observe atmospheric composition, such as to resolve the structure of the climate system. But we must understand that this is a common astronomical instrument — of course, there is so much competition, and KT them. Hubble: someone galaxy wants to watch someone stars someone something. It is planned a number of specialized missions to study extrasolar planets, such as NASA’s TESS (Transiting Exoplanet Survey Satellite). In fact, in the next 10 years we can expect substantial progress in our knowledge about extrasolar planets in General and about potentially habitable extrasolar planets like Earth in particular.