How are they looking for planets in the habitable zone, what conditions are necessary for the formation of life and what is interesting about the discovery of the exoplanet Proxima b
The habitable zone, which in English is called the habitable zone, is an area in space with the most favorable conditions for life of the terrestrial type. The term habitat means that almost all conditions for life are met, we just do not 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 required amount of energy.
Study history: terrestrial planets
From the point of view of astrophysics, there were several stimuli for the emergence of the concept of a habitable zone. Consider our solar system and four terrestrial planets: Mercury, Venus, Earth and Mars. Mercury has no atmosphere, and it is too close to the Sun, therefore, 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 star's corona.
Consider the rest of the terrestrial planets in the solar system - these are Venus, Earth and Mars. They arose practically in 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 of the space age, when we have advanced in the study of 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 - these are temperatures at which many substances even melt. And from the first panoramic shots of the surface, we saw that it is completely inanimate and practically not adapted to life. The entire surface is one continent.
Terrestrial planets - Mercury, Venus, Earth, Mars
Promotional video:
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. The carbon dioxide atmosphere is very thin; if the water was there, then it was all frozen. Mars has a polar cap, and recent results from a mission to Mars suggest that ice exists under the sandy cover - the regolith.
And the Earth. Very favorable temperature, 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 see a small part of the solar system, in which three planets, called terrestrial planets, were formed, but their evolution is completely different. And on these first ideas about the possible paths of evolution of the planets themselves, the idea of the habitable zone arose.
Habitable zone boundaries
Astrophysicists observe and study the world around us, the outer space that surrounds us, that is, our solar system and planetary systems in 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 have always believed that other stars should have planets, but instrumental power allowed us to discover the first exoplanets - planets located outside the solar system - just 20 years ago.
How are the inner and outer boundaries of the habitable zone determined? It is believed that in our solar system, the habitable zone is located at a distance of 0.95 to 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. That is, 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 luminosity ratio of this star and refer to the luminosity of the Sun, that is:
Rae = (Lstar / Lsun) 1/2.
Here Rae is the average radius of the habitable zone in astronomical units, and Lstar and Lsun are the bolometric luminosities of the sought star and the Sun, respectively. The boundaries of the habitable zone are set 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 boundary of the habitable zone, the planet does not receive enough solar radiation to compensate for radiation 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, as a result of which the water will evaporate.
More strictly, the inner boundary is determined both by the distance of the planet from the star and by the composition of its atmosphere, and in particular by the presence of the so-called greenhouse gases: water vapor, carbon dioxide, methane, ammonia and others. As you know, 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 planet's surface and loss from the atmosphere.
The external border is already the other side of the issue. It can be much farther when there is little energy 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 the actual greenhouse gases 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 is a zone of potential habitability, that is, conditions are met in it that are necessary, but not sufficient for the formation of life. Here we need to talk about the viability of the planet, when a number of geophysical and biochemical phenomena and processes come into play, such as the planet's magnetic field, plate tectonics, the duration of planetary days, 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 if they are in the habitable zone. From astronomical observations, you can see where this planet is located, where its orbit is. If in the habitable zone, then immediately interest in this planet increases. Next, you need to study this planet in other aspects: atmosphere, chemical diversity, the presence of water and the source of heat. This already slightly takes us outside the brackets of the concept of "potential". But the main problem is that all these stars are very far away.
It's one thing to see a planet near 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 exceeding the radius of the Earth. Astrophysicists study them by studying the atmosphere, we do not see surfaces - 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.
Exoplanet (red dot on the left) and brown dwarf 2M1207b (middle). First image taken using direct imaging technology in 2004
ESO / VLT
In a broad sense, the search for life outside the solar system, and in the solar system, is the 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 there was very little oxygen on the early Earth. The simplest, primitive life arose early, multicellular life arose quite late, not to mention intelligent. But then, due to photosynthesis, oxygen began to form, 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,let's say the decomposition of water vapor under the influence of stellar ultraviolet radiation. Therefore, all the enthusiasm that, as soon as we see molecular oxygen, it will be a biomarker, is not entirely justified.
Mission "Kepler"
The Kepler Space Telescope (CT) is one of the most successful astronomical missions (of course, after the Hubble Space Telescope). It aims to find planets. Thanks to the Kepler CT, we have made a quantum leap in exoplanet research.
The Kepler CT was focused on one way of discovery - the so-called transits, when a photometer - the only instrument on board the satellite - tracked the change in the brightness of a star at the moment the planet passed between it and the telescope. This gave information about the planet's orbit, its mass, temperature regime. And this made it possible to identify about 4500 potential planetary candidates during the first part of this mission.
Space Telescope "Kepler"
NASA
In astrophysics, astronomy and, probably, in all natural science, it is customary to confirm discoveries. The photometer records that the brightness of the star changes, but what can this mean? Maybe the star has some kind of internal processes leading to changes; the planets pass - it is darkened. Therefore, it is necessary to look at the frequency of changes. But in order to say for sure that there are planets there, it is necessary to confirm this in some 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 was received of the presence of a planet named Proxima b near the star Proxima Centauri. Why is it so interesting to everyone? 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). This is the closest exoplanet to us and, possibly, the closest celestial body to the solar system, 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 taken. And a number of scientific teams from the European Southern Observatory (ESO) have observed the star for several years. They made a website called Pale Red Dot (palereddot.org - ed.), That is, a 'pale red dot', and posted observations there. Astronomers attracted different observers, and it was possible to track the results of the observations in the public domain. So, it was possible to follow the very process of the discovery of this planet almost online. And the name of the observing program and website goes back to the term Pale Red Dot coined by renowned American scientist Carl Sagan for images of 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 how our planet looks from the depths of space. This project was named Pale Blue Dot ('pale blue dot'), because from space, due to the luminosity of the atmosphere, our planet is visible as a blue dot.it was possible to follow the very process of the discovery of this planet almost online. And the name of the observing program and website goes back to the term Pale Red Dot coined by renowned American scientist Carl Sagan for images of 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 how our planet looks from the depths of space. This project was named Pale Blue Dot ('pale blue dot'), because from space, due to the luminosity of the atmosphere, our planet is visible as a blue dot.it was possible to follow the very process of the discovery of this planet almost online. And the name of the observing program and website goes back to the term Pale Red Dot coined by renowned American scientist Carl Sagan for images of 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 how our planet looks from the depths of space. This project was named Pale Blue Dot ('pale blue dot'), because from space, due to the luminosity of the atmosphere, our planet is visible as a blue 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 how our planet looks from the depths of space. This project was named Pale Blue Dot ('pale blue dot'), because from space, due to the luminosity of the atmosphere, our planet is visible as a blue 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 how our planet looks from the depths of space. This project was named 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, the 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 fainter, it is colder, and already at such distances it falls into the so-called zone of tidal capture. 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 warmed up, and the other is cold.
The alleged landscape of Proxima Centauri b as seen by the artist
ESO / M. Kornmesser
There are such climatic conditions, a system of winds that exchanges heat between the heated 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. There are stellar flares, coronal mass ejections, and so on. Quite a lot of scientific articles on this system have already been published, where, for example, they say that, unlike the Earth, the level of ultraviolet radiation there is 20-30 times higher. That is, in order to have favorable conditions on the surface, the atmosphere must be dense enough to protect against radiation. But this is the only exoplanet closest to us,which can be studied in detail with the next generation of astronomical instruments. Observe its atmosphere, see what is happening there, whether there are greenhouse gases, what is the climate there, whether there are biomarkers there. Astrophysicists will study the planet Proxima b, a hot object for research.
Perspectives
We are waiting for several new ground-based and space telescopes, new instruments to be launched. In Russia, this will be the Spektr-UF 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 im. Hubble. Its resolution will be much higher, and we will be able to observe the composition of the atmosphere in those exoplanets of which we know, somehow resolve their structure, climatic system. But we must understand that this is a common astronomical instrument - naturally, there will be very strong competition, as well as at the CT. Hubble: someone wants to watch the galaxy, someone - the stars, someone else something. Several specialized missions to explore exoplanets are planned,e.g. NASA's TESS (Transiting Exoplanet Survey Satellite). Actually, in the next 10 years, we can expect a significant advancement in our knowledge about exoplanets in general and about potentially habitable exoplanets like Earth, in particular.
Valery Shematovich, Doctor of Physics and Mathematics, Head of the Department of Solar System Research, Institute of Astronomy, Russian Academy of Sciences