Exoplanets are planets orbiting other stars. As soon as ideas arose that the stars of the night sky were distant suns, they began to talk about the possibility of the existence of planets around them and life on these bodies. However, one thing is to reason, and another is to discover. Since planets are much lighter than stars and emit less light, it is very, very difficult to open them. Despite individual attempts, success only came at the end of the 20th century.

By the end of the 1980s, several observation methods had attained the required accuracy, and in the early 1990s, exoplanets began to be discovered. Now there are several thousand planets known, and their number is growing. It turned out that other planetary systems and their members may differ significantly from what we see in our system. The abyss has opened, the exoplanet is full. And now we need to understand how all these objects are arranged, how they were formed, how they will evolve. About this, perhaps the most "live" area of ​​astrophysics, our course.

Lecture # 1: How are exoplanets recorded?

As soon as people began to guess that the stars are distant suns, a natural idea appeared: once the planets are spinning around our Sun, then there must be planets around other stars. Now these are the planets we call exoplanets - the extra solar planet (extrasolar planets). And, in principle, people began to think long ago how to discover planets in other stars. This is difficult to do for two reasons: firstly, the planet itself is small, it shines very badly and only with reflected light, it is difficult to notice, it is even harder to notice because it is located next to a very bright star. It often happens that we could see the exoplanet itself if it were in exactly the same place with exactly the same brilliance, but the bright light of a very close star prevents us from doing so.
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The question of which exoplanet was discovered first is quite complicated. It simply does not have a clear answer. On the one hand, we can say that the first exoplanet around a star, somewhat similar to the Sun, was discovered in 1995 - the reliable discovery of an exoplanet in a solar-type star. However, in 1992, using a completely different technique, people discovered several exoplanets, but not around an ordinary star, but around a neutron star - a radio pulsar. Someone thinks that from this moment on you can count the exoplanetary history, someone thinks that these are still completely different animals, because the star is not like the Sun, and the planets most likely formed after the outbreak of a supernova a substance that was ejected by a supernova. But even this does not end the story. In 1988, a statement by a group of astronomers appeared that they might have discovered an exoplanet, but there was not enough observation accuracy. And only about 15 years later, this discovery was confirmed when the planets were already counting on many dozens. So, on the one hand, we can say that in 1988 the first exoplanet was discovered, but there was no certainty. And the story does not end there. In 1989, people stated that they definitely see a low-mass satellite in a normal star, similar to the Sun. But the difficulty is in determining what a planet is. There are planets, there are stars, between them there is another type of objects that are called brown dwarfs - they are heavier than planets, but lighter than stars, there are thermonuclear reactions, but these are not reactions of turning hydrogen into helium, which allow normal stars to emit a lot of energy. So, we still do not know whether the object discovered in 1989 is a very heavy exoplanet or a very light brown dwarf. So the story is quite confusing. Surprisingly, in the 1980s – 1990s, several methods simultaneously allowed the discovery of exoplanets. Pretty amazing coincidence. We will talk about these different methods.

First method

The first reliable exoplanet of a star of the Sun type was discovered in this way: usually, if we come to someone on the street and ask: “What does the spin revolve around: the Earth around the Sun or the Sun around the Earth?” If we are answered: “The Earth around the Sun ", We nod, and if they say:" The sun around the Earth ", we will point and laugh, but we can run into the answer that both of them rotate around a common center of mass. And this is really the right answer! We often do not think about the fact that the Earth attracts us, but we attract the Earth with exactly the same force. It's just that the Earth is very heavy, and we, having jumped, slightly displace the Earth, but if it increases your self-esteem, then, really, jumping, you slightly displace the Earth. In the same way, a planet, rotating around a star, makes the star move slightly, and this can be noticed. People understood this for a long time. What would you like to watch? We get the spectrum of a star with very high accuracy, we see spectral lines, if the star moves towards us, then all the lines shift to the blue region of the spectrum, if from us, then to the red. This is in the simplest case when there is a star and there is one planet or, if you like, the hardest. If you look at the solar system from a distance, Jupiter affects the Sun more than all other planets. We would see that with Jupiter’s orbital period, the Sun is approaching a distant observer, it is being removed, and according to the strict periodicity of the process, we could guess that this is really some kind of invisible satellite, and not, for example, the Sun pulsation or something else. . So, it was necessary to learn how to accurately measure these speeds and, moreover, to do it for a long time. What is the problem with time? For example, you have a watch that goes very accurately. Let's say it takes less than one second per day. Good. It takes a year. Can you, without stopping the clock - for example, you are sitting on a desert island and you don’t have any means of communication - it’s still accurate to determine the time with an accuracy of one second? It would seem, for example, they are lagging behind you for a second. A year has passed. You have counted 365 seconds and again think that you know the time to within a second. This is not at all the case, as there can be some non-uniformity of the course. It is not always exactly a second per day. The same with the measurement of spectra. Finally, in the late 1980s, people learned how to solve this problem in 1995; this is how the first exoplanet was discovered. Then people measured speeds of about 5–10 meters per second. By modern standards it is quite a lot.

Second method

The second way to discover exoplanets. Imagine that at some point the planet passes exactly between us and the star. In approximately the same way as, observing the Sun , once in quite a long time we see Venus or Mercury pass through the disk of the Sun. What will happen? We do not see the planets, we do not see any dark spots on the star, but we see that the star’s brightness has diminished a little. The star's disk is bright, and the planet is dark. Imagine that we are observing a star and accurately measuring its brilliance. We do not see the disk of the star, we do not see any details, but, measuring the brilliance with high precision, we see that suddenly the brilliance drops slightly. Indeed a little - it could be one ten-thousandth or several ten-thousandth. If this happens periodically, then the only reasonable reason is the passage of the planet through the star's disk. Such planets are called transit ones, and the phenomenon itself is called transit, and this is a very good way to discover exoplanets. The difficulty lies only in the need to measure their brilliance for a long time and very accurately. And on Earth, the atmosphere begins to interfere. Therefore, such observations are usually carried out from space. A telescope that is launched into space may not even be very large. It is important that it can measure the brightness of stars with high accuracy, because it does not interfere with the atmosphere. In this way, the Kepler satellite has already quite reliably discovered almost a thousand exoplanets. There are several thousand very reliable candidates, of which 90% over time will be confirmed. Now these are the two main ways of discovering exoplanets, but there are still some rather interesting ones.

Third method

The third method is associated with such a very beautiful phenomenon, as gravitational lensing. All the good modern theories of gravity, including the general theory of relativity, are geometric theories of gravity, where this lensing effect can be especially clearly explained. Heavy bodies distort the space around them, and this, depicting black holes, paint as such a hole in the plane on which a rectangular grid is drawn. The light, moving in such a space (let us directly imagine this plane, along which the light moves), will feel this hole, it will deflect. Thus, any heavy body effectively works as a collecting lens, that is, if we look at some distant star, measure its brilliance, any massive body flies between us and the star, another star, a neutron star, a black hole - what you want, the star's brilliance will increase slightly. Suppose another star flies, we see that the star's brilliance increases, and suddenly we see another additional “spikes” on the light curve, there is a second small lens. In this way, planets that revolve around stars are discovered. That star, which is a lens, possessing a planet, will give an additional "spikes". This is a very good way to register the planets, because this way you can register quite distant planets. See what the problem is: if we measure speeds or measure transits, we need several attempts to make sure that the effect is there, that these are not some random pulsations of the star, not some kind of spot appeared on the star, like sunspots, but the process is periodic. Observing 5 years, you cannot open a planet with a circulation period of 6 years, and by observing an event by spinning, you can immediately discover a planet whose revolution period around the star is years or decades, as you wish, since this method is not tied to repeatability therefore, it complements the first two well.

Fourth method

The following method for detecting exoplanets is associated with observations of any periodic processes. Imagine that you have a watch, they have to go exactly, and you see that they periodically go astray. If this is a mechanical watch, you may suspect that something is inside, something is interfering. They lubricated their watches with oil, and bread crumbs got there, and now the gears are not spinning so well or you have cockroaches that periodically prevent the gears from moving. In the same way, if there is some periodic process in the star system, for example, there is a double star (two stars revolve around each other), and an eclipse occurs, the luster periodically changes. This should be a very strict periodic process. We observe and see that failures sometimes occur. How to explain this? Something extra body in the system. You can simulate this system and determine the mass of this excess, interfering body. If it turns out to be planetary, then you have discovered the planet. And in this way open up quite unusual planets. For example, there is such a planet, a pulsar, a white dwarf rotates around it, and a planet rotates around this whole system, that is, by observing a pulsar, it was possible to understand that in addition to an invisible white dwarf, there is another extra component - a planet that revolves around this beautiful pair. .

Fifth method

Next way. Recall the very first thing - about changing speeds. So, the star moves around a common center of mass. We can directly see this movement. If we measure the coordinates of a star very accurately, imagine an image of a star and a cross that measures its position accurately. We measured today, we measured in half a year and saw that the star had shifted. They measured another six months later, and another, and another - they saw that it was a periodic movement. In this way, people discovered invisible satellites. For example, the very first white dwarf satellite of the brightest star in the night sky of Sirius was discovered in this way. We saw that Sirius is moving and has an invisible companion, but the white dwarf is a heavy object, it can have a mass equal to or equal to half the mass of the Sun. Finding an exoplanet in this way is difficult, but in principle possible. By the way, hundreds of years ago people stated that they see the movement of stars to corresponding small satellites, then, unfortunately, it turned out that this was due to some inaccuracies in observations, some errors, but Guy’s satellite, which specifically designed to measure the position of a huge number of stars with very high accuracy. It is not intended to search for exoplanets, but there is no doubt that a side result of the work of the satellite Guy will be the detection of about a thousand exoplanets.

Sixth method

Finally, we are getting close to the most understandable way of discovering exoplanets, as the mass media say, with a stingy journalistic line, “astronomers have seen.” Indeed, in some cases we can directly see the exoplanet, but before we get to this, there are two more ways that are similar to “see.” We can not directly see, but highlight the light of exoplanets. Often exoplanet emit quite a lot. For example, an exoplanet can be quite close to a star and heated to a high temperature, or it has just formed and continues to shrink and has its own energy source. Then, having an event like transits, we will see that the total brightness of the system changes or the planet will have phases, such as Venus, and we will see that the total brightness of the entire system changes. In this way, we will select extra light and will be able to attribute it to an invisible exoplanet. A similar way - is to see some parts in the spectrum, unusual for stars. They will periodically move, appear, disappear, and we can also attribute them to the exoplanet, also highlight its light.

And finally, really the last, most beautiful way is to get live images. Now there are several dozens of good direct images of exoplanets, when we actually see these objects and in rare cases we directly see how, as we observe observations, the exoplanet shifts in its orbit.

Thus, there are several beautiful, to varying degrees reliable methods for discovering exoplanets, which in general perfectly complement each other.

Sergey Popov - Doctor of Physics and Mathematics, Leading Researcher of the SAI MSU

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