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This is a new series, although it is called "vulgar planets", but instead of talking about the celestial bodies in the solar system, we focus on the field of exoplanets and take a look at the strange planets outside the solar system.

Unlike the abstract feeling of the universe, exoplanets are more concrete and more imaginable. So the content of this series should not have any threshold, whether you are an astronomy enthusiast or a rookie who knows nothing about it, I believe it is easy to accept.

In order to watch the series more easily in the future, as the first issue, I think it is necessary to introduce some basic knowledge, such as how many light-years of those distant planets and how many light-years did writers observe that day? How far have humans been in the exploration of exoplanets?

Exoplanets, that is, planets outside the solar system that revolve around other stars. Due to distance and technical limitations, almost all exoplanets found so far are in the Milky way. The number of stars in the Milky way is currently estimated at hundreds of billions, which means that the number of exoplanets may also be on the same scale. Although the number is large, this is only the total number speculated in theory, and only more than 5,000 have been confirmed in practice.

Of these identified exoplanets, about 1max 3 is a gaseous giant (a gaseous planet like Jupiter), a Neptune-like object (a gaseous planet like Neptune in size), and a super-Earth (a pure rocky or mixed planet with a mass larger than Earth but smaller than Neptune). Except for these three, the rest are basically Earth-like planets similar to Earth.

The type and number of exoplanets have long been thought to exist outside the solar system, but limited by technology, they were not really discovered until the end of the last century.

The first discovery of exoplanets dates back to 1984. A huge disk of gas and dust was first discovered around a star 63 light-years away. This means that the star should have just been born, and a lot of matter around it has not had time to gather into celestial bodies, so the place where the planet was born is called the protoplanetary chart.

Later, the European Southern Observatory (ESO) confirmed the existence of two planets here through direct imaging. However, the ability to image directly is obviously a later thing, because the direct imaging method has only been used in recent years. In addition to the accuracy requirements of the equipment, this observation method is very difficult and is only suitable for very special occasions. For example, the planet needs to be very large and far enough from the star.

Being large means that it reflects more light, so it is easier to see. It is far enough away from the star because the light of the star is so bright that it looks like a high beam, so it is usually necessary to use a coronometer to cover the star so that it does not go dark under the lamp.

So the direct imaging method does not mean that a very clear image of the planet is taken, it only determines whether there are planets here by observing whether the infrared light in the star is reflected. This approach requires not only the equipment, but also the environment of the planet. To this day, only a handful of exoplanets have been identified in this way.

Previous methods of observing exoplanets rely on the radial velocity method, also known as the apparent velocity method or Doppler spectrum, such as the discovery of the first exoplanet Pegasus 51b.

Here, by the way, we mention the naming rules of exoplanets: we often see XXXb, XXXc, and XXXd, whose names are not numbered according to the distance from the star, but in the order in which they were discovered. For example, Pegasus 51b, this "b" means that it is the first planet we found around the star Pegasus 51. Since a usually refers to the star itself, the first planet uses b, then c, d, e, and so on.

The return radial velocity method, the so-called radial velocity method, is judged by observing whether the star is affected by the gravitational pull of the surrounding planets. If there are planets around, the stars and planets are actually revolving around their common center of mass. For a star, it has a very weak wobble. Such a faint appearance cannot be seen directly, but it will be reflected in the star's spectrum because of the Doppler effect. Because the mass of the planet is much smaller than that of the star, this subtle change in the spectrum still requires high-precision detection equipment, so it is difficult to detect if the mass of the planet is too small.

Radial velocity method at the end of the last century, the radial velocity method was widely used by astronomers to study exoplanets. At the beginning of this century, with the development of space telescopes, we can finally get rid of the interference of the earth's atmosphere and observe from space. So people began to try to find exoplanets by measuring changes in the brightness of stars.

When a planet passes in front of a star, the star is slightly less bright because it is obscured, similar to a solar eclipse. Of course, it's not as much as the moon blocks the sun, but it's just a dot like the transit of Mercury. Because the occlusion of the planet to the star is periodic, we observe a periodic change in the brightness of the star, which is called the transit method or the transit method.

Compared with the radial velocity method, the advantage of the transit method is that it can directly calculate the size of the planet. after all, how much light it can block is directly related to the diameter of the planet, so the larger the planet is, the easier it is to find it.

For example, we mentioned last year that astronomers suspected of discovering an "extragalactic planet" the size of Saturn through the transit method. That's right, not outside the solar system, but outside the Milky way! Most of the exoplanets observed are no more than a few thousand light-years, the Milky way is 100, 000 light-years in diameter, and the galaxy M51 where the extragalactic planet is located is 2800 light-years away. Does it feel like a big deal?

The extragalactic planet in galaxy M51 (pictured), but the transiting method also has its drawbacks: first, it requires an angle of observation, and as observers we must be on the side of its orbit. In addition, there are many other reasons for stellar luminosity changes in addition to being obscured by planets, so the false alarm rate is relatively high. This is why the identification of exoplanets generally requires a variety of ways to cross-verify.

And cross-validation has another advantage: the Ling star method can calculate the size, and the radial velocity method can calculate the mass, so we can know the density of the combination of the two, so we can further infer its internal structure.

The transiting star method and the radial velocity method are the main methods to find exoplanets. Of the more than 5,000 exoplanets identified so far, about 95% have been found in these two ways. The rest is done in other ways, in addition to the direct imaging method just mentioned, such as microgravitational lenses, pulsar timing, and so on.

Even this year's Astrophysical Journal KuaiBao mentioned a new way to discover exoplanets, which is to predict the location of exoplanets by observing Trojan asteroids and combining Lagrangian points.

Prediction of exoplanets through Trojan asteroids most of the known exoplanets except data from ground-based telescopes come from observations from Kepler and TESS space telescopes. The Kepler telescope was launched in 2009 and the search was taken over by the TESS telescope when it was retired in 2018. As can be seen from this picture, the number of planets discovered by the transit method also began to soar at that time.

You may wonder: why are so many exoplanets concentrated in this small area of the sky (the purple region on the left)? Because this is the main observation area of Kepler. Why do you just shoot here? Because the transit method requires continuous observation in order to see the periodic luminosity changes of stars, the observation strategy of this telescope is to concentrate the observation area as much as possible. Even the TESS survey telescope, which is 400 times larger than Kepler's coverage area, has concentrated observation time in some areas.

Some people say why not use the Weber telescope to observe the sky with the TESS Space Telescope. Isn't infrared its strong point? Weber can certainly observe, and the observation of exoplanets is one of Weber's main tasks. But just like looking at microbes with a microscope, you need to use a low-power mirror to find the target, and then switch to a high-power mirror. Weber is like a high-power mirror on a microscope, although he can see it more clearly, but using it to find exoplanets is not nearly as efficient as the TESS survey telescope. So Weber's previous observations focused on identified exoplanets and focused on the analysis of their atmospheric composition. By analyzing the composition of the planet's atmosphere, we can speculate on the general situation of the planet and whether there is the possibility of life.

Well, in the next issue, we will officially begin to explore those strange and magical planets.

This article comes from the official account of Wechat: Linvo says ID:linvo001, author: Linvo

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