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In our last talk, we mentioned that the surface temperature of the hottest stars may exceed 200,000 degrees, and even for small stars like the sun, the surface temperature is several thousand degrees. Stellar heat is normal, but can you imagine a planet hotter than some stars? But if a planet is really so hot, can it still be a planet? shouldn't it be a star?

Note: stars refer to celestial bodies that can glow and heat themselves through fusion reactions. So since it can be classified as a planet, it means that no matter how hot it is, it comes from the outside world, not from itself.

Someone might say, "No, it's hot inside the planet, too. This heat doesn't come from the outside."

The internal structure of the earth (schematic diagram) Yes, you are like the earth, it does have geothermal, but it is not caused by fusion reaction! Geothermal mainly comes from radioactive decay and residual heat at the beginning of planetary formation, which is extremely limited compared with fusion reaction. For example, the core of the earth is only a few thousand degrees, and it is even less that it can reach the surface and radiate outward. Even a giant planet like Jupiter has a core of only tens of thousands of degrees. By comparison, the interior of the sun is 1500 million degrees.

Jupiter's internal structure (schematic) so if a planet has an extremely high surface temperature, the heat must come from the outside world, such as its host star. But even the smallest red dwarf has a surface temperature of two or three thousand degrees. If a planet wants to have a temperature of thousands of degrees, its host star must be much hotter than a red dwarf.

But hotter means that stars are brighter and bigger, which is bad for searching for exoplanets.

Most of the exoplanets found so far are found around small stars like the sun. As dwarfs, such stars are relatively low in temperature and low in brightness, so it is easier for us to see planets around them. You see, if the star is particularly bright, then the planetary transit is not obvious, and the transit method is very difficult to find.

Secondly, the larger the mass of the star, the weaker the gravitational influence of the surrounding planets, and the weaker the Doppler effect of the star spectrum, so that the apparent velocity method is difficult to play a role.

The apparent velocity method (radial velocity method) and the transit method and the apparent velocity method are the most important ways to discover exoplanets at present. Of the more than 5,000 exoplanets identified so far, 95% have been discovered in these two ways. So it is difficult to find planets around massive stars in the first place by means of observation.

In addition, for massive stars, their radiation is very intense, which directly affects the formation and evolution of the surrounding planets. And the lifespan of massive stars is so short that even if there are planets, they will soon experience various explosions at the end of their evolution and even be swallowed up directly by inflated giants.

Therefore, the search for exoplanets has not focused on massive stars from the very beginning. In fact, there is another fundamental reason for this. What do you think we're doing looking for exoplanets? I just want to see if there is extraterrestrial life. Modelled on the earth, it takes a long time to evolve life. So for "short-lived ghosts" who only have a life span of tens of millions (or even millions), we don't have to spend too much time on it.

However, in recent years, this rare exoplanet has been discovered by us.

In 2017, astronomers discovered a special planet, KELT-9 b, 670 light-years away. Before, planets were rarely seen near type A stars, and the planets around type B stars were unheard of. The planet revolves around a massive star between a type A star and a type B star, with a mass 2 to 5 times that of the sun and a surface temperature of tens of thousands of degrees.

Even if KELT-9 b's transit and apparent velocity data are just stellar heat, the key is that the planet is still very close to the star, with only 0.03 astronomical units, which is about 1 inch 10 from Mercury to the sun. You can imagine how hot the surface of the planet has to be.

KELT-9 data show that the planet has been locked by the tide of the star at such close range that one side of it will be scorched by the star for a long time. After calculation, the "sunny side" temperature of the star can reach 4000 degrees! This number has exceeded the temperature of most red dwarfs (M-stars) or even orange dwarfs (K-stars)! In some parts of its atmosphere, the temperature may even be as high as tens of thousands of degrees!

Surface temperature of KELT-9 b although this is a gaseous giant planet twice the mass of Jupiter, its atmosphere is rapidly dissipating in such an environment that is scorched by stars at close range.

Interestingly, the planet's atmosphere is very special. Due to the extremely high temperature and long-term exposure to high-energy rays, the normal molecular structure no longer exists and is replaced by torn atoms. And in addition to hydrogen, helium and other elements, scientists have also found iron, titanium and other metal elements here. These elements combine to form metal oxides or hydrides as they move to colder nights, so it may rain like lava in the planet's sky.

KELT-9 b (concept map) some people may wonder how far away an exoplanet is, how do astronomers know how many degrees it is?

There are many ways to calculate the surface temperature of exoplanets, but these are the main factors involved: the first and foremost is, of course, how much energy it can receive from the star, so the star's light intensity is one of the first factors to be considered.

Second, the planet does not receive as much as it comes. After all, it is not an ideal blackbody that can absorb 100% radiation, and it will reflect some of it, so the reflectivity of the planet's surface is also one of the factors to be considered.

In addition, the distance between the planet and the star is also critical. After all, the radiation intensity is inversely proportional to the square like gravity, that is, if the distance is doubled, the radiation intensity will be reduced to 4 times the original one, the distance is 10 times, and the intensity is 100.

Based on this information, we can roughly calculate a base temperature, also known as the equilibrium temperature of the planet. This temperature is a very rough theoretical value and is not usually mentioned as the average temperature of the planet, because it ignores a very key factor-the planetary atmosphere.

A planet with an atmosphere is like a quilt, and the heat preservation effect can be said to be immediate. For example, the equilibrium temperature of Venus is only about-13 ℃, but in fact, we know that the atmosphere of Venus is extremely thick, and Greenhouse Effect explodes, so its average temperature can actually reach more than 400 degrees.

In addition to the atmosphere of Venus, the orbital shape and inclination of the planet will also directly affect the surface temperature. Considering that there is a certain error in the calculation of the temperature of the star itself, plus the distance error between the planet and the star, the reflectivity error. It can be seen that the estimation of the temperature of exoplanets is actually very rough, and it is difficult to say who is the real "hottest planet".

For example, the next planet, Kepler-70 b (KOI-55 b), may even have a higher surface temperature than the sun.

Unlike KELT-9 b, Kepler-70 b is a rocky planet with a density close to that of Earth, but less than half the mass of Earth. According to a rough calculation by scientists, the surface temperature of the planet may have exceeded 7000 degrees.

The concept map of KOI-55 b (Kepler-70 b) has such an adverse temperature that its host star must be very hot.

Yes, Kepler-70 is a type B dwarf with a surface temperature of 27000 degrees. But the temperature of the dwarf star is not high, why is the temperature of this dwarf star so high? Because it's actually the core of a red giant. Because the outer layer of hydrogen has been spilled out, the star is left with a hot core rich in helium. When helium is almost burned in the future, it will collapse into a white dwarf.

In addition to the hot stars, the planet is very close to the star. The KELT-9 b just mentioned is 0.03 astronomical units from the star, while this planet is only 0.006 astronomical units from the star, which is less than the one from Mercury to the sun.

Did the KOI-55 (Kepler-70) system find anything? So close to the star, and the star has experienced the red giant phase, that is to say, the planet may have entered the interior of the star!

Scientists speculate that the planet could have been a gaseous giant. Later, as the host star expanded into a red giant, the planet's atmosphere was gradually evaporated and eventually swallowed by the star as a whole. Then the planet's core shattered inside the red giant, leaving only a few fragments left to become today's lava planets.

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

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