In the vast universe, black hole is like a devouring beast. No matter can escape from its huge gravitational claw, even light. Recently, a team of researchers proposed that there are planets around the supermassive black hole, which are similar to the planets around the stars. How did such planets form? Is it widespread? How were they discovered? Is there life on these planets? With these questions in mind, the reporter of science and technology daily interviewed Professor Wu Qingwen, vice president of the school of physics, Huazhong University of science and technology. Is human being the only life in the universe? To answer this question, scientists have been searching for exoplanets. Two of the three winners of the 2019 Nobel Prize in physics were awarded for the breakthrough in the study of exoplanets. < / P > < p > “exoplanets are one of the most hot and cutting-edge research fields in astronomy. Scientists mainly look around stars like the sun.” Searching for more exoplanets is conducive to understanding the mysteries of the formation of the solar system, the evolution of the earth and the origin of life, Wu said. Previously, scientists have observed the formation of the early solar system using the Atacama large millimeter / submillimeter telescope array (ALMA). First of all, gas and dust collapse to form stars under the action of self gravitation; as time goes on, the disk-shaped gas and dust around the star will gradually gather to form larger masses, and further condense to form planets. < p > < p > Japanese astronomers have proposed that gas and dust are not unique to young stars. There are also a lot of gas and dust around the black hole in the center of the galaxy. The mass of the gas and dust can be billions of times larger than that of the original stellar disk. A dark and cold supermassive black hole like the central black hole of the Milky way may also be an ideal place for breeding planets. The formation of ice and condensation of more gases and dust are the key conditions for the formation of planets. Wu Qingwen said that in the gas disk around the black hole, the closer to the black hole, the higher the gas temperature, but beyond a certain distance, the temperature is low enough to turn some water into ice. According to the calculation, for a supermassive black hole with a mass of 4 million times the mass of the sun in the center of the Milky way, about 10 light years away from the black hole is enough to condense gas such as dust and water, and form planets several to thousands of times the mass of the earth. Such planets are called black hole planets. < / P > < p > living near black holes, black hole planets are somewhat “scared”. Black holes have strong gravity, and even light is hard to escape within the horizon. Even if it is a little farther away, the powerful tidal force around the black hole may tear up the celestial bodies. This may sound terrifying, but as the distance increases, the gravity of the black hole will decrease rapidly. Beyond a certain distance (about a few light years), the movement of hot gas is almost no longer controlled by the black hole gravity. Therefore, the conditions for the formation of black hole planets have determined that they will not be swallowed up by black holes. Wu Qingwen said that there are hundreds of billions of galaxies in the universe, and there are hundreds of billions of stars in each galaxy. Therefore, earth like planets must exist, and black hole planets may also exist universally. Because of the long distance, it is difficult for us to directly detect the planets around the supermassive black hole in the center of the galaxy. Previously, Keiichi Keiichi Keiichi Keiichi, a professor of astrophysics at Kagoshima University in Japan, said that the development of modern astrophysics shows a tendency, that is, to develop in an unfamiliar, unexpected, or even impossible direction. < / P > < p > “as intelligent creatures living on planets, humans have always been interested in planets that revolve around other stars.” Wu Qingwen introduced that there are different black hole systems in the universe, such as active galactic nuclei, low luminosity galactic nuclei and X-ray binaries. According to the research model of Keiichi Huitian’s team, black hole planets generally appear around AGNs with short life cycle and relatively low luminosity. < p > < p > generally speaking, for the common planetary systems that revolve around the stars, after the formation of stars, the dust and gas will cool in the original disk around the new star, and the dust will condense into larger particles, which will form asteroids in collision and compression. Asteroids collide with each other to form larger planetary embryos, and finally form different physical processes Such as a gas giant similar to Jupiter, or a rocky planet similar to earth. < / P > < p > like stars, black holes have similar nurseries for planets. Wu said that supermassive black holes are usually located in the center of galaxies and provide energy to galaxies through accretion. Accretion is a kind of high-efficiency process in which a dense central celestial body captures the surrounding medium by gravitational action. The accreted matter rotates around the central black hole to form an accretion disk due to its initial angular momentum. In addition to the accretion disk, there are also huge dust rings around the black hole. The matter in the outer region of the accretion disk and in the dust ring is similar to the original astrolabe. It is reasonable to consider whether there are planets around the black hole on this basis. According to the model calculation results of Keiichi keida’s team, the mass of the most likely black hole planet is tens to hundreds of times that of the earth, and may even reach the upper limit of planetary mass. Under the condition that the average internal density of the two planets is similar, the maximum radius of the black hole planet can reach 10 times that of the earth. < / P > < p > “the origin of life is an important scientific proposition. It is worth studying whether there is life form on black hole planets.” Wu Qingwen said that life is great, but it is also fragile. A suitable and safe environment plays a vital role in nurturing life. Wu Qingwen pointed out that although planets can be formed around a supermassive black hole in the center of a galaxy, there are great differences between such planets and those around stars. < / P > < p > similar to the earth, planets that revolve around sun like stars can get sunlight, but black hole planets may not have this condition. Although it is not ruled out that stars and planetary systems similar to the solar system may be formed around black holes, the distance between black hole planets and black holes is much larger than that between the earth and the sun. For isolated black hole planets, they do not necessarily have sunlight like the earth. However, < / P >, which means that the solar radiation of black hole is mainly in the solar center. In addition, there are many high-energy activities around the supermassive black holes. These high-energy x-ray radiation or ultraviolet radiation will ionize and blow away the atmosphere on the planet surface. The surface atmosphere of Mars is extremely thin because almost all of its atmosphere is stripped away by the solar wind. Wu Qingwen said that the existence of life requires specific conditions, such as appropriate temperature and water. After the formation of a planet, according to the luminosity of the central star and the distance from the planet, we can estimate the surface temperature of the planet and judge whether it is livable. If the black hole planet is isolated, it is difficult to meet the above conditions. In addition, considering the high-energy events such as X-ray burst around the black hole, tidal collapse of the black hole, and supernova explosion in the central region, it is very difficult for life to survive here. < / P > < p > in the search for extraterrestrial life, we usually consider whether the planet is in a habitable zone, and whether water can exist in liquid form within a suitable distance around the star, so as to have the possibility of breeding life. < / P > < p > it should be pointed out that, in addition to meeting the conditions for the existence of liquid water, more factors must be taken into account in searching for signs of life in black hole planets. If the matter such as stars is relatively dense in the central region of the galaxy, black hole planets are easily affected by some catastrophic events, such as a large number of high-energy rays generated by the black hole activities in the center of the galaxy, and some close-up stars are disintegrated by the supermassive black hole in the center of the galaxy. These high-energy events may occur every thousands of years, which will bring devastating damage to the life of the planets around the black hole Strike. Wu Qingwen said that at present, everything about black hole planets is preliminary exploration and calculation, and perhaps in the near future, the mysterious veil of black hole, the “demon” of modern astronomy, will eventually be revealed. < / P > < p > “the possibility of looking for signs of life near a black hole is unpredictable, but if we never explore it, the possibility is zero.” Wu Qingwen said that should be the case when exploring interstellar space, although it is not known whether life is possible on black hole planets. Global Tech