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In 1846, French astronomer Urbain Le Verrier (Urbain Le Verrier) used mathematical calculations to determine the exact location of a planet. Soon, Neptune-the last known planet in the solar system-was observed under Levier's guidance.
And mankind will find a new planet again, it will have to wait until 1992. This time, the human vision rushed out of the solar system. In Virgo, 2800 light years away, radio astronomers Aleksander Wolszczan and Dale Frail discovered that there are two planets orbiting around the pulsar PSR 1257+12. It is also the first time a planet has been found outside the solar system. From Neptune to the first exoplanet (exoplanet for short), people have been searching for nearly a century and a half; and from the latter to finding the first exoplanet, it may take us less than 30 years. A recent paper published on the preprint platform arXiv announced: Using data from the Chandra X-ray telescope, researchers have discovered a planet slightly smaller than Saturn in a spiral galaxy 28 million light years away. Search in the Milky Way After the first confirmation of the existence of exoplanets in 1992, astronomers' search for exoplanets has never slowed down. So far, 4,348 exoplanets have been confirmed to be found in 3,213 star systems, and there are more than 5,000 to be confirmed. Among them, the transit method and the radial velocity method are the main methods to search for exoplanets. The Transit Method is based on the scenario: when a planet passes over the surface of a star, the star will naturally become dim in the eyes of earth observers because the planet blocks part of the light emitted by the star. Of course, the principle seems simple, but actual observation is not easy. Because the volume of stars often far exceeds that of their planets, the decline in star brightness is minimal. For example, when an earth-like planet passes by a sun-like star, the brightness of the star will only drop by about one ten thousandth. Therefore, only a sufficiently precise detector can capture such faint changes. It is the Kepler Space Telescope that was launched in 2009 that can achieve this type of observation. During the observation lifetime of nearly 10 years, the Kepler telescope discovered 2662 exoplanets. After its decommissioning, the Transiting Exoplanet Survey Satellite (TESS) became the successor, continuing to search for exoplanets in a wider field of view and at a greater distance. Another important observation method is the radial velocity method. If there are planets orbiting around a star, the gravitational force of the planet will cause the speed of the star to move away from or approach us to change. According to the Doppler effect, we can find such changes in the spectrum of stars. Although the signal is also weak, astronomers have already gained something through this method. In 1995, Michel Mayor and Didier Queloz used this strategy to detect the first exoplanet orbiting a sun-like star. Won the Nobel Prize in Physics last year. However, the nearly 10,000 planets (and candidate celestial bodies) found so far are all located in the Milky Way Galaxy. When these methods that have achieved fruitful results for us in the galaxy are applied outside the galaxy, they all become powerless. This dilemma is not difficult to understand: whether it is the transit method or the radial velocity method, the magnitude of these changes is inherently weak, and it is more difficult to observe when placed in other galaxies farther away. X-ray transit method(Hermes Constance 24cm For Sale) In this research led by astronomer R. Di Stefano of the Harvard Smithsonian Center for Astrophysics, they considered another type of signal. The principle of this observation is actually similar to that of the Kepler telescope, both of which are transiting-but the signal source is changed from visible light to a bright X-ray source. Outside the Milky Way, bright X-ray sources mainly originate from the X-ray binary star system. This type of system consists of an ordinary star and the remains of a massive star (such as a black hole or a neutron star). The latter's huge gravitational force can accrete the material of the companion star, and in this process, the accretion disk will release X-rays. One reason why such X-ray signals can be used to search for planets outside the Milky Way is that they contain huge energy (for example, the brightness of this signal is about 1 million times the sum of the sun's various bands); more importantly, in X In the phenomenon of ray transit, the brightness of stars changes very obviously. In a normal transit, the entire star emits radiation, so the passing planet can only block a small part of the light. In contrast, the emission area of X-rays is concentrated in a small accretion disk. When a planet passes by, it can even completely block the X-rays. At this time, the X-ray source has experienced a "total solar eclipse." With this strategy, the research team used data from the most advanced contemporary X-ray telescope, the Chandra X-ray Astronomical Telescope, and found an expectation in the Whirlpool Galaxy, which is more than 28 million light-years away. Long signal. This group of signals belongs to a binary star system named M51-ULS-1: During the duration of 3 hours, the X-ray brightness is shown in the figure below, showing a U-shaped curve, which is the characteristic of the transit phenomenon. In addition, the X-ray signal completely disappeared within 20-30 minutes. Of course, researchers are also aware that in addition to passing planets, there are other factors that may cause similar brightness changes. For example, the accretion process itself is disturbed, resulting in changes in brightness; changes in the nature of the X-ray binary stars cause the X-ray source to be turned off for a period of time; or, the X-ray source is not a planet, but a smaller size Stars (Hermes Constance 18cm Bag On Sale) However, based on the characteristics of the brightness curve and other astronomical limitations, these options were excluded one by one by the researchers. Therefore, the greatest possibility has surfaced: a planet (named M51-ULS-1b) is orbiting this binary star system in an orbit with a radius of billions of kilometers. According to calculations, the volume of M51-ULS-1b is slightly smaller than that of Saturn. It is worth mentioning that this can be said to be a discovery nearly 8 years late. This signal was captured by the Chandra telescope as early as 2012, but it was buried in a large amount of data and received no special attention. It was not until Di Stefano and others began to study the planets outside the Milky Way that its significance became apparent. However, it is still too early to say that we have found an extragalactic planet. At present, this paper has just been uploaded to the preprint website, and it has not yet been formally published after peer review. If finally confirmed, this will be an important progress in our understanding of the universe, and the scope of our search for planets will also be greatly expanded. At the same time, this also provides new ideas for finding terrestrial planets in the Milky Way.
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