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让·亨德里克·奥尔特(1900～1992年)根据彗星围绕太阳运行的周期,天文学家将其分为两大类:周期在200年以上的长周期彗星和周期在200年以下的短周期彗星。按理说,由于行星的干扰作用,彗星应该均衡分布,周期长短不一。然而,丹麦天文学家让·亨德里克·奥尔特却发现,周期超过100万年的彗星在数量上占有明显优势。1950年,他指出这种现象的根源在于包围着太阳系的巨大的球状云团。该云团以太阳为中心的延伸半径达到10000～100000个天文单位(一个天文单位等于地球和太阳之间的平衡距离,星体和太阳之间的最近距离为270000个天文单位)。根据他的计算,云团中的彗星数目大约为1000亿颗。
According to Jean Hendrick Oort (1900-1992), according to the cycle of comets orbiting the sun, astronomers classify them into two categories: long-period comets with a period of 200 years or more, and short-cycles below 200 years. Cycle comet. It is reasonable to say that due to the interference of the planets, comets should be evenly distributed and their periods vary. However, the Danish astronomer Jean Hendrick Oort discovered that the comet with a period of more than 1 million years has a significant advantage in terms of quantity. In 1950, he pointed out that the root of this phenomenon is the huge spherical cloud that surrounds the solar system. The radius of the cloud centered on the sun reaches 10,000 to 100,000 astronomical units (one astronomical unit equals the equilibrium distance between the earth and the sun, and the closest distance between the star and the sun is 270,000 astronomical units). According to his calculations, the number of comets in the cloud is about 100 billion.