Today in the history of astronomy, the first person to propose the Oort Cloud is born.
NASA's illustration here depicts the positioning of Voyager 2 relative to the Oort Cloud in 2018. Voyager 1 was in a similar location in 2013. Despite how close they appear, the scale of the solar system means it'll take about 300 years for the Voyagers to reach the inner edge of the Oort Cloud. Credit: NASA/JPL-Caltech
Born Oct. 23, 1893, in Estonia, Ernest Opik was based at the Astronomical Observatory in Tartu from 1921-1944. It was there in the 1920s that he began to research meteors and comets, their frequencies, and their speeds. In 1932, he proposed a reservoir of long-period comets that formed a large cloud, orbiting the Sun at the farthest reaches of the solar system. Opik’s theory was based on the fact that comets rapidly burn out once they swing into the inner solar system; therefore, he concluded, there must be a source out there somewhere to continue providing all the new comets we observe. In the 1950s, Jan Oort used cometary orbits to provide the mathematical evidence for this fresh-comet reserve. Although still a theoretical structure, this bubble or shell around our solar system is widely accepted as the explanation for long-period comets and is today known as the Oort Cloud – or, sometimes, as the Opik-Oort Cloud.
Elisa Neckar is senior production editor of Astronomy magazine, and the editor of the Today in the History of Astronomy feature on Astronomy.com.