In 1922 he correctly predicted the frequency of craters on Mars long before they were detected by space probes. In 1932 he postulated a theory concerning the origins of comets in our solar system. He believed that they originated in a cloud orbiting far beyond the orbit of Pluto. This cloud is now known as the Oort cloud or alternatively the Öpik-Oort Cloud in his honour. From October 1931 to the end of July 1933, Öpik, Harlow Shapley and Samuel L. Boothroyd headed the Arizona meteor expedition, which detected approximately 22,000 meteors. He also invented a rocking camera for the study of meteors. In 1951 he published a paper concerning the triple-alpha process, describing the burning of helium-4 into carbon-12 in the cores of red giant stars. However, this achievement is often overlooked because Edwin Salpeter's paper on the same subject had already been published by the time Öpik's paper reached Britain and the United States.