Dutch astronomer Jan Oort, born April 28, 1900, studied physics and astronomy and went on to a long career at Leiden University. Oort was a pioneer in 20th-century astronomy, revolutionizing our understanding of galaxy rotation, radio astronomy, and dark matter.
In 1950, Oort proposed the existence of a huge group of trillions of pieces of icy debris at the outer edge of our solar system. This massive collection, which we today call the Oort Cloud, begins 1,000-5,000 astronomical units from the Sun, and extends almost halfway to Alpha Centauri. Building on the work of Ernst Opik by finding mathematical evidence of cometary orbits, Oort showed that because long-period comets lost material every time they pass close to the Sun, they couldn’t possibly have existed on their current orbits since formation of the solar system. Therefore, long-period comets must be routinely “replenished” from a collection of existing bodies. Recognizing that these comets come from all directions, Oort further reasoned that the cloud must be more of a bubble than a disc. Today, the Oort Cloud (sometimes called the Opik-Oort Cloud) is widely accepted, despite never having been directly observed or imaged.
