Trillions of icy bodies mark the edge of the solar system. They form a shell that extends one or two light-years from the Sun in every direction. A passing star may sometimes give some of them a nudge toward the Sun. When they get here, they become comets – visitors from the icy deep.

That distant region is known as the Oort Cloud. It’s named for Dutch astronomer Jan Oort, who was born 125 years ago today. He proposed the existence of the cloud in 1950. And today, that’s his best-known accomplishment.

Yet it’s far from his most important work. Early in his career, he confirmed that the Milky Way is a wide, flat, spinning disk. And he showed that, instead of inhabiting the center of the galaxy, the solar system is in the hinterlands – many thousands of light-years outside the heart.

Oort spent most of his career at Leiden University in the Netherlands, including decades as director of Leiden Observatory. When Germany invaded the country, though, he left his post instead of working with the Nazis.

When he returned, after World War II, he became a pioneer in the new field of radio astronomy. He mapped giant clouds of gas and dust throughout the galaxy. Their distribution provided even more proof of his picture of the Milky Way.

Oort continued his research until shortly before his death, in 1992. Scientists have named quite a few things in his honor, including an asteroid – and the icy shell known as the Oort Cloud.

Script by Damond Benningfield