Comets from sister stars

Upating Comets

Did our comets come from sister stars?

How did the Solar System acquire its never-ending supply of comets to keep startling us? An explanation comes in today’s Science magazine, from Harold Levison and David Kaufmann of the Southwest Research Institute in Boulder, Colorado, working with Canadian and French colleagues.

The presumed source of supply is a very distant cloud of 100 billion or more comets, loosely bound to the Sun, called the Oort Cloud. The new report suggests that, in the tight cluster of stars in which the Sun was born, comets were scattered hither and yon in close encounters between stars, and many of our comets were captured from the Sun’s sisters.

In this extract from Comets I am at pains to stress that Jan Oort wasn’t the inventor of the distant comet cloud.

Ernst Öpik is an Estonian astronomer and musician who has recently been running the Armagh Observatory in Northern Ireland. For most of his long life he has adopted the role of cosmic garbage-sorter, concerning himself with the stray material of the Solar System. In 1932 he calculated that an invisible cloud of comets and meteors, surrounding the Sun at enormous distances, could survive throughout the long lifetime of the Solar System. In 1950 the doyen of Dutch astronomers, Jan Oort of Leiden, who is better known for classic work on the nature of galaxies, reworked Öpik’s idea. He emphasised a different aspect of it, namely that passing stars would cause a few of the objects to fall out of the cloud and into the heart of the Solar System, to become observable as ‘new’ comets.

Thus was the fabulous Öpik-Oort Cloud conceived, as the source of the comets. I abridge the name to the Öoo Cloud and defend this coinage on grounds of sight and sound. It looks like an untidy collection of roughly round objects of various sizes, and it is pronounced ‘Er, oh!’ – just what a neophyte comet lover is liable to utter when he is first told that there are many billions of the things out there.

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