They were postulated around 1900 by two University of Chicago professors, geologist T. C. Chamberlain and astronomer F. R. Moulton. As planetesimals grow ever more massive, their gravitational attraction to the sun and to one another becomes more effective than the collisions they encounter as they move in the solar nebula. Here a savvy reader might well object. A few more numbers may serve to orient. The cosmic space we can survey offers us a view of fewer than a trillion galaxies and of fewer than half a trillion stars that make up our own galaxy.
Comets arrive nearby after falling in as far as a light-year to approach the sun.
New comets have straggled sunward from remote darkness every year for five billion years. In itself that would imply tens of billions stored, thereby allowing for losses. Long study of the orbits of comets shows that the gravitational disturbances from passing stars and clouds of gas only rarely nudge a comet out of the cold attic, so the original comet inventory must rise to many trillions. That comets come in trillions verifies the planetesimal idea, for comets are but a species of planetesimal from uncrowded icy orbits. The densely pockmarked faces of the moon and the airless planets, moreover, record the waning barrage that closed the curtain on planet formation, a process that is now all but extinct.