A new study suggests that most of the comets now orbiting the Sun in fact originated in other star systems. The study tries to put a number on the comet population of the Oort Cloud, a huge shell around the Sun far beyond the orbit of Pluto, and comes up with 400 billion. Using the current model, which assumes virtually all comets are left over pieces from the formation of the Solar System, the number comes to only about 6 billion.
The huge discrepancy probably stems from the birth circumstances of the Sun, researchers say. Astronomers think the Sun developed in a loose star cluster that contained perhaps between one hundred and one thousand members. The cluster broke apart, of course, since the Sun now flies through space essentially alone, but while the stars were close, they could have traded comets, and the Sun could have acquired a huge retinue.
Astronomers generally argue that studying comets can tell us a lot about the early Solar System. If this new study is correct, studying comets could also shed light on the formation of other solar systems-- a remarkable bonus for science.