Dutch astronomer
Jan Van
Oort studied the
orbits of 19
long-period
comets and
proposed in 1950 that the comets which are visible from
Earth come from a huge comet 'cloud' of perhaps a trillion
objects.
From the comets' orbits, Oort estimated
the size of this cloud to be about 100,000
astronomical units
(AU) in
diameter; about half the distance to the nearest stars. By
comparison, the orbit of Pluto, is only 40 AU in
diameter.
