Debris Disks
Debris disks are formed when a planetary system has been build, and
when there are regions in the system where not all the material has
been collected to form big planets. In our own solar system, there
are two main regions where this has been the case. The asteroid belt
is the region between the planets Mars and Jupiter. Jupiter formed
very quickly around the young sun, and it stirred up the region next
to it so much that the growing bodies could no longer continue to
grow. Collisions became violent, whenever two bodies would meet,
destruction would follow. Still today, such collisions occur and
produce a spray of dust grains. We can see these grains in a dark
night, through reflected sun light above the horizon, as seen in the
picture to the right.
The other region where lots of material was left over is the Kuiper
Belt, outside the orbit of planet Neptune. It seems that this is also
the case in many other stars, because many stars show infrared
radiation from this cold dust, long after the main protoplanetary disk
has disappeared. Sometimes we can image this dust as well, the figure
to the right shows one example.
We can imagine that in the earliest phases of a planetary system, the
production of debris is much more violent that it is now in our solar
system, or in the debris disks around many stars. When planets are
forming, they can actually move around in the system, disturb the
orbits of leftover material and start the production of a lot of
dust. Some young stars seem to have a torus of sphere shaped clouds of
dust particles close to the star, and one way to produce such a
structure would be to have a planet migrate through the planetesimal
disk and stir up particles so much that they leave the plane of the
planetary system and even fly over the poles of the star. The figure
to the right, taken from Krijt & Dominik (A&A 531, A80), show how an
inward-migrating planet can do just that.