CHAPTER 19 | THE ORIGIN OF THE SOLAR SYSTEM 415
comets, and Kuiper belt objects, and
astronomers have evidence that the solar
system’s Kuiper belt extending beyond the
orbit of Neptune is an example of an old
debris disk.
Some examples of debris disks are
around the stars Beta Pictoris, HD 107146,
and Epsilon Eridani (■ Figure 19-13). Th e
dust disk around Beta Pictoris, an A-type
star more massive and luminous than the
sun, is about 20 times the diameter of our
solar system. Th e dust disk around Epsilon
Eridani, which is a K-type star somewhat
smaller than the sun, is similar in size to the
solar system’s Kuiper belt. Like most of the
other known low-density disks, both of
these examples have central zones with even
lower density. Th ose inner regions are
understood to be places where planets have
fi nished forming and swept up most of the
construction material.
Few examples of planets orbiting stars
with debris disks have been detected, but
the presence of dust with short lifetimes
around old stars assures you that small bod-
ies such as asteroids and comets must be
present as sources of the dust. If those small
objects are there, then it is likely that there
are also planets orbiting those stars.
Infrared observations reveal that the star Vega, easily visible
in the northern hemisphere summer sky, also has a debris disk,
and detailed studies show that much of the dust in that disk is
tiny. Th e pressure of the light from Vega should blow away
small dust particles quickly, so astronomers conclude that the
dust being observed now must have been produced by a big
event like the collision of two large planetesimals within the last
million years (■ Figure 19-14). Fragments from that collision
are still smashing into each other now and then and producing
more dust, continuing to enhance the debris disk. Th is eff ect
has also been found in the disk around the faint star known as
HIP 8920. Such smashups probably happen rarely in a dust
disk, but when they happen, they make the disk very easy to
detect.
Notice the diff erence between the two kinds of planet-re-
lated disks that astronomers have found. Th e low-density dust
disks such as the ones around Beta Pictoris, Epsilon Eridani, and
Vega are produced by dust from collisions among comets, aster-
oids, and Kuiper belt objects. Such disks are evidence that plan-
etary systems have already formed (■ Figure 19-15). In
comparison, the dense disks of gas and dust such as those seen
round the stars in Orion are sites where planets could be form-
ing right now.bipolar fl ows (page 223) in that they focus the gas fl owing way
from a young star into two jets shooting in opposite directions.
In addition to these dense, hot planet-forming disks around
young stars, infrared astronomers have found cold, low-density
dust disks around stars much older than the newborn stars in
Orion, old enough to have fi nished forming. Th ese tenuous dust
disks are sometimes called debris disks because they are evi-
dently made of dusty debris produced in collisions among small
bodies such as comets, asteroids, and Kuiper belt objects, rather
than dust left over from an original protostellar disk. Th at con-
clusion is based on calculations showing that the observed dust
would be removed by radiation pressure in a much shorter time
than the ages of those stars, meaning the dust there now must
have been created relatively recently. Our own solar system con-
tains such “second-generation” dust produced by asteroids,
Infrared images■ Figure 19-12
Dark bands (indicated by arrows) are edge-on disks of gas and dust around
young stars seen in Hubble Space Telescope near-infrared images. Planets
may eventually form in these disks. These systems are so young that mate-
rial is still falling inward and being illuminated by light from the stars.
(D. Padgett, IPAC/Caltech; W. Brandner, IPAC; K. Stapelfeldt, JPL; and NASA).