The Solar System and Its Place in the Galaxy 7
FIGURE 1 False color images of the dust disk around the star ß Pictoris, discovered by theIRAS
satellite in 1983. The disk is viewed nearly edge on and is over 900 AU in diameter. The gaps in
the center of each image are where the central star image has been removed. The top image
shows the full disk as imaged with the Wide Field Planetary Camera 2 (WFPC2) camera onboard
theHubble Space Telescope (HST). The lower image shows the inner disk as viewed by the Space
Telescope Imaging Spectrograph (STIS) instrument onHST.The orbits of the outer planets of
our solar system, including the dwarf planet Pluto, are shown to scale for comparison. There is
evidence of a warping of the ß Pic disk, possibly caused by perturbations from a passing star.
Infrared data show that the disk does not extend all the way in to the star, but that it has an inner
edge at about 30 AU from ß Pic. The disk interior to that distance may have been swept up by the
accretion of planets in the nebula around the star. This disk is a possible analog for the Kuiper
belt around our own solar system.
eroded out to a distance of∼100 AU from the Sun, but that
considerably more mass may still exist in orbits beyond that
distance.
Although gravity is the dominant force in determining
the motion of bodies in the solar system, other forces do
come into play in special cases. Dust grains produced by
asteroid collisions or liberated from the sublimating icy sur-
faces of comets are small enough to be affected by radiation
pressure forces. For submicron grains, radiation pressure
from sunlight is sufficient to blow the grains out of the so-
lar system. For larger grains, radiation pressure causes the
grains to depart from Keplerian orbits. Radiation effects
can also cause larger grains to slowly spiral in toward the
Sun through the Poynting–Robertson effect, and meter- to
kilometer-sized bodies to spiral either inward or outward
due to the Yarkovsky effect.
Electromagnetic forces play a role in planetary magne-
tospheres where ions are trapped and spiral back and forth
along magnetic field lines, and in cometary Type I plasma
tails where ions are accelerated away from the cometary
coma by the solar wind. Dust grains trapped in planetary
magnetospheres and in interplanetary space also respond to