SCIENCE BACKGROUND:THE DYNAMICS OF THE 
BETA PICTORIS DISK
                                
                                 
Why Hasn't The Warp Been Detected Previously?

Though the Beta Pictoris Disk has been studied intensively for more 
than a decade the inner region of the disk is very hard to see with 
ground-based telescopes because of the glare from the central
star.  Also, the visible disk is faint because it consists of microscopic 
grains of ice and dust that shine only by reflecting light from the star. 
Hubble Space Telescope concentrates the star's light and produces an 
image that is ten times sharper than can be obtained from the ground 
under good conditions.


How Is The Lifetime of The Warp Determined?

Because the microscopic dust particles in the disk collide, in about a 
million years they either fall into the star or get broken up and blown 
out of the system by radiation pressure.  Any warp in the visible disk 
will straighten out in even less time because of the same viscous 
processes.  This means some continuous source of both particles and 
the warp must be in operation.  The particles are probably the result 
of collisions within a belt of unseen larger comet-like objects which are
tens of kilometers in diameter (like objects in the Kuiper belt around 
our own Solar System). In the absence of a planetary perturbation, 
gravity would straighten out any warp in this "Kupier belt"
region within less than ten million years.


How Exactly Does a Planet Warp the Disk?

In 100 million years, a Jupiter-sized planet in a Jupiter-sized orbit 
would produce and maintain the warp Hubble sees. The Hubble results 
predict the planet's orbital plane is inclined by about three degrees to 
the outer disk.  The comet-like bodies within the warped area precess, 
or wobble, around the planet's orbital plane and this leads to the inner 
disk being fattened and aligned with the planet's orbit. Material outside 
that radius has not time for its orbit to precess significantly,
so appears in its original plane.  


Why Is There a Central Clear Area in the Beta Pictoris Disk?

The central clear area, approximately the diameter of our Solar System, 
has long been suspected of harboring one or more planets which 
coalesced out of the disk.  After planets form, they are expected to 
rapidly clear the visible disk in their vicinity.  However, an alternative 
explanation was that the clear zone is the result of ices melting 
(sublimation).


What Was Known Previously about the Beta Pictoris Disk?

Discovered in 1983, the Beta Pictoris disk has long ben been considered 
a relic of planet formation.  In 1775 philosopher Immanuel Kant 
proposed the nebular hypothesis of planet formation to explain the fact 
that the orbits of the planets almost lie in the same plane.  He considered 
these coplanar orbits a "skeleton" of a primordial disk where the planets 
grew from smaller particles that stuck together to "snowball" into 
larger bodies -- a process called agglomeration.  (Hubble observations 
of the Orion star forming region find these disks are common in early 
stages of star formation.)

The disk about beta Pictoris was deduced from infrared observations 
obtained with NASA's Infrared Astronomical Satellite (IRAS).  The 
discovery image was obtained by Brad Smith (University of Arizona) 
and Richard Terrile (JPL) in 1984 using a Charge Coupled Device 
(CCD) electronic camera with a coronagraph to block out the light 
from Beta Pictoris to reveal the faint disk.  Such ground-based 
telescopic images of Beta Pictoris have revealed a nearly 
edge-on disk extending at least 100 billion miles from the star 
(1,000 times the distance between the Earth and Sun).