Two independent teams of astronomers are presenting the discovery of new features in an edge-on disk around the
nearby star Beta Pictoris at the Gillett Symposium on “Debris Disks and the
Formation of Planets” in Tucson, Arizona.

Infrared images from the W.M. Keck Observatory reveal an important clue in
the configuration of dust confined to a solar-system sized region close to
the star: the dust orbits in a plane that is offset by approximately 14
degrees from that of the outer disk. Moreover, the offset is in the opposite
direction from that of a larger scale warp detected previously by Hubble
Space Telescope. This double warp is believed to be due to the presence of
one or more unseen planets and constitutes one of the strongest pieces of
evidence yet which links observations of circumstellar disks to the actual
formation of planets.

At the Keck II telescope at Mauna Kea, Hawaii, Prof. David Koerner and
graduate student Zahed Wahhaj of the University of Pennsylvania led a team
of astronomers from NASA’s Jet Propulsion Laboratory (JPL), Franklin and
Marshall College, and Caltech in observations of Beta Pic with MIRLIN, a
mid-infrared camera from JPL (
Alycia Weinberger, now at the Carnegie Institution of Washington, and Eric
Becklin and Ben Zuckerman from UCLA carried out observations with the Long
Wavelength Spectrometer at Keck I (LWS)
( Both
telescopes have 10-meter (400-inch) apertures. Both MIRLIN and LWS work at
wavelengths between 8 and 20 microns.

Prof. Koerner reported, “We’ve seen disk features before that could be due
to planets — inner holes, narrow rings, and variations in azimuthal
brightness. To date, however, most of these were discovered far outside the
region where planets reside in our own solar system, and plausible
non-planetary explanations have been found for some of them. In contrast,
the distorted disk plane in Keck images occurs at Jovian-planet distances
from the star (from 5 to 30 Astronomical Units or AU; 1 AU is the average
distance between the Earth and the Sun). Moreover, no obvious explanation
exists for its origin other than the gravitational influence of planets. The
different inclinations of dust grain orbits around Beta Pic bear a
resemblance to those of planetary orbits in our own solar system. Pluto’s
orbit is inclined by 17 degrees compared to Earth’s, and Mercury’s differs
by 7 degrees, for example. The new Keck images may be interpreted as
circumstantial evidence for a similarly organized planetary system.”
Dr. Weinberger added, “The images show the power of large ground-based
telescopes, like Keck, to reveal disk details in the hot inner portions of
disks.” In addition to imaging, Weinberger and colleagues obtained spectra
at different locations along the disk using the same Keck instrument (LWS).
Spectroscopy spreads the disk radiation into component wavelengths, much the
same way that a prism divides up visible light. The result enables
astronomers to study composition as well as geometry. Weinberger’s group
found that, at the position of the newly discovered warp, the disk is
composed of small silicate particles that are hotter than expected.
Weinberger says, “It may be that as a planet warps the disk, it also causes
more collisions of rocks in its neighborhood.” The very small grains
produced in collisions would tend to be hotter, at the same distance from
the star, than larger dust grains. Outside the warp, in the outer part of
the disk, the disk light appears to come either from larger grains or from
dust that is composed of something other than silicates.

To ensure that the observed offset was not the product of optical distortion
in either the atmosphere or telescope, Zahed Wahhaj carried out computer
modeling of the Keck image using a disk model and images of a nearby star
that were taken at the same time. His analysis provides an estimate of the
uncertainty in the measured value of the offset. “We generated millions of
different computer models of disks and used them to simulate images of Beta
Pic as observed with the Keck telescope. Computational comparisons of the
models with the images showed that the inner disk is offset from the outer
disk by an angle somewhere between 10 and 18 degrees. This is in good
agreement with a value between 11 and 15 degrees, as determined by the other

Beta Pictoris is a young star about 20 million years old that is located 63
light years away in the constellation Pictor (the painter’s easel). The star
is located too far south to be visible from the continental United States,
but it can be seen in winter from Hawaii where it rises just 20 degrees
above the horizon. In 1983, astronomers discovered dust radiation, first
from Vega, and later from Beta Pictoris using the Infrared Astronomical
Satellite (IRAS).

The Gillett Symposium commemorates Fred Gillett’s role in the discovery of
the first IRAS disk detection around Vega and is being held in his memory
one year after his death. Subsequent telescope observations of Beta Pic
yielded the first image of a protoplanetary disk. Like all observations
carried out at visible wavelengths, it required a coronagraph to block out
the glare from the central star. As a consequence, the region of the disk
corresponding to our solar system was not discernible for study. The human
eye is insensitive to the infrared light collected in the new Keck
observations of Beta Pic. The contrast between star and disk radiation is
more favorable, however, so the Jovian planet region was discernible for the
first time.

The W.M. Keck Observatory provides astronomers from associated institutions
access to two 10-meter telescopes, the world’s largest. Each telescope
features a revolutionary primary mirror composed of 36 hexagonal segments
that work in concert as a single piece of reflective glass to provide
unprecedented power and precision. Each telescope stands eight stories tall
and weighs 300 tons, yet operates with nanometer precision. The observatory
is operated by the California Association for Research in Astronomy, a
partnership of the California Institute of Technology, the University of
California, and the National Aeronautics and Space Administration (NASA),
which joined the partnership in October 1996. For more information, visit
the W.M. Keck Observatory Web site at or send e-mail

Dust around the young
nearby star, Beta Pictoris. This image was made with the Keck II 10-meter
(400 inch) telescopes using an infrared camera operating at 18 microns.
The inner contours are misaligned with respect to the outer disk, and provide
evidence of a newly discovered warp in the disk (labeled as “A”). For comparison,
an image of reflected light from Beta Pic is shown, as it appears in observations
taken with the Space Telescope Imaging Spectrometer (STIS) on board the
Hubble Space Telesope (HST). The HST/STIS image is exaggerated in vertical
scale to show a warp which occure further out and in the opposite direction
from that seen in the Keck infrared image. This morphology can be reproduced
as an inner disk with radius 5 to 30 AU and an orbital inclination that
is offset 14 +/- 4 degrees from the large outer disk, and in the opposite
sense of the HST/STIS warp. “B” refers to lobes equidistant from the star
that are consistent with a 40-AU-radius ring or bright inner edge of the
outer disk. “C” is a peak that could be associated with a ring further
out that is not azimuthally symmetric (i.e., its counterpart on the other
side of the star is not very prominent).

Examples of computer representation
of the infrared emission from Beta Pic, before the images were blurred
for comparison to Keck results. Viewing angles are 10 degrees above the
disk plane (upper) and along the line of sight from Earth to Beta Pic (lower).