Evidence for young planets found in dusty orbits about nearby stars

Discussion in 'Astronomy, Exobiology, & Cosmology' started by thed, Apr 23, 2002.

  1. thed IT Gopher Registered Senior Member

    Messages:
    1,105
    W.M. Keck Observatory
    Kamuela, Hawaii

    Media Contact:
    Laura K. Kraft, (808) 885-7887, lkraft@keck.hawaii.edu

    April 11, 2002

    EVIDENCE FOR YOUNG PLANETS FOUND IN DUSTY ORBITS AROUND NEARBY STAR

    TUCSON, Arizona -- 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
    (http://cougar.jpl.nasa.gov/mirlin.html). 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)
    (http://www2.keck.hawaii.edu:3636/realpublic/inst/lws/lws.html). 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 team."

    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 www.keckobservatory.org or send e-mail to:
    www@keck.hawaii.edu .

    IMAGE CAPTIONS:

    [Image 1: http://www.hep.upenn.edu/~davidk/bigpress2.gif (731KB)]
    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).

    [Image 2: http://www.hep.upenn.edu/~davidk/bigmod80.gif (94KB)
    Image 3: http://www.hep.upenn.edu/~davidk/bigmod89.gif (21KB)]
    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 [Image 2])
    and along the line of sight from Earth to Beta Pic (lower [Image 3]).
     

Share This Page