EVIDENCE FOR YOUNG PLANETS FOUND

david koerner, univ. of pennsylvania and w. m. keck observatory

Dust around the young nearby star, Beta Pictoris. This image was made 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).

MAUNA KEA, Hawaii (April 11th, 2002) 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.