Dramatic pictures of eerie disks of dust encircling young stars are giving astronomers a new look at what may be the early formative stages of planetary systems.
Although these pictures from NASA's Hubble Space Telescope don't show planets, the edge-on disks seen by Hubble provide some of the clearest views to date of potential planetary construction zones, say researchers. The images also offer a peek at what happened 4.5 billion years ago when the Earth and other planets in our Solar System began to condense out of a pancake-like disk of dust and gas centered on the young Sun.
Although more than a dozen possible extrasolar planets have been discovered (though not imaged) over the past few years, astronomers lack detailed pictures of environments around newborn stars where planets form. Even in nearby star-forming regions, circumstellar disks are hard to see largely because the glare of the central star overpowers the feeble reflected light from the disk. An exception is when the disk is close to edge-on, eclipsing the infant sun.
"While the existence of these disks has been known from prior infrared and radio observations, the Hubble images reveal important new details such as a disk's size, shape, thickness, and orientation," said Deborah Padgett of Caltech's Infrared Processing and Analysis Center, Pasadena, CA.
Padgett's group used Hubble's Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) to peer through obscuring dust clouds surrounding six extremely young stars 450 light-years away in the constellation Taurus. Evidence for dusty disks was found in all six, in the form of dark bands (dust lanes) crossing bright areas around each star. The presumed disks have sizes 8-16 times the diameter of Neptune's orbit.
"The NICMOS images show dark clumps and bright streamers above and below the dust lanes, suggesting that raw material is still falling into these disks and driving outflowing jets of gas from the forming stars," Padgett said. Padgett's results are reported in a paper to appear in the March 1999 issue of The Astronomical Journal.
Another group using Hubble has taken extremely sharp visible-light pictures of disks in the same region. John Krist of the Space Telescope Science Institute, Baltimore, MD, found that the young star Haro 6-5B is actually a small nebula crossed by a dust lane 10 times the size of Neptune's orbit.
Karl Stapelfeldt of NASA's Jet Propulsion Laboratory, Pasedena, CA, used Hubble's Wide Field and Planetary Camera 2 to spot the first example of an edge-on disk in a young double star system. This disk is centered on the system's faint companion star, and has a diameter of only 3.5 times the diameter of Neptune's orbit.
"The Hubble images of this disk offer further evidence that planet formation should be possible in binary star systems", said Stapelfeldt. Theory suggests that gravitational forces in binary star systems tend to tear apart fragile planet-forming disks. Fitting theoretical models to the color and reflectivity of the dust in the disks observed, the team found evidence the dust grains were larger than those found in interstellar space, suggesting that the dust is clumping together and beginning to make larger bodies.
Computer modeling of the Hubble images makes it possible to estimate how much material is available in these disks to form planets. These estimates show the disks are about 1/200th to 1/10,000th the mass of the Sun (by comparison, the combined mass of the planets in our solar system today is about 1/1,000th of the Sun's mass).
Only Hubble and the new generation of telescopes with adaptive optics are able to see visible and near-infrared light evidence of objects as small as our Solar System around young stars in the nearest stellar nurseries in the constellation Taurus.
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