In 1999, some of the clearest views yet seen of protoplanetary disks came from observations by the HST. A group at the California Institute of Technology's Image Processing and Analysis Center, in Pasadena, used NICMOS to peer through obscuring dust clouds surrounding six extremely young stars in a stellar nursery in Taurus, 450 light-years away. Evidence for dusty disks, 8 to 16 times the diameter of Neptune's orbit, was found in all six in the form of dark bands, believed to be dust lanes, crossing the bright areas around each star. Dark clumps and bright streamers above and below the dust lanes, apparent in the NICMOS images, suggest that raw material is still falling into the disks and driving outflowing jets of gas from the young stars. At about the same time, another group using Hubble took very sharp visible-light pictures of disks in the same region. John Krist of the Space Telescope Science Institute in Baltimore found that the young star Haro 6-5B is actually a small nebula crossed by a dust lane about 10 times the size of Neptune's orbit. Also in 1999, Karl Stapelfeldt of the Jet Propulsion Laboratory used Hubble's Wide Field and Planetary Camera 2 to spot the first example of an edge-on disk in a young binary star system. The disk is centered on the fainter component and has a diameter of only 31 times the diameter of Neptune's orbit. This offers further evidence that, in spite of theoretical predictions that the gravitational forces in binaries ought to tear apart fragile protoplanetary disks, planetary formation may well be able to occur in two-star systems.
Some young protoplanetary disks are thought to have a mass of 0.01 to 0.1 solar masses, or more than 10 times that needed to make a planetary system like our own. Much of this material will eventually be blown away by the strong stellar wind from the central star. Dust accounts for about 1 percent of the disk's initial mass, the rest being made up of gas, mainly hydrogen and helium. Young disks imaged by the HST, in Orion and Taurus, are seen at many different angles, from edge-on to nearly face-one, and are typically a few hundred astronomical units in diameter. Two of the most spectacular edge-on disks surround HH 30 and the "Butterfly Star," both of which are narrow in their central parts but gently flare at distances of about 100 astronomical units. Such flaring was predicted as a result of heating by the host star by Harvard-Smithsonian astronomers Lee Hartman and Scott Kenyon in the late 1980s.
As accretion continues within a protoplanetary disk, sizable objects known as planetesimals form which, after several million years, give rise to small, rocky planets close to the host star. Further out, where it is cold enough for ice to form in the disk, more solid material is available for world-building. Gas giants, like Jupiter and Saturn, may start with cores of rock and ice of about 10 Earth-masses and then sweep up large quantities of light gases to form thick atmospheres. This should result in the creation of a central cavity within the circumstellar disk, similar in size to the solar system, and a drastic depletion of the disk's gas content. Recent observations, for example of the 10-million-year-old star HR 4796A, provide evidence for this view.
Studies of more mature stars, around which dust rings have been detected, including Beta Pictoris, Vega, and Epsilon Eridani, suggest that the circumstellar material is in the form of regenerated disks. There is also indirect evidence of planets in these systems.
Related categories EXTRASOLAR PLANETS AND SUBSTELLAR OBJECTS
PLANETS AND MOONS
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